DEPARTMENT OF THE INTERIOR

BULLETIN

OF THE

UNITED STATES

GEOLOGICAL SURVEY

No. 145

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1896

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

THE

POTOMAC FORMATION

IN

VIRGINIA

WILLIAM MORRIS FONTAINE

WASHINGTON

aOVEKNMENT PBINTING OFFICE 

' 1896

CONTENTS.

Letter of transmittal ....................................................... 9

lutroduction................................................................ 11

Location and boundaries of the Potomac terrane in Virginia................. 14

Location and boundaries of the subordinate areas ........................... 17

The Nottoway area ..................................................... 17

The Petersburg area .................................................... 18

The Hanover area....................................................... 19

The Fredericksburg area................................................ 20

The geologic character of the Potomac formation at different localities....... 24

Exposure near Weldon.................................................. 24-

Exposure at Boilings Bridge ............................................ 25

Exposure at Petersburg ................................................. 26

Exposure at Point of Eocks ............................................. 27

Exposures near Dutch Gap.............................................. 31

Exposures in Trents Eeach.......................................... 32

Exposures on Dutch Gap Canal ..................................... 35

Exposures above Dutch Gap ........................................ 36

The cobblestone bed near Dutch Gap ................................ 37

Exposures at Deep Bottom on the James ................................ 40

The artesian well at Fort Monroe........................................ 44

Exposure at Chaffins Bluff.............................................. 46

Exposure at Drurys Bluff ................................................ 46

Exposure at Wilton farm................................................ 48

Exposures at Richmond ................................................. 48

The Tertiary at Richmond .............................................. 50

Approximate section at Richmond................................... 54

Exposures near Hanover Junction ....................................... 54

Exposure near Morris Ford.............................................. 55

Exposures at Hanover Junction ......................................... 56

Exposure at Massaponax River ........................................... 57

Exposures on the Rappahannock below Fredericksburg.................. 58

Exposures at Fredericksburg............................................ 59

Section near Hazel Run.............................................. 66

Exposure at Alum Spring................................................ 68

Exposures between Fredericksburg and Falmouth........................ 72

Exposures near Falmouth ............................................... 73

Exposures on Potomac Run............................................... 75

Exposures at Potomac church ........................................... 75

Section on Potomac Run near Potoinac church ....................... 76

Exposure at the railroad bridge on Potomac Run ........................ 77

Exposures on the Telegraph road at Potomac Run........................ 79

Exposures on the Woodchopping road................................... 85

Exposures near Brooke.................................................. 85

Section at Brooke................................................... 86

5

	j
6

The geologic character of the Potomac formation, etc. Continued. Page.

Exposures on the railroad near Brooke .................................. 87

Section at the seventy-second milepost .............................. 89

Exposure at Aquia Creek................................................ 93

Exposures near Stafford................................................. 94

Quarries on Aquia Creek .............................. m. ..... i......... 95

The lower quarry ................................................... 95

The upper quarry ................................................... 97

Exposures on the Telegraph road at Aquia Run .......................... 99

Section along the Telegraph road south of Aquia Creek .............. 99

Exposures at Quantico .................................................. 100

Section on Neahsco Run..................................».......... 101

Exposure on Powells Run ................................... i........... 102

Section on Powells Run ............................................. . 103

Exposures along the railroad from Powells Creek to Cockpit Point....... 104

Exposures at Cockpit Point............................................. 106

Section in railroad cut at Cockpit Point............................. 106

Lignite deposit near Neabsco Run ....................................... 109

Exposures on the Colchester road near Neabsco Run ..................... 110

Exposures at Freestone Point ........................................... Ill

Exposures on the Telegraph road from Neabsco Run to Occoquan ........ 113

Exposures on the Telegraph road from Occoquan to Telegraph Station

(now Lorton)......................................................... 115

Exposures between Telegraph Station (Lorton) and Woodbridge......... 115

Exposure in the cut on Pohick Run ...................................... 118

Section near Pohick Church................................i............ 121

Exposures on the Accotink road......................................... 122

Exposure at Freestone Point.......:...........---................,...... 123

Exposure at Stony Point fishing shore................................... 123

Exposure above Indian Head............................................ 124

Exposure at Gunston Cove Bluff......................................... 125

Exposure at White House Bluff.......................................... 126

Exposure at Red Bank.................................................. 133

Exposure at Mount Vernon....... 1...,................_..._ ^ ........... 134

Exposure at Fort Washington........................................... 135

Section at Fort Washington......................................... 135

Exposures from Accotink to near Alexandria ............................ 137

Exposures near Seminary Station........................................ 138

Section on the road between Alexandria and the Episcopal high school... 139

Exposures on the railroad near Seminary Station ............. ^.......... 140

Exposures on the Little River turnpike.................................. 141

The Potomac formation at and between Washington and Baltimore ...... 142

The geologic position of the Potomac formation.............................. 142
ILLUSTRATIONS.

Pago. 

PLATE I. Map of portions of Maryland, Virginia, and District of Columbia,

showing distribution of Potomac formation .................... 14

II. Figs. 1-4. Modes of occurrence of clay layers at Dutch Gap....... 36

FIG. 1. Erosion horizon in the Tertiary at Richmond...................... 53

2. Lenticular patches of color at Fredericksburg...................... 62

3. Elliptical streaks of color near Fredericksburg.................... 67

4. Irregular stratification near Fredericksburg....................... 68

5. Current-bedding near Fredericksburg............................. 68

6. Current-bedding outlining lenses near Fredericksburg............. 70

7. Pre-Eoceue ravines near Brooke................................... 89

8. Clay pockets near Brooke......................................... 90

9. Clay pockets near Brooke......................................... 90

10. Clay pockets near Brooke......................................... 91

11. Erosion horizons at the upper quarry on Aquia Creek............. 98

12. Eroded clay layer near Powells Creek............................. 105

13. Remnant of clay layer at White House Bluff........................ 130

7

LETTER OF TRANSMITTAL.

UNIVERSITY OF VIRGINIA, January 2,1896.

SIR : I transmit with this my report on the geology of the Potomac 

formation in Virginia. The facts embodied therein were obtained by 

detailed work carried on during 1886 and 1887 at a large number of 

localities in the different areas occupied by the formation. The local 

detailed work was supplemented by a comparative examination of the 

entire formation from its northern to its southern limits. The manuscript was prepared in 1887, but the nomenclature has since been 

modified.

The Potomac formation is of special interest, both because it fills a 

gap hitherto thought to exist in the geologic column of the eastern part 

of the United States and because recent investigations show that 

strata of like age are widely distributed and cover extensive areas in 

different portions of the United States.

 Eespectfully, WM. M. FONTAINE.

To the DIRECTOR UNITED STATES GEOLOGICAL SURVEY.

9

THE POTOMAC FORMATION IN VIRGINIA.

By WILLIAM M. FONTAINE.

INTRODUCTION.

Most writers in referring to the Mesozoic strata of Yirginia have 

spoken of them as belonging to the Triassic formation. They have assumed that the beds of all the detached areas have the same general 

character, and also that these areas show strata similar in age 

to those occurring in the long belt extending from middle Virginia to 

the Hudson Eiver in New York. These assumptions are erroneous, as 

Prof. William B. Rogers showed long ago. The annual reports of his 

survey of the formations of Yirginia, made during the years 1835-1841 

have been recently reprinted by Mrs. Eogers, and with them a number 

of independent papers on the geology of the Virginias. All these are 

embraced in a volume entitled Geology of the Virginias. 1 In the frequent mention herein of the geological work of Professor Rogers reference will be made to this publication.

In his annual reports Professor Rogers regarded the Mesozoic areas 

of Virginia as composed of strata belonging to. two different epochs not 

remote from each other. The older strata he called "Middle Secondary," and the younger "Upper Secondary." In his Middle Secondary 

he included all the areas except the most easterly ones and the northern 

detached portion of the Richmond coal field. The Upper Secondary 

areas of Professor Rogers are those numbered 6 and 7 by Prof. James 

D. Dana,2 with also .the northern portion of No. 5, these being the 

portions mentioned above as excepted.

In his later writings Professor Rogers still maintained the difference 

in age of the areas, as indicated above, but expressed the opinion that 

the older areas contain strata of both the Triassic and Jurassic age. 

The younger ones, however, in his. opinion, range in age from the 

Jurassic into the lower' Cretaceous. Accordingly he called the older 

beds "Jura-Trias," and the younger ones " Jurasso-Cretaceous." These 

younger beds form the subject of this memoir.

'A reprint of Annnal Reports and other Papers on the Geology of the Virginias, by the lato William 

Barton Eogers; by Emma Eogers, 832 pp., 1884. 

2 Dana's Manual of Geology, 3d ed., p. 405.
12 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

As the typical strata of this younger series are shown in greatest 

force along the Potomac Biver, the name "Potomac" has been proposed for them by Mr. W. J. McGee, and this name will be used in this 

report.

It would seem that Professor B-ogers was induced, both by the general geologic character of the Potomac beds and by a few fossil plants 

found in them, to maintain that they differed in age from those of most 

of the Virginia Mesozoic areas. The geologic features, in most cases 

striking, will be noticed later.

I have not been able to obtain any definite information concerning 

the fossil plants seen by Professor Bogers. It appears that they were 

found in the beds near Fredericksburg. In his Geology of the Virginias l he says of these plants:

In some instances, however, very distinct impressions have been met with, unequivocal in character, and of such a nature as clearly to show that the sandstones 

in question were deposited during a comparatively late period of the series of 

secondary formations, and are to be regarded as of more recent production than 

the various slates, shales, sandstones, and conglomerates described under the name 

of the Middle Secondary formation in my last year's report. They may, therefore, for the present, with great propriety, be designated as the Upper Secondary 

sandstones.

He again alludes to the Fredericksburg plants,2 but not with any 

more defmiteness. He says,3 in speaking of the Potomac beds:

These are the siliceous, argillaceous, and pebbly beds which, underlying the Tertiary in Virginia and the well-marked Cretaceous formation farther north, have, 

in the latter region, been regarded as belonging to the base of the Cretaceous series 

of the Atlantic States. In Virginia the formation consists typically of a rather 

coarse and sometimes pebbly sandstone, in which the grains of quartz and feldspar 

are feebly cemented by kaolin derived from the decomposition of the latter, and of 

argillaceous and siliceous clays, variously colored and more or less charged with 

vegetable remains, either silicified or in the condition of lignite. These constitute 

the group of beds designed in the Virginia geological reports as the Upper Secondary sandstones, and referred by me long since (1842) to the upper part of the 

Jurassic series, corresponding probably to the Purbeck beds of the British geologists. From the Potomac northward this group of deposits, as exposed in the deep 

railroad cuts between Washington and Baltimore and on to Wilmington, is made up 

of variegated, soft, argillaceous, and siliceous beds, which, from the preponderance 

of the ferruginous coloring toward the Delaware, has been called by Professor 

Booth the "Bed Clay formation." At a few points only toward the bottom of the 

deposit it brings to view a bed of feldspathic sand, or crumbling sandstone, above 

referred to.

How far we may consider this group of sediments in Maryland, Delaware, and 

New Jersey as merely a continuation of the Virginia formation above described 

can be determined only by further investigation; but the discovery in them at Baltimore, by Professor Tyson, of stumps of cycads would seem to bring them into 

near relation with the formation at Fredericksburg containing similar remains, and 

would favor their being referred, at least in part, to the horizon of the Upper Jurassic rocks. Possibly we may find here a passage group analogous to the Wealden 

of British geology.

1 Geology of the Virginias, pp. 437-438. "Ibid., pp. 709-713. 'Ibid., pp. 711-712.
FONTAIME.] INTRODUCTION. 13

In his annual reports Professor Eogers makes his Upper Secondary 

stop at Mount Vernon. This results from his first limiting the formation to the sandstone member. From, the quotation given last it will 

be seen that more recently he is disposed to extend the formation into 

New Jersey. It appears, also, that in his still later publication he fixes 

the age more definitely.

In his annual reports he notes the fact that there is no exposure of 

Potomac beds between Chesterfield Station, in Caroline County, and 

Massaponax Creek, in Spottsylvania County. On the geologic map of 

the Virginias, which he prepared long after the publication of these 

reports, he colors as Jurasso-Cretaceous an unbroken strip extending 

from Hanover County through Caroline County to Fredericksburg and 

beyond. So far as I know, he has published no explanation of this 

change of opinion. As will be seen in the course of this memoir, the 

older view is more nearly correct.

Notwithstanding the fact that the difference in age of the Virginia 

Mesozoic strata was clearly and positively asserted, both in the annual 

reports of Professor Eogers and in his later and more accessible papers, 

the opinion has long been generally held that these beds are all of 

Triassic age. The obvious stratigraphic and lithologic differences 

noted by me in my examination of these areas prepared me to admit 

the correctness of Professor Eogers's view. Paleontologic evidence, 

however, was at first wanting. For a considerable period it seemed 

that the apparently younger beds contained nothing but vague impressions of stems and fragments of lignite.

To Prof. P. E. Uhler, of Baltimore, I owe the first sight of recognizable plant fossils obtained from these beds. Several years ago, when 

visiting that city, I saw a small collection of fossil plants obtained by 

Professor Uhler from the Potomac beds at Fredericksburg, and as they 

were very unlike those that I had collected from the beds called Middle 

Secondary by Professor Eogers, I wished to collect and study them. 

Professor Uhler was kind enough to point out the exact locality where 

they were to be found. When the place was examined the outcrop in 

a limited space gave indications of good specimens. I had the rock 

quarried out and succeeded in obtaining a number of fairly well preserved impressions, most of which belong to new genera and species 

and none of which occur in the Middle Secondary beds of Professor 

Eogers. Encouraged by my success here, I began to look elsewhere for 

fossil impressions. Strata of apparently the same age as the Fredericksburg beds extend for a long distance in the banks of the James 

Eiver below Eichmond. The canal cut across Dutch Gap during the 

war of the rebellion runs through these strata, and the recent widening of this canal gave opportunity for the examination of the material 

taken out. A visit to this place showed that impressions of plants occur 

in considerable numbers, both at the canal and in the river banks in the 

vicinity. Continued search in other portions of the Potomac area has
14 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

brought to light at several localities a large number of plaiit fossils, 

many of which are new to science.

None of these fossils have been found in the Richmond coal field, and, 

so far as known, none of the supposed older Mesozoic areas contain any 

of them. It is sufficient to say here that this flora indicates that the 

Potomac beds were laid down in a period decidedly more recent than 

that in which the Middle Secondary strata of Rogers were deposited. 

It is proposed to give descriptions and illustrations of these fossil plants 

in a future publication. 1 The object of this memoir is to give some 

account of the geology of the lower Potoinac formation in Virginia, 

where it is perhaps most typically developed.

It should be stated that there is reason to think that the extensiveformation of clay and fine sand known in Maryland as the "variegated clay formation," or the "iron-pre clays," may belong to the 

same general epoch as the Potoinac of Virginia, forming an upper 

member of the group of which the Virginia Potoinac is the lower. The 

Virginia beds and those of Maryland can not now be certainly separated 

by any sharp differences; hence, for the present, the Virginia strata 

must be regarded as lower Potomac, and the Maryland formation as 

upper Potomac. ,

LOCATION AND BOUNDARIES OF THE POTOMAC TERRANE IN

VIRGINIA.

The Potomac strata occur in several apparently disconnected areas 

that for the most part lie in lines of prolongation of one another running nearly north and south. They form the most easterly of the 

Mesozoic tracts and rest upon the eastern margin of the pre-Oauibrian 

terrane, sinking to the east beneath the Tertiary. Unlike the older 

Mesozoic beds, they in no case occupy areas within the pre-Cambrian.

The north-and-south trend of the exposures of the Potomac strata is 

due to the fact that they were probably laid down as shore deposits 

along the margin of the pre-Cambrian rocks. At a later period they 

were covered by other deposits, and now appear only where these have 

been removed by erosion. The overlap of the Tertiary and more recent 

deposits upon the pre-Cambrian in the State of Virginia takes a direction almost due north and south, and in consequence of this the Potomac 

exposures have a similar direction.

In the northern outcrops, from Fredericksburg to Alexandria, the 

Potomac beds reach a height above tide considerably greater than that 

attained by those farther south, aud the depression of their present 

surface increases as we go southward. In the Fredericksburg area the 

Tertiary and later deposits have been removed from a long strip of 

country several miles wide, so that now the Potomac beds, with the 

products of their decay, occupy the surface. In the region from

'Mon. TJ. S. Geol. Survey, Vol. XV, The Potomac or Younger Mosozoic Flora, 2 vols. (377 pp. text 

and (JLXXX plates in separate volumes), 1889, 4°.
U.S.GEOLOGICAL SURVEY. BULLETIN NO. 145 PLATE I.

eirnesvUie

Sao/fhersbt)

/ / / ,~~/

/ HernSon

________

arlborcr7

ce_A . . , 

. Frederick

MAP

OF PORTIONS OF

MARYLAND, VIRGINIA

DISTRICT OF COLUMBIA

SHOWING DISTRIBUTION 

OF THE

FOTOMAC FORMATION

IN PART OVERLAIN BY COLUMBIA AND LAFAYETTE FORMATIONS

FROM DATA FURNISHED 

BY

N.H.DARTON

8 miles = one inch

8 16

. J SCALE 500.000

IxA 876543E I 0

LATER CRETACEOUS AND TERTIARY FORMATIONS 

(EXCEPTING LAFAYETTE FORMATION)

POTOMAC FORMATION (CRETACEOUS) 

NEWARK FORMATION fjURA TR/AS ) 

PRE CAMBRIAN CRYSTALLINE ROCKS
FOOTAINE.] LOCATION AND BOUNDARIES IN VIRGINIA. 15

Hanover Junction to Petersburg the Potomac shows itself nowhere 

except in the banks of the streams or in natural and artificial excavations of considerable depth. In these exposures only a few feet of 

Potomac material can be seen. In the most southern exposure on the 

Nottoway the top of the Potomac is barely reached by the deepest 

waterways.

The eastern margin of the pre-Cambrian rocks is often at the head 

of tidewater in the principal streams that cross from the pre-Cambrian 

to the Tertiary terrane. In this latter the streams, being tidal, soon 

fail to cut down to the Potomac strata, which decline in the direction 

of their now, and it is therefore useless to seek for Potomac strata in 

place except in the vicinity of the junction of the Tertiary and the 

pre-Cambrian. It is true that debris, mostly gravel derived from Potoinac beds, may be often found at a considerable distance from this line 

of contact a fact whose significance will be noticed presently 5 we are 

now dealing with the occurrence of the Potomac strata in situ.

It is clear that the Potomac beds, as now shown, by no means represent the formation in its original thickness and extent. Potomac debris 

may be found over a wide belt of the pre-Cambrian terrane to the west 

of the strata in situ, and the character and amount of this material 

found on the pre-Cambrian and on the surface of the Potomac strata, 

where they form, the country rock, show that a great thickness of Potomac beds must have been removed by erosion. This erosion, however, 

in part at least, has been produced in recent times, and, indeed, is going 

on now. There is unmistakable evidence that the Potomac formation 

had suffered greatly from erosion before the deposition of the Tertiary. 

Professor Rogers calls attention to this fact in his annual reports. He 

states that the oldest Tertiary beds rest on the deeply channeled surface of the Potomac sandstone, especially in the region near and south 

of James Eiver. He says, further, that this worn surface under the 

Tertiary has often a layer of coarse pebbles which marks " an epoch 

of violence" prior to the deposition of the Tertiary. 1 The expression 

11 deeply channeled," used by Professor Eogers,2 correctly describes the 

surface of the Potomac beds on which the Eocene rests. So marked 

are the inequalities of the surface that the level of an Eocene exposure 

may be many feet lower than some adjacent outcrop of the Potomac 

beds. "Coarse gravel" hardly describes much of the larger material 

resting on the surface of the Potomac beds and lying under the Eocene. 

Not uncommonly the stones are large enough to be called cobbles.

The extensive erosion to which the Potomac formation has been subjected has not only greatly diminished its thickness where it remains 

in situ, but has also removed it more or less completely from areas 

once covered by it. There are indications that the Potomac formation 

extended farther westward than it now does, and that it once covered 

a considerable belt of the pre-Cambrian that now shows no Potomac 

in place. Over a belt of the pre-Cambrian, of varying width, lying

1 Geology of tlio Virginias, p. 261. 2 Uriel., p. 204.
16 . THE POTOMAC FORMATION IN VIRGINIA. \VUVL. u&.

immediately to the west of the present outcrops of Potoniac unmodified 

strata, we find various clays, sands, and cobblestones that, for the most 

part, are similar to the structureless materials which rest upon surfaces 

of Potomac beds, and which represent the products of the erosion 

and disintegration of these beds. The character of these materials 

found on the pre-Cambrian indicates that they are the debris resulting 

from the destruction of Potomac strata which at one time covered the 

surface where they lie.

Along the Potomac terrane extending from Petersburg to Hanover 

County these unstratified materials spread over a band of the preCambrian from 10 to 15 miles wide. In the vicinity of the northern 

Potomac aijea, extending from Fredericksburg to Alexandria, the belt 

of the pre-Cambrian covered by them is not so wide, having a width 

of but 5 to 7 miles. There can be hardly a doubt that the Potomac 

formation once extended westward at least as far as this debris is found.

It may be mentioned in this connection that red clay and cobblestones, the latter precisely like those resulting from the erosion of 

Potomac strata, may be found up the James River as far as Lynchburg. 

The cobbles are on top of the hills bordering the river. They do not 

extend far back from the river, but descend the slopes toward the 

stream. This lower position is due to surface degradation. Without 

further examination the significance of these cobbles and clay can not 

be stated. . , ' 

The location and boundaries now to be given apply to the Potomac 

strata in situ. No Potomac beds are positively known to exist south 

of the Nottoway Eiver in Sussex County, but it is quite probable that 

they are to be found buried under the deep covering of later deposits 

which occur in that region. The exposures there are very poor, for 

only a few of the streams cut through the Tertiary strata, and these 

deposit so much alluvial matter that any Potomac material that might 

be reached would be hidden. At best, any exposure of these strata 

that occurs must be slight, and could be found only by careful and 

detailed examination of the region.

We have, however, some evidence that the Potoniac strata extend 

into North Carolina. On the banks of the Eoanoke Eiver at W^ldon, 

in the bed of a little stream, there is a small exposure of a coarse sand 

with cobblestones, forming a material that has all the lithologic characters of the Potomac formation. This will be described more particularly in another connection (see p. 25).

In his annual report for 1835 ] Professor Eogers says that the strata 

now called Potomac are to be found in the counties of Greenville and 

Brunswick. As in his later reports and writings he does not repeat 

this assertion, it may be inferred that on further examination he gave 

up that belief. Some of the post-Mesozoic strata in the counties above 

named have a striking likeness to some of the Potomac beds, and this, 

no doubt, was the cause for their being mistaken for the latter.

1 Geology of the Virginias, p. 70.
FONTAINE.] THE NOTTOWAY AREA. 17

LOCATION AND BOUNDARIES OF THE SUBORDINATE AREAS. 

THE NOTTOWAY AREA.

The most southern known area of Potomac strata occurs along the 

Nottoway Eiver, in Sussex County, about 25 miles south of Petersburg. 

This locality of Potomac beds did not escape the notice of Professor 

Eogers. He says of it,1 speaking of the Mesozoic sandstone, his Upper 

Secondary:

Only in one instance has rock resembling it been found in the country lying south 

of the neighborhood of Petersburg. Neither, so far as I know, does it make its 

appearance in North Carolina. The insulated locality just referred to is in the bed 

of the Nottoway Eiver about 4 miles above Boilings Bridge, where a rock, forming 

the natural foundation of the bridge, is exposed for a short distance nearly on a level 

with the water, bearing a close resemblance to the formation referred to, and which 

there is strong reason for believing is geologically the same.

On his geologic map lie colors as u Jurasso-Cretaceous " a small oval 

patch at this place.

The exposure of Potomac strata in Sussex County, on fthe Nottoway 

Eiver, is more extensive than is indicated by the above-quoted statement. A qursory examination of this locality shows that the formation 

certainly underlies the surface at Boilings Bridge as well as at the 

precise spot mentioned by Professor Eogers. Its character at both 

localities renders it almost certain that these strata may be found 

beneath the Tertiary throughout the interval between the bridge mentioned by Eogers and Boilings Bridge. Indeed, it is highly probable 

that the formation extends far beyond these two outcrops. In this portion of the State the Potomac formation is buried beneath a thick covering of more recent deposits. We may expect to find it only in the 

vicinity of the margin of the pre-Cainbrian rocks, where the streams 

have cut down through more recent beds. The Nottoway is the first 

river south of Petersburg which has cut its channel deep enough to 

touch the Potomac beds where they are covered by the later-formed 

strata. The river in the vicinity of Boilings Bridge has a considerable 

fall, which gives it a decided current. It is now running on the top 

of the Potomac sandstone, having excavated its bed 2 or 3 feet in 

this material. It also forms "riffles" over harder and projecting portions of the sandstone. This being the case, and a great thickness of 

post-Mesozoic material existing along the river, no exposures of Potomac beds can be found except in the immediate banks and bed of the 

Nottoway. Even here, as the river has cut s"o slightly into the formation, exposures would not be conspicuous, and must be carefully sought.

In the main the characters and modes of occurrence of the Potomac 

beds on the Nottoway and in the area next to the Petersburg area are 

very similar, so much alike, indeed, that one must think that the 

strata of both areas were laid down under like conditions and that

1 Geology of the Virginias, p. 216.-

Bull. 145 2
18 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

they form parts of the same region of deposition; but this does not 

appear from connecting exposures. No Potomac strata are shown 

north of Boilings Bridge until Petersburg is reached, 22 miles distant 

in an air line. In this interval the post-Meso/oic strata are very thick; 

and as there are no streams capable of cutting channels deep enough 

to pass through them, it is evident that no exposures of Potomac 

strata can be found. Indeed, the erosion is rarely sufficient to penetrate through the Miocene beds.

THE PETERSBURG AREA.

The Petersburg area begins at the city from which it is named and 

extends northward to Richmond. It might have seemed fitting to 

give the name of Richmond to the area were it not for the danger of 

confounding it with the belt of older Mesozoic lying to the west of 

Richmond, known as "the Richmond coal field."

In the interval between Petersburg and Richmond the western limit 

of the Potoniac area is not well defined, and outcrops of strata in situ 

can be found* only in the immediate vicinity of the larger creeks that 

cross from the terrane of the pre-Cambrian and older Mesozoic into 

that of the Potomac and later formations. In this region not only the 

Potomac but the more recent formations have suffered greatly from 

erosion, which has produced heavy deposits of unstratified sand, 

gravel, and clay that usually conceal all strata in place.

The line of the Richmond and Petersburg Railroad, which forms a 

part of the Atlantic Coast Line, maybe taken as pretty closely approximating the western boundary. The eastern limit of Potomac outcrops 

may be given as follows: Exposures, very slight in extent, occur in the 

immediate vicinity of Petersburg on the north side of the Appomattox. 

They extend in greater force down this river to a place about 3 miles 

above City Point, the confluence of the Appomattox and the James. 

The exposures are found mostly on the eastern side of the river down 

to Point of Rocks, 4 miles above City Point. Point of Rocks is on the 

western side of the river, and forms a bold bluff immediately on 

the stream. Here may be found the most extensive vertical exposure 

of Potomac strata visible in the Petersburg area. From this place 

down the river toward City Point for about a mile outcrops may be 

found mostly on the right or east bank.

A line drawn from a place on the Appomattox about 3 miles above 

City Point, in a direction a little east of north, to Deep Bottom, on 

James.River, will touch the most easterly exposures on these streams. 

From Deep Bottom to Richmond exposures are numerous on James 

River. In the triangular space between the rivers the thick postPotomac deposits hide nearly everywhere the older strata, and the 

same is true north of James River. The nature of the material shown 

on the two rivers indicates not only that the formation is continuous, 

underlying the area included between the railroad and the rivers, but 

that it extends north, south, and east of the last exposures. Between
FONTAINE.] THE PETERSBURG AND HANOVER AREAS. 19

the rivers and in the terrane under which the Potomac probably lies, in 

a few places, may be seen the marl bed, which in Virginia forms one of 

the lowest of the Eocene strata. Heavy deposits of gravel and cobbles 

composed of quartz and Cambrian quartzite occur along the rivers, 

resting often on the Potomac beds. These materials are much more 

extensive near the western margin of the formation, and here the coarser 

deposits are associated with red and variegated clays.

The coarse and heterogeneous character of the Potomac material, as 

it is exhibited in the most eastern exposures seen on the two rivers, 

gives us strong reason to think that the true eastern margin of the 

formation is to be found much farther east than the points at which it 

was last seen. The indications given in the amount and character of 

the most easterly outcrops of this group of beds are confirmed by the c 

disclosures made in boring an artesian well at Fort Monroe. The 

results obtained from an examination of the material taken out of

this well, which will be given farther on (see p. 45), show that there 

can be no doubt that the Potomac formation extends in force at least 

as far east as that place.

Fort Monroe is 60 miles in an air line southeast of the last exposures 

on the James and the Appomattox. The direction of this air line is 

very nearly the same as that of the average direction of declination 

of the formation. How-much farther eastward beyond this point it 

extends we have no means of knowing. At Eichmond the Potomac 

may be seen underlying the Tertiary and resting on the pre-Canibrian. 

It shows only a thin layer, and being near the original western boundary of the formation, it has suffered much from erosion.

THE HANOVER AREA.

North of Eichmond no outcrops appear until we reach the vicinity of 

the confluence of the North Anna and the South Anna rivers. Here, 

in the neighborhood of Hanover Junction, a thin remnant of Potomac 

beds may be seen at several points. They rest unconformably on the 

older Mesozoic or Ehetic beds, which belong to the northern end of the 

Eichmond coal field. The line of the Eichmond, Fredericksburg and 

Potomac Eailroad may be taken as approximately the western boundary of the strata in place. This may be called the Hanover area. The 

fact that exposures of the Potomac do not appear in the interval 

between this region and Eichmond does not necessarily imply that the 

formation is absent. In this quarter the post-Potomac deposits are 

thick and conceal everything, for there are no rivers with sufficient 

erosive power to cut down to the older formation.

Between Hanover Junction and North Anna Eiver the last exposures 

in this neighborhood occur. The easternmost outcrop of the Potomac 

in the vicinity of Hanover Junction occurs a little above the confluence 

of the North and South Anna. Here the Potomac sinks under the 

Tertiary. At Morris Ford, a little more than a mile above the junction
20 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 146.

of the rivers, there is a good exposure, showing the Eocene 1 greensand marl bed resting on the Potoinac, with a bed of cobblestones 

intervening. Professor Eogers noted the disappearance of the Potomac 

in this region beneath the Tertiary. * The occurrence of the formation 

and its mode of disappearance under the Tertiary, both in this region 

and at all points south of it, indicate that the group, as a whole, sinks 

in a southeasterly direction.

It may be repeated here that Professor Bogers in his reports considered all of the Mesozoic strata north of the Chickahominy Elver as 

belonging to his Upper Secondary. He does not seem to have found 

any fossil plants in them, and in the absence of fossils this mistake was 

very natural, for some of the older Mesozoic beds here are lithologically much like those of the Potomac. I was fortunate enough to find 

fossils at several localities, and they show that the greater portion of 

the Mesozoic beds in this region are older Mesozoic or Ehetic in age.

North of North Anna Eiver, toward Fredericksburg, there are no 

exposures of Potomac strata. It is probable that the formation exists 

and is continuous in this interval, but it is too deeply buried under the 

latter formations to be reached by the erosion of the streams. In the 

vicinity of Milford Station the head streams of the Mattapony do not 

cut down through the Eocene; and farther west, along the junction of 

the pre-Cambrian terrane with that of the Tertiary, where outcrops of 

Potomac might be supposed to exist, the covering of post-Pototnac 

material hides everything. In this interval of about 25 miles, measured on an air line, the lowest stratum visible in place is the greensand 

marl bed of the Eocene, and this is seen only in the lowest depressions 

made by the streams.

THE FRBDERTCKSBURG AEEA.

After passing over the interval north of the North Anna we find the 

first exposures of Potomac strata on Massaponax Eiver 5 miles south 

of Fredericksburg. From this point to the Potomac Eiver and beyond 

the outcrops are continuous. This area, which we may call the Fredericksburg area, is much the most important one in Virginia. It is 

unlike those farther south in that the Potomac beds form the surface 

rocks over a considerable belt, and they generally are cpvered only by 

material derived from themselves.

The exposures begin on the Massaponax near where that stream is 

crossed by the road leading from Port Eoyal to Fredericksburg. From 

this point the western margin of the Potomac terrane may be given 

approximately as follows:

Beginning on the Massaponax, it sweeps around so as to cross 

Hazel Eun and the narrow-gauge railroad from Fredericksburg to 

Orange, 2 miles west of the first-named place. It crosses the Eap-

'The Eocene deposits of Virginia and Maryland are now designated the Pamunkey formation oa 

the maps of the United States Geological Survey.
FONTAINE.] THE FEEDERICKSBURG AREA. 21

pabanriock Eiver 1 mile above the village of Falmouth. From this 

place to Occoquan the Telegraph road runs in general parallel with 

the western margin and about 1 mile east of it. From Stafford to 

Occoquan this road has a direction nearly parallel with the course of 

the Potomac River, and the general direction of the Potomac area now 

in question shows a striking parallelism with the course of that 

stream. Opposite Stafford the Potomac Eiver bends abruptly eastward and no longer follows the direction of the Potomac formation, 

hence exposures of the strata are not found in that river below the 

mouth of Aquia Creek. The general course of the Telegraph road 

is such that it crosses all the streams that empty into the Potomac Eiver between Stafford and Occoquan, near the head of tidewater in each. In local usage, the tidal portion of each of these tributaries is called the " creek," while the upper portion is called the 

"run."

, At the village of Occoquan, where the Telegraph road crosses Occoquan Eiver, mica-schist and gneiss are extensively displayed, while in 

the tops of the hills some small patches of Potomac occur associated 

with a large amount of gravel, sand, and cobblestones, which resulted 

from the destruction of Potomac beds. These evidently once formed 

here a continuous covering over the schists. Two miles below Occo. 

quan, on the river of the same name, at the crossing of the railroad 

between Alexandria and Fredericksburg, the Potomac has thickened 

sufficiently to form a continuous covering for the crystalline rocks.

At the village of Occoquan the western margin diverges more widely 

from the Telegraph road, for while it continues its northeastward curvature to conform to the course of the Potomac River, the road turns 

more sharply eastward and penetrates farther within the Potomac 

area. North of Occoquan Eiver the western margin runs northward 

and then northeastward, crossing Pohick Run about 1 mile west of 

Pohick church. Thence it continues so as to cross the Virginia 

Midland Railroad and Little River turnpike about 7 miles west of 

Alexandria. It here bends rapidly eastward and crosses the Potomac 

Eiver a little west of Washington. The formation thence continues on 

to Baltimore, following the line of the Baltimore and Ohio Eailroad.

The eastern margin of the area in which the Potomac beds, with the 

products of their erosion, form the surface material is very irregular 

and vaguely defined. Where the streams have sufficient erosive 

power we observe in a number of places that they cut down to the 

Potomac and expose it at points farther east than the general eastern 

boundary of the belt of which the formation constitutes the surface 

rock.

The Potomac beds are covered eastward by the Tertiary. Along the 

Potomac River the Tertiary has been often, and the Potomac sometimes, 

removed and replaced by the Pleistocene.J All these materials are very

'The Pleistocene deposits of this region are now designated the Columbia formation on the maps of 

the United States Geological Survey.
22 THE POTOMAC FORMATION IN VIRGINIA. . [BULL. 145.

friable and easily removed by erosion. Each of the three formations, 

the Potomac, the Tertiary, and the Pleistocene, is often buried beneath 

a thick covering formed of its own ruins. These materials blend in 

such a manner and are often so much alike that it is difficult to detect 

the exact boundaries of the different formations. Some patches of the 

lowest Tertiary are left uneroded well within the belt in which the 

Potomac forms the principal surface rock.

The eastern limit of exposures of the formation from Alexandria to 

near the mouth of Aquia Creek is the Potomac Eiver. Between the 

mouth of Aquia Creek and the southern end, on the Massaponax, the 

limit will be approximately determined by a line connecting these 

two points which curves westward so as to pass by Brooke, on the 

Eichmond, Fredericksburg and Potomac Eailroad, and old Potomac 

Creek church, on Potomac Creek, half a mile below the railroad bridge. 

It should be added, however, that on the Potomac Eiver the Potomac 

formation is often concealed by a thick mass of Pleistocene deposits. 

It shows itself on both sides of the river, in Maryland as well as in 

Virginia, but in Maryland it is covered by the Tertiary, or by the Tertiary and the Pleistocene, and is visible only in the lower portion of 

some of the bluffs immediately on the Potomac Eiver.

On the Virginia side the formation constitutes the surface rock in a 

continuous belt running from the Potomac Eiver north of Alexandria 

to the Massaponax Eiver 5 miles south of Fredericksburg. The width 

of this belt ranges from 3 to 7 miles, the maximum width being near 

Alexandria. The average width is 3 or 4 miles. In the vicinity 

of the Telegraph road the Potomac, with the products' of its erosion, 

forms the entire surface rock, but away from this road toward the 

Potomac Eiver the formation is sometimes covered by the Tertiary or 

the Pleistocene.

It may be well, before leaving the description of the eastern limit of 

the formation, to mention here some of the more important localities 

along this margin that show outcrops of the Potomac. Going northward, after leaving the Massaponax we find extensive exposures in the 

banks of the Eappahannock a short distance above the mouth of the 

Massaponax. Here the material is a pretty firm sandstone, which was 

formerly quarried for building purposes. We next find a good exposure 

on Potomac Creek, about half a mile below the railroad bridge, on the 

road from Fredericksburg to Brooke, where it leaves the creek. The 

ruins of the colonial church, named for Potomac Creek, are close by.

The next locality worthy of note is Brooke station. Both here and 

at Potomac Creek church the Potomac disappears under the Eocene. 

We find the next most easterly outcrop on Aquia Creek, about half a 

mile above its mouth, according to Professor Eogers. The exposures 

are extensive where the railroad crosses the creek, but from that point, 

which is about 3 miles up from the Potomac Eiver, the Pleistocene 

generally conceals the formation. Then, proceeding northward along
FONTAINB.] THE FREDERICKSBURG AREA. 23

the Potomac Elver, we find that for some miles the Pleistocene cuts 

out the Potomac formation immediately on the river. At the mouth 

of Quantico Creek, at Shipping Point, and'elsewhere, however, extensive exposures also occur. From Shipping Point northward on the 

Virginia side the outcrops are extensive in all the high bluffs and 

hills near the Potomac Eiver almost to the mouth of the Occoquan. 

In these places the Potomac formation contributes most of the material, 

but it has usually a remnant of the Pleistocene resting upon it. This 

is the condition of things at Cockpit Point and Freestone Point. In 

Masons Neck and the promontory of High Point the Pleistocene cuts 

out the Potomac, immediately on the river. On the Maryland side, 

about 1£ miles above High Point, there is a fine exposure of Potomac 

material capped by the Eocene.

North of Hallowing Point, Pohick and Accotink creeks have a common tidal portion which separates Masons Neck from White House 

Point. On both the southern and the northern side of the common 

estuary of these creeks there are high bluffs, composed almost wholly 

of Potomac material. The bluff on the northern side, situated at White 

House Landing, displays the greatest thickness of Potomac strata to be 

seen on the Potomac River. Between White House Bluff-and Mount 

Vernon the Pleistocene, for most of the distance, cuts out the Potomac 

beds; but at the latter place these appear in considerable force.

At Fort Washington, on the Maryland side of the river, and a little 

above Mount Vernon, occurs the last exposure of the formation on the 

Potomac Eiver. It shows here only a few feet, barely rising above high 

tide. There is no doubt that the formation seen along the Potomac 

Eiver is continued far eastward under the Tertiary. The Potomac 

strata in these, the most easterly exposures, are far from giving any 

indication of thinning out or of becoming finer. Some of these exposures show the formation at its maximum thickness, and in all of them 

the material is often coarse and heterogeneous in composition. At the 

same time the bottom of the formation is revealed at none of the abovenamed places, while its top always shows signs of great erosion.

The Fredericksburg area has a large amount of gravel and cobblestones spread over its surface in connection with mottled sandy 

clay. Southward of Pohick the larger waterworn material is mainly 

quartz, but northward the proportion of pre-Cambrian quartzite in the 

gravel and cobblestones is suddenly and greatly increased and the 

stones become larger. Toward the Potomac, and along that' stream, 

the rounded material is composed mainly of Cambrian quartzite. The 

gravel and cobblestones which cover the surface of the Potomac formation are extended westward over the pre-Cambriau without material 

change in lithologic character. It is this material that forms the belt 

of loose matter mentioned before as skirting the Potomac on the west 

in a band of varying width.

On the western margin of the Fredericksburg area and over the
24 THE POTOMAC FORMATION IN VIRGINIA. [BULL.US.

adjoining pre-Oambrian rocks the waterworn stones are not associated 

with clay. Westward of the area, in the vicinity of Hanover Junction, 

and westward of the Petersburg area, deep-red clay is commonly found 

with the stones. The stones over all the Potomac areas are well 

rounded. As we pass from the Potomac formation westward into the 

pre-Cambrian terrane, they gradually give way to the more or less 

angular drift that comes from the degradation of the pre-Cambrian. 

These rounded stones evidently came originally from the Potomac 

beds, for they abound in the vicinity of outcrops of that formation 

and diminish as we depart from its exposures.

It is a remarkable fact thai? cobblestones of Cambrian quartzite are 

common in the gravel that lies upon the pre-Cambrian along the 

western margin of the Potomac on the North Anna River. We might 

explain the existence of such material in the vicinity of the James and 

the Potomac by assuming that it was brought down by these streams, 

for west of the Blue Ridge they cross the Cambrian -formation; but 

this is not the case with the North Anna.

The above account gives approximately the location and limits of the 

several detached Potomac areas.

THE GEOLOGIC CHARACTER OF THE POTOMAC FORMATION AT 

DIFFERENT LOCALITIES.

We may now turn our attention to the geologic character of the formation. It has been found to be impossible to give a general account 

of the geology of this group of beds. The materials that form the 

Potomac change so rapidly that representative sections do not exist. 

In any given exposure, if vertical sections should be made at intervals of, say, 50 feet, they would be as a rule widely different. The 

changes, it is true, do not have large range, but they occur suddenly 

and without any evident law. It is therefore impossible to get connected sections over any considerable space. Then, again, the formation has everywhere suffered much from erosion, and it is usually 

buried under debris formed by its own destruction or by the decay of 

the younger rocks which once covered it.

It seems best, then, to give the character of the material exposed at 

localties which, as nearly as possible, may be representative of the 

regions in which the exposures occur. In doing this I shall begin at 

the south and proceed north.

EXPOSURE NEAR. WELDON.

Concerning the formation in the Weldon locality, there is but little 

to add to what was said'in giving the location of the Potomac areas. 

At Weldon there is a thick deposit of post-Miocene material which 

apparently rests immediately on the crystalline schists. The precise 

age of these beds has not been determined, but they are certainly 

younger than the Miocene, and are separated from the latter by an
FONTAINB.] EXPOSURE NEAR WELDON. 25

interval of time in which the Miocene strata were subjected to considerable erosion. These beds are confusedly stratified sands and clays, 

usually of heterogeneous composition, often pebbly, and mottled in color. 

They contain no recognizable fossils, only drifted fragments of stems of 

plants being seen. The material is exhibited in great force along the 

junction of the pre-Cambrian and Tertiary terranes, and it extends far 

eastward over the Tertiary region, but with a more uniform character 

to the east and with a larger proportion of clay than can be found in 

the vicinity of the pre-Cambrian.

The name Lafayette' has been given to this formation. As a rule it 

forms the surface in the more southerly Potomac areas, the exceptions 

being along the Appomattox and the James, where the Pleistocene 

appears in places to rest upon its eroded surface.

As in giving an account of the local sections frequent mention must 

be made of this group of beds, it appears proper to give the preceding 

account of their character,

On the north bank of the Eoanoke, opposite Weldon, in the cut 

made for the approach to the railroad bridge, and also in the hills 

flanking the river, the Lafayette beds are shown in force. They seem 

to be fully 100 feet thick. The river has barely cut through them, and 

is now flowing on the surface of the gneiss. If the marine Tertiary 

was ever present at this locality it has been wholly removed by erosion. 

The Potomac also, if ever present, has probably been removed. The 

exposures near the top of the gneiss, the horizon where the Potomac 

may be looked for, are very poor. In the mouths of the streams that 

enter the river and cut down to this level there is a thick deposit of 

recent alluvium that hides everything older.

In only one place, and in a very limited space, did I find the surface 

of the gneiss laid bare. This was in the channel of a small run that 

entered the river on the north side of and near the bridge head. Here 

a deposit, a few inches thick, of coarse, gray grit, with occasional large 

pebbles, is displayed. The material has all the lithologic characters 

that distinguish the Potomac formation. It seems to owe its preservation to its firm union with the uneven surface of the gneiss.

EXPOSURE AT BOLLINGS BRIDGE.

North of Weldon, as far as the vicinity of Stony Creek, the thick 

covering of the Lafayette formation conceals everything older. About 

2 miles from Stony Creek the road leading from Weldon to Petersburg 

crosses the Nottoway Eiver by means of Boilings Bridge. Here, 

almost immediately under the bridge, in the bed of the river, there is 

an exposure of typical Potomac sandstone. Between 2 and 3 feet of it

i This formation was discriminated by McGee and named Appomattox. Am. Jour. Sci., 3d set., vol. 

35., pp. 328-330, 1888. In 1891 it was agreed by a conference consisting of Dr. Hilgard, Dr. Safford, 

Dr. Smith, Prof. J. A. Holmes, L. F. Ward, E. T. Hill, and W. J. McGee to adopt the name Lafayette 

mainly on the ground of priority. Am. Geologist, vol. 8, pp. 129-131, 1891.
26 THE POTOMAC FORMATION IN VIRGINIA. [nun. 145.

are shown, forming the bed of the river, it being the lowest rock seen. 

The material is a coarse, dark brownish-gray grit, with small gravel and 

some pebbles that attain a maximum diameter of 3 inches. These 

pebbles are composed mostly of quartz, or occasionally of a siliceous 

schist. . The most striking peculiarity of this grit is the presence in it of 

scattered, large, rounded masses of highly plastic clay. As we shall 

see later, the occurrence of such clay balls in coarse grit is highly 

characteristic of the Potomac in all the areas to the north of this one. 

It is interesting to find this feature present in the isolated Nottoway 

area. In these clay balls, in the more northern areas, fragments of 

plants are often found, but none were seen here. The clay masses 

sometimes have a diameter of 12 or 18 inches. The clay is quite constant in character. When moist, it is dark gray or almost black; 

when dry, a dark brownish-gray. The masses come from layers of clay 

intercalated in the grit, which were deposited locally in pauses in the 

more violent movements of the waters that spread out the sand. In 

the renewal of the agitation of the waters these clay layers were often 

torn up and the fragments were rolled away to greater or less distances.

The close resemblance of this clay to much of that occurring in a 

similar way in the Potomac areas farther north is remarkable. No difference in their character or their mode of occurrence can be found 

except in the preservation of organic remains, as mentioned. The 

locality at Boilings Bridge jnay be taken as giving exposures typical 

of the Kottoway area, so far as that has been examined.

The spot mentioned by Professor Rogers, where the Potomac forms 

the natural abutment of a bridge, is found about 4 miles farther up 

the river. The Potomac material shown here is of the same nature as 

that at Boilings Bridge, except that no clay is embedded in the stony

grit. About 3 feet of this is visible. It would seem that at both of

these localities the river in cutting its channel has barely reached the 

top of the Potomac, and of course its thickness is not disclosed.

A careful examination in this region showed no Cambrian material 

in the gravels and cobbles embedded in the Potomac grit and none in 

the loose d6bris lying on the surface in the vicinity of its outcrops. 

This is in marked contrast to the condition of things in the Petersburg 

area. At Boilings Bridge a foot or two of a dark-brownish, friable 

sandy clay is visible on the top of the Potomac grit. It shows indistinct impressions of shells which are Tertiary. This is the only spot 

in this area where anything like the Tertiary beds was seen.

EXPOSURE AT PETERSBURG.

From Boilings Bridge northward to Petersburg nothing but the 

Lafayette formation.is visible along the line of the county road, which 

coincides closely with the route taken by the Petersburg and Weldon
PONTAINE.] EXPOSURE AT PETERSBURG. 27

Eailroad. The first exposure of the Potomac formation is found in the 

base of the hill ascending from the north end of Pocahontas Bridge at 

Petersburg. Here is visible on the roadside a small outcrop of incoherent sandy clay belonging to this formation, about 15 feet in thickness. The material is a loose sandy clay, light gray to nearly white in 

color, containing a large proportion of white clay resembling kaolin. 

The finer matter seems to come from the decomposition of granite or 

gneiss, and appears to be deposited without sorting and stratification. 

This finer matter is full of rounded stones of various sizes. The largest 

ones measure 6 to 8 inches in diameter, and one was seen having the 

dimensions 12 by 7 by 4 inches. Much the greater number have a diameter of 3 inches or less. They are well rounded and smooth, and are scattered irregularly throughout the mass. The material composing these 

stones is quartz and Cambrian quartzite, a large number being formed 

of the latter. As is commonly the case in the Petersburg area, the

largest Cambrian cobbles are formed of a very compact, dark-gray 

quartzite. Over the Potomac at this locality there is a thick mass of 

the Lafayette formation, and in the top of the hill the Pleistocene is 

found.

For 10 or 15 miles west of the line of the Eichmond and Petersburg 

Eailroad there is a heavy deposit of sandy clay and gravel, forming the 

surface material and overlying all the older rocks which occur in that 

belt. With these finer materials a notable, amount of quartz and Cambrian cobbles occurs. Some of the Cambrian stones are a foot or more 

 in diameter. The gravel and sandy material may be found on the surface of both the granite and the older Mesozoic of tbe Eichmond Coal 

Field. The creeks that flow out of this belt into the Appomattox and 

the James bring down into the Potomac terrane large quantities of 

rounded stones. Where the railroad crosses these creeks such cobbles 

are conspicuous.

The Appomattox Eiver, from Petersburg to near City Point, gives a 

number of exposures of the Potomac beds. The most notable of these 

occurs at Point of Eocks, 4 miles above City Point.

EXPOSURE AT POINT OF ROCKS.

The exposure of the Potomac formation at Point of Eocks is much 

the most extensive one on the Appomattox, and may be taken as giving 

. a fair representation of the character of the formation in its most eastern outcrops in the Petersburg area. At this place a precipitous bluff 

nearly 100 feet high, composed wholly of Potomac, abuts on the river. 

It lies on the western or convex side of the sudden bend made by the 

river here. The exposure shows for 75 to 80 feet above the water a 

mass of coarse grit,-mostly of a light-gray color, but in places where 

the proportion of kaolin is large it is nearly white. The material in its 

finer portions is evidently derived from the decay of granite or gneiss. 

The greater part of this is quartz in rather large grains, and this is
28 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

*

mixed with a varying amount of kaolin. This latter in some portions 

is so large in amount that it would seem capable of utilization if separated from the quartz by washing. Throughout this finer material, 

which forms much the greater part of the mass, there are many small, 

more or less rounded stones scattered in an irregular manner. These 

stones are mainly of quartz, and are 2 or 3 inches or less in diameter, 

but we sometimes find larger stones, which may be called cobbles, and 

these are often composed of Cambrian quartzite. These are not uncommonly 5 or 6 inches in diameter; some are 8 inches, and one 10 inches 

in diameter was seen. The smaller stones of quartz, 3 inches or less in 

diameter, are derived from granite or gneiss.

None of this material is distinctly bedded. The nearest approach to 

stratification is an arrangement of some parts of the material in irregular, interlocking, lenticular layers, a structure characteristic of the 

Potomac everywhere. These layers vary so much in texture and 

composition that no two vertical sections of the bluff would be alike. 

Current-bedding on a minor scale is visible in much of the material. 

These structural features, common everywhere in the Potomac formation, make it impossible to get reliable dips and strikes. The more 

siliceous parts of the material are very firmly consolidated, and some 

portions of the rock where the grain is most nearly uniform have been 

used in constructing rough walls. Where much kaolin is present the 

matter is incoherent, and in outcrop usually forms a loose, sandy clay. 

About midway up the bluff several angular masses of pale chocolatecolored shale, occurring close together, are embedded in the coarse grit. 

The largest one of these masses measured 30 by 24 by § inches in the 

portion visible, and a part of it had been broken off. The masses are 

angular in form, and have been torn from some bed of clay deposited 

in the grit, formed in the same manner as the clay masses mentioned 

as occurring near Boilings Bridge. Here, however, they are not rolled 

into balls.

While most of the finer materials of the Potomac here were evidently derived from the decay of the granite and gneiss which lie 

west of the Potomac terrane, the Cambrian material must have come 

from areas west of the Blue Ridge, for no rock like that of these 

larger cobbles is found east of that range. The material attributed 

to the Cambrain varies somewhat. That giving the largest cobbles 

is a compact, dark-gray quartzite. A common kind is a light-gray 

quartzite, often somewhat friable, sometimes with visible quartz grains. 

This rock often contains a good deal of white clay with the quartz, 

and by the comminution of this variety some, of the quartz grains 

and kaolin-like clay of the Potomac may have been produced. The 

cobbles formed of this kind of stone may often .be split into plates 

with a hammer. The splitting takes place along the bedding planes of 

the quartzite. This stone when transported was apparently firm and 

tough, as is indicated by the well-rounded, smooth cobbles which it
FONTAINE.] EXPOSURE AT POINT OF BOOKS. 29

furnishes. The friability now shown seems to have been produced by 

long weathering.

Both kinds of Cambrian cobbles abound at the top of the Potomac 

grit when this is found in place, and they are numerous along both 

the Appomattox and the James. There seems to be in this region a 

continuous bed of these coarse stones separating the Tertiary and 

Lafayette formations from the Potomac. In these scattered stones 

casts of Scolithus linearis are often found, but none were seen in the 

cobbles of the bluff at Point of Eocks.

The top of the grit shown in the bluff has evidently lost much from 

erosion. It is cut down till it presents an uneven, undulating, and 

sometimes channeled surface. At the top of the coarse grit which 

forms the greater portion of the bluff there is a noteworthy bed of 

cobbles and coarse gravel. This is the bed mentioned above as 

extending widely under the surface of Post-Potomac formations. It 

appears to result from the erosion of the Potomac, which causes a concentration of the larger stones on the top of the formation where it 

remains in place. The process of erosion removes the finer matter.

The stratum is composed mainly of rounded stones of considerable 

size. These are packed in a sandy matter that is the residue of the 

finer material of the grit upon the top of which the cobble bed rests. 

In short, this stratum differs from the underlying normal Potomac 

chiefly in the greater proportion of large stones. Its thickness varies 

greatly, for it also has lost much from erosion. At Point of Eocks it 

varies in thickness from 5 to 12 feet, being quite commonly 8 to 10 feet 

thick. With the rounded stones, which are of quartz and Cambrian 

quartzite, there are occasional masses of partly rounded pre-Cambrian 

material, such as granite, gneiss, and mica-schist. The proportion of 

Cambrian quartzite is great, and the cobbles composed of this material are generally larger than those formed of quartz. Many of the 

former are as much as 6 or 8 inches in diameter, and some go up to 12 

inches, the dark-gray quartzite furnishing the largest. All are well 

rounded and smoothed, as if they had been long subjected to water 

action. The material of this bed is without stratification, the stones 

being confusedly heaped together in it.

Prof. William B. Eogers, in his Virginia reports, made forty years 

ago, mentions this locality as noteworthy for the number and size of 

the stones composed of his No. 1 formation, which is of Cambrian age. 

He alludes to the resemblance which this bed bears to glacial drift, 

or, as it was then called, " diluvium," saying that it resembles " an 

ancient diluvium." This cobble bed appears to be considerably older 

than the Lafayette formation, for this latter rests upon it at the ocher 

"works a few hundred yards farther up the river, and is totally different 

from it, being a plastic clay and ocher. The material exposed in the 

bluff' at Point of Eocks is of an unusual character, however, in showing 

no intercalated layers of clay. So far as seen it is all grit and pebbly 

matter.
30 THE POTOMAC FORMATION IN VIRGINIA. [BOLL. 145.

In the exposures above and below Point of Bocks, while the greater 

portion of the material is a coarse, pebbly grit like that at the Point, 

yet we often find intercalated in the coarse sandy matter thin sheets 

of clay that have a limited horizontal extent or are lenticular in form. 

These clays occur without rule and at different levels. In rare cases, 

as between the Point and theocher works, the clay thickens up suddenly 

so as to form a considerable portion of the exposure. The clay mass 

then has the form of a disk-shaped cake that thins out rapidly on all 

sides, giving way to the grit. These clays in the exposures on the 

Appomattox often show unrecognizable bits of plants, but in no case 

well-preserved fossils.

It is from the erosion of the clays in the process of depositing the 

sand and grit that the isolated clay masses and balls are formed. 

Whenever found in the Potomac two colors characterize them, the one 

being, when the material is damp, dark-gray to black and the other 

chocolate. It is not an unusual thing to find a thin clay layer inclosed 

on all sides by coarse sand which is sharply distinct from it. The beds 

are thin, ranging generally from 4 to 12 inches. Sometimes they are 

cut away abruptly at the ends, apparently by an eroding current. The 

peculiar clay balls that are so common on the James Eiver and in all 

the Potomac areas appear to be rare on the Appomattox Eiver.

About 200 yards above the Point an unusually large proportion of 

clay is present. The banks of the river are here much lower than at 

the Point, rising only about 30 feet above the water. Nearly all of this 

material is a reddish-brown or chocolate-colored, thinly laminated, 

plastic or sandy clay, with some layers of fine, sandy matter. A considerable portion of the clay is plastic, and obscure fragments of plants 

occur in some of the layers. A short distance up or down the stream 

no trace of this clayey material can be found.

About 400 yards above the Point nearly 40 feet of the Potomac grit 

is exposed, and here there is a bed of highly ferruginous clay, from 

4 to 5 feet thick, resting upon it. This latter stratum has so much 

iron oxide in one portion that it is worked as an ocher. The ocher bed 

is probably a member of the Lafayette group.

The exposures in the vicinity of the Appomattox show that the 

Potomac had lost much from erosion before the deposition of the 

Eocene. In places its surface had been cut down much lower than in 

others. This is shown by the position of the Eocene at some localities 

in this region.

In the Potomac areas south of Fredericksburg the lowest Eocene 

bed visible is a greensand marl. Two miles northwest of Point of 

Eocks this bed crops out in a ravine called Deep Bottom, a name also 

given to a locality on the James Eiver below Dutch Gap. The top of 

the greensand bed here is considerably below the level of the top of the 

Potomac at Point of Eocks.

Between Point of Eocks and City Point, where it enters the James 

Eiver, the Appomattox flows due east and soon passes so far- within 

the Tertiary that exposures of the Potomac cease.
POHTAINK.) EXPOSURES ON JAMES RIVER. 31 

EXPOSURES NEAR DUTCH GAP.

The exposures next to be noticed occur on the James River in the 

vicinity of Dutch Gap. About 17 miles below Richmond the James 

Eiver makes a bend in the shape of a long, narrow horseshoe whose 

heels almost touch. The narrow neck which separates the heels, barely 

100 yards wide, has long been known as Dutch Gap. Through this the 

noted Dutch Gap Canal was cut. The promontory around which the 

river runs has its longest dimension turned to the southwest, and is 

called Farrars Island. The southern portion of the bend in the river 

around Farrars Island is known as Trents Reach. The length of the 

horseshoe is 7 miles.

Exposures of the Potomac formation occur in the left bank of the 

river for 2 miles above the canal. They are found also at several 

points in the bend, being most extensive at the'northern entrance of 

the convex portion of the horseshoe at a locality called Osbornes. At 

the eastern entrance of the horseshoe. there is an outcrop that has 

yielded many fossil plants. Many have been obtained also from the 

banks of the canal and from the outcrops above the canal. The lower 

portion of the neck through which the canal was cut," for a height of 

10 to 15 feet above the water, is composed of Potomac material. The 

upper portion is Pleistocene. The canal is cut wholly in the Potomac, 

and does not pass below it.

The finer Potomac material in the vicinity of Dutch Gap is, as usual, 

composed of sand and clay, the latter generally forming thin, lenticular 

layers in the former. Rounded stones and clay balls often occur in 

this, especially toward the top. This matter is extremely variable from 

place to place and shows no regular bedding. The structure, so far as 

seen, is a sort of current-bedding, which does not anywhere permit a 

determination of dip and strike. This heterogeneous matter forms the 

lower part of the bank of the river, and its base is nowhere exposed. 

Material on an average from 10 to 15 feet thick is exposed above the 

water. The top, however, is uneven, and the formation has evidently 

lost much by erosion in pre-Eocene times. Near Osbornes as much as 

30 to 40 feet is shown.

On top of the Potomac formation we usually find a mass of gravel 

and cobbles, often containing large fragments of pre-Oambrian rock. 

This cobblestone deposit is in rare cases absent, but it generally ranges 

in thickness from 4 to 10 feet. Above the cobblestone bed we often see, 

in variable thickness, a loam that resembles the clay used in a number 

of places for the making of bricks. Both the cobblestone bed and the 

loam are apparently Pleistocene in age. This arrangement of material 

is not confined to the vicinity of Dutch Gap, but extends down the 

river to Deep Bottom, and up to Richmond.

The sand which forms much the larger part of the Potornac, and 

which may be taken as the typical rock, is sometimes locally sufficiently
32 THE POTOMAC FORMATION ON VIRGINIA. [BULL. 145.

indurated to form a sandstone, but as a rule, in weathered outcrops at 

least, it is incoherent. It differs in no essential from the grit of Point 

of Eocks and other places on the Appornattox. The color at Dutch 

Gap is often brownish-gray or yellowish-gray, with patches and blotches 

of light-gray or white, the latter being caused by a larger proportion of 

kaolin-like clay. The texture of the sand is generally quite coarse. 

Most of it is quartz in coarse grains. Intimately mixed with the quartz 

grains there are in some places many more or less fully kaolinized particles of feldspar, a little mica, and a varying quantity of a nearly white 

clay. The amount of mica is remarkably small if this Potomae material 

comes principally from the pre-Cainbrian on the west, for that is composed of micaceous rocks, such as granite, gneiss, and mica-scntst. 

Some of the light-colored clay is derived, no doubt, from the comminution of Cambrian material, for some of this, now found in cobbles, has 

a great deal of light-colored clay with tbe quartz. We find very generally scattered through the sand a number of more or less rounded 

quartz pebbles 3 inches or less in diameter. These seem to come from 

the pre-Cambrian. In most of the exposures large cobbles of Cambrian 

quartzite and quartz do not occur in the body of the sand. Those in 

Trents Beach are exceptional in this respect.

It would be futile to attempt to use the barometer in taking the dip 

of this material. Its surface is determined by the accidents of erosion, 

and no single layer maintains the same character long enough to be 

traced from one outcrop to another. The dip as a whole sinks gently 

to the southeast.

EXPOSURES IN TRENTS REACH.

In-Trents Eeach, near Osbornes, 40 to 50 feet of the sand is exposed, 

giving much the most extensive outcrops in the vicinity of Dutch Gap. 

Here very striking and sudden changes of structure, texture, and 

composition are shown, Lenticular masses and irregularly shaped 

portions of the sand are fine-grained and comparatively uniform in 

texture. These are surrounded by material of very heterogeneous 

composition and varying coarseness, so that the finer material may 

abut against portions that are full of large cobbles. It is impossible, 

without tediously minute description, to give an adequate idea of the 

heterogeneous character of the sand in Trents Eeach. Current-bedding and beach structure are very marked, and, so far as any separate 

portions can be distinguished, the material is arranged in irregular, 

interlocking, lenticular layers. In some places here the structure 

seems to indicate that the matter was deposited spasmodically, with 

intervening periods of erosion, a feature very generally characterizing 

the Potomac. The freshly deposited sand seems to have been cut away 

in the body of the mass from time to time, forming pits and trenches, 

and in these depressions was deposited a mixed mass composed of sand 

and stones, large and small.

In the confused material above described nests and pockets of
FONTAINE.] EXPOSURES ON JAMES RIVER. 33

gravel and large cobbles of quartz and Cambrian quartzite occur, generally in the form of short, lenticular layers. The cobbles range in 

diameter usually from 3 to 8 iuches, but some are 10 to 12 inches. At 

one point near the mill on the bank of the river in Trents Reach a 

stratum of coarse sandstone occurs near water level, which, for the 

space of about 100 yards, is packed full of large cobbles. This cobble 

bed has a maximum thickness of about 6 feet. On each side the stones 

diminish in size and number until they give way to the usual sand or 

grit. The cobbles for a considerable space are so numerous as to touch 

one another, and are remarkable for their uniformly large size and for 

the large proportion of those composed of Cambrian quartzite. Many 

of the stones are from. 6 to 10 inches, and some 12 inches, in diameter. 

They are smooth and mostly elliptical in shape. Most of them are 

composed of Cambrian quartzite, the largest being formed of the coinpact, dark-gray quartzite. This rock is found west of the Blue Ridge, 

underlying the typical Cambrian, with Scolithus. These cobble beds 

never contain any pre-Cainbriaii masses. They are integral portions 

of the Potomac formation, and must not be confounded with the cobble 

stratum which rests upon the Potomac sand, and which frequently contains pre-Cambrian material. Probably the cobbles of this latter came 

originally from such deposits as have been.described above. Cases are 

very rare where the sand shows any tendency to lamination or to graduate into clay, and when clay is found it is confined to very limited spots, 

forming, indeed, lenticular pockets in the heterogeneous matter. Clay 

beds are very rare in Trents Reach, except at and near its southeastern 

entrance, where the proportion of this material becomes unusually 

large. In the vicinity of Osbornes, however, clay balls of remarkable 

size occur embedded in the sand.

Before describing the various modes of occurrence of the clay in 

Trents Reach it will be necessary to make some general statements 

concerning the character and occurrence of this material at Dutch Gap, 

and what will be said of the Dutch Gap clay applies most remarkably 

to the Potomac clay of the areas farther north.

The clays of Dutch Gap are mostly plastic, and have the usual two 

colors so commonly found in the formation. They are, when moist, 

either dark-gray to blacker dark-brown to reddish-brown; when the 

clay is dry the colors are paler. Clay of both colors is found in the 

original layers, and in balls or more or less rounded masses that have 

been torn off from the original layers and transported some distance 

while sand was being deposited. The original clay beds have sometimes been wholly removed, and are represented now solely by masses 

of clay that are isolated and embedded in the sand. The balls then 

represent erosion horizons, and they may occur at any level, but are 

most common near the top of the Potomac. They are often associated 

with cobblestones. The way in which they are probably formed has 

been explained in another connection. 

" Bull. 145 3
34 THE POTOMAC FORMATION IN VIRGINIA. [BULL-. 145.

The clay layers, when found as originally deposited, nearly always 

occur isolated and embedded in the sand. They are seldom more than 

2 feet thick, generally 10 to 12 inches. They show very great and often 

sudden changes in thickness, and may split up and graduate into sandy 

matter, but most commonly they have the form of sharply distinct 

thin cakes or lenticular layers of very limited extent that disappear by 

thinning out on all sides. They may occur at any level and may show 

any inclination in the sandstone. In extreme cases "we may even find 

several radiating from a common center. When partially eroded, very 

many modifications may occur. Portions may remain buried in fragments of clay, but this is not so common here as in the Fredericksburg 

area. The ends may be cut squarely off and abut against coarse sand 

and even gravel. Frequently the layers split before thinning out. In 

very rare cases we find the layer thickened abruptly, so as to give a 

mass of clay 8 to 10 feet thick. This is the case at the southeastern 

entrance to Trents Eeach.

All the well-preserved fossil plants found in the Potomac, both here 

and elsewhere, occur0 in these clay layers. The plants are much more 

common in the chocolate-colored clay than in the dark clay.

A remarkable feature of these clays in many cases is the sudden 

transition from the clay to- the embedding sand. In most cases there 

is no gradation in passing-from one to the other. I have seen perfectly 

pure and plastic clay resting on coarse sand, and even pebbles, with 

the line of junction so sharply defined that the pebbles and grains of 

sand could be picked out from the surface of the clay. The dark clay is 

frequently pyritiferous, and it is then difficult to preserve the fossils 

found in it. This clay, while not commonly showing good impriuts of 

leaves, very often contains much lignite. This generally occurs in the 

form of small fragments, but sometimes large logs are found. A portion of a log of lignite was found embedded in the banks of Dutch 

Gap Canal, indicating a coniferous tree originally 18 inches thick, and 

it was stated that at least 40 feet had been removed from it.

Nearly all of the undisturbed clay layers show some cleavage, but 

they are never distinctly laminated when unrnixed with layers of sand. 

Here there is little of the uptorn and redeposited clay beds so common 

in the Fredericksburg area.

Isolated clay balls and masses are among the special features of the 

Potomac formation everywhere. They are common at Dutch Gap, and 

sometimes attain huge dimensions. They range in size from pellets 

less than an inch in diameter up to masses 4 or 5 feet through. These, 

as before stated, are derived by erosion from the clay layers. They 

sometimes give recognizable fossil plants, but generally the plants are 

reduced to small bits and the clay is contorted, crushed, and very much 

affected, with rubbed surfaces or slickensides. AtOsbornes a number 

of large balls of dark clay maybe seen embedded in the sand, arranged 

in a row on nearly the same level. They occur at intervals in the face
EXPOSURES ON DUTCH GAP CANAL. 35

of the cliff about 30 feet above the water. Some of them are from 3 

to 5 feet iii diameter. The dark color of the clay contrasts strongly 

with the light color of the inclosing sand. A remarkable clay ball, 

abounding in impressions of plants, was found in the banks of Dutch 

Gap Canal. This will be described further on.

At the southeastern entrance of the bend around Farrar Island, in 

the right bank of the fiver, there is an exposure of the Potoinac noteworthy for the large amount of clay that is shown in one portion and 

for the number of fossil plants occurring in another. This locality is 

almost immediately south of the canal at a point where a small stream 

enters the river. The banks here range in height from 20 to 40 feet, 

the upper portion being composed of the cobblestone bed in a large 

deposit and the lower portion of Potomac material. The exposures of 

Potoinac seen lowest down the river here are composed almost wholly 

of clay, having a maximum thickness of about 20 feet. This is reddishbrown in color, and is cleavable. JSTo plants are found in it. About 30 

yards farther up the river the clay splits up and becomes mixed with 

fine sand, the material being confusedly mingled. The top of the 

Potomac here is composed of a stratum of rather uniform clay 6 to 8 

inches thick. This last is full of fragments of plants, a large number 

of them being very well preserved and readily identified. A number 

of new plants were found in this layer, such as Williamsonia mrginiensiSj several species of Baieropsis, Frenelopsis parceramosa, and many 

specimens of Dioonites bucliianus. The greater portion of the exposure 

is composed of mixed sand and clay. A number of plants are irregularly distributed in this, and they are mostly of species different from 

those found in the top layer. A little farther up the stream a lignite 

log was found embedded in this material. The log is lignitized at one 

end and silicified at the other. A heavy bed of cobbles, in places 10 

to 15 feet thick, overlies the Potomac in these exposures. The cobbles 

are of very large size, many of them being 8 to 10 inches in diameter, 

and some of them a foot.

EXPOSURES ON DUTCH GAP CANAL.

The banks of the canal expose the Potomac for the height of 8 to 10 

feet, and give a good illustration of the exceedingly heterogeneous 

character of its material. The material composing the opposite sides 

of the canal shows great difference in composition and structure. In 

the left bank the clay is, in places, unmixed with sand, and rises 6 to 8 

feet above the water; but the pure clay soon gives place to clay with 

seams of sand. In some parts the entire bank is composed of the usual 

coarse, sandy grit, in which, toward the top, large clay balls are 

embedded. With these, at the lower end of the canal, many large 

Cambrian and quartz cobbles are associated. The right bank also is 

composed of very variable materials. Portions of this bank are formed 

wholly of grit or sand and small stones. In other portions clay is
36 THE POTOMAC FORMATION IN VIKGINIA. [BULL. 145.

irregularly mingled with the sand. In the upper end of the canal 

the bank is formed of alternations of sand and clay. % It is said that the 

canal below the water line was dug wholly in these. The clay in 

the right bank is mostly dark-colored; that in the left bank is mainly 

chocolate-colored or reddish-brown.

Fine fossils are sparingly and irregularly scattered in both banks of 

the canal. .There is no regular layer containing them, and they can be 

found only by accident, as it were. Specimens of several conifers with 

acicular and scale-shaped leaves are found, with sometimes scattered 

fragments of Baieropsis, but Dioonites buchianus is by far the most 

common plant. Impressions of very large leaves ' of this fine cycad 

were seeu on the surface of the dark clay in the right bank, in some 

places reaching the length and width of about 18 inches. As these 

were mere fragments, they indicate that the entire leaves must have 

been at least a yard long.

The clay here, although in some places abounding in specimens of 

Dioonites, has but few species. While searching the exposure in a horizontal direction foot by foot for plants I came suddenly upon a rounded 

mass of pure, chocolate-colored clay embedded in the iuterstratified 

sand and dark clay. This, unlike the surrounding material, was somewhat crushed and showed slickensides. It was full of impressions of 

plants totally different from those in the surrounding material, and the 

number of different species was remarkably large. At first I thought 

that I had discovered a rich fossiliferous layer, from which a large 

quantity of good material could be taken, but on working in this 

material it disappeared as abruptly as it had appeared. For a time 

this was a puzzle, but it proved to be a ball of reddish clay embedded 

in the normal heterogeneous material composing the banks of the canal. 

This was an unusual mode of occurrence, for in all other known cases 

in this area the clay balls occur in the sand where it is unmixed with 

clay. This clay ball was about 5 feet in diameter, and was probably 

torn off from some richly fossiliferous older clay layer nowhere visible 

now. It yielded some fossils that were not found anywhere else.

EXPOSURES ABOVE DUTCH GAP.

For about 2 miles above the canal exposures of the Potomac occur 

in the lower portion of the river bank on the left-hand side. The thickness ranges from 10 to 20 feet. , The material is almost wholly a coarse 

grit or sand similar to that found near Osbornes and at Point of 

Eocks, on the Appomattox, except that large cobbles are rare, while 

small gravel, large clay balls, and irregular pockets of clay are common. 

In many places thin layers of clay of the usual kind are found, and in 

some of them plant impressions may be seen. The sand has the usual 

heterogeneous composition and mode of arrangement, being disposed 

in irregular, interlocking layers, showing current bedding and beach 

structure. The clay layers show many irregular features, and they are 

variously affected by partial erosion.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 145 PL. II

Fig. 1.

Fig. 2.

:.''^:-:-':^^-? 

^'^^^j^iifei^^

Fig. 3.

Fig. 4.

MODES OF OCCURRENCE OF CLAY LAYERS AT DUTCH GAP.
FONTAINE.] EXPOSURES NEAR DUTCH GAP. 37

In the figures given on PI. II, I have attempted to represent some of 

the varying modes of occurrence of these layers. Fig. 1 gives three 

large clay masses embedded in the sand. Fig. 2 represents two clay 

balls and two layers that extend to the top of the Potomac and are 

there cut away by the erosion to which it has been subjected. Fig. 3 

gives two layers that are cut away at both ends by erosion. Fig. 4 

partially represents the condition of things at the plant locality near 

a fishing hut about a mile above the canal. Here three clay layers 

diverge from near a common point a few feet above the level of the 

water. I have represented these as abruptly cut off on the right, but 

they really extend beyond the limits of the figure. The upper one is 

very soon thinned and lost in the sand. The two lower extend some 

20 or 30 yards before disappearing in a similar manner, and they approach each other quite closely and run parallel for some distance. 

Both of these contain impressions of plants. A large number of good 

fossils were obtained here. Above this point and on toward a large 

spring that issues from the bank of the river the sand has some irregular pockets of clay, 5 to 10 feet in length near the level of the water. 

Some of these contain plant impressions.

The clay layers in Fig. 4 do not have the plant impressions throughout their mass. The upper layer has but few, and these are unevenly 

dispersed. The lower layer has the fossils in two thin seams that 

run in a very irregular manner through the bed. In places these are 

rich in fossils. This bed is remarkable for the great variety of plants 

that it contains. The chief of these are Dioonites luchianus, Sequoia 

Reichenbachi, Equisetum virginicum, and several species of Baieropsis 

and L;iricopsis. A number of ferns and many other plants are found. 

This is the richest deposit of fossils at Dutch Gap.

THE COBBLESTONE BED NEAR DUTCH GAP.

At nearly all the Dutch Gap localities a bed of coarse stones caps the 

Potomac sand. It will be more convenient to give a general description 

of this and omit the details for each locality, for local descriptions 

would involve much repetition.

Sometimes the cobblestone bed is separated from the Potomac sand 

by a thin layer of chocolate clay, but generally it rests immediately 

upon the sand and is mixed more or less with it, the sandy matter serving as a cementing matrix, in which the stones are packed. This 

matrix, however, although derived from the Potomac, is apparently 

later in deposition, and comes from the ruins of the older bed. Indeed, 

most of the stones seem to have the same origin.

As was stated before, the stratum of cobbles is widespread. It 

appears all along the Appomattox and the James from Deep Bottom 

to Eichmond. It probably underlies the surface deposits between the 

two rivers, and is certainly found several miles north of the James 

wherever the streams cut down to its level. The larger material of
38 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

the bed consists of rounded stones of all sizes under 10 inches, with 

occasional masses of pre-Oambrian rock. While the cobbles are generally under 10 inches in diameter, they sometimes reach 12 inches, 

and in a very few cases masses of Cambrian quartzite having the 

diameter of even 24 inches may be seen. These larger stones are subangular, while those from 10 inches in diameter down are well rounded 

and smooth. Tbis rounded material is sometimes quartz, but more 

often Cambrian quartzite, the larger stones being composed of the 

latter. In some cases markings made by Scolithus are quite distinct. 

In one or two localities near the banks of the river partly rounded 

slabs 10 to 12 inches in diameter have been picked up. These appear 

to be Devonion in age, since they show impressions of crinoid stems 

and ..shells characteristic of the Chemung formation. On the surface 

of the Potomac sand, in the left bank of the canal, a fragment of 

brownish sandstone 5 to 6 inches in diameter was found showing 

imprints of Chemung shells.

It seems certain that a large portion of the coarse material comes 

from the Cambrian, and some at least from higher Paleozoic formations. None of these formations in Virginia are found east of the 

Blue Eidge. One piece of Cambrian quartzite found on the banks of 

the canal was planed off smooth at one end and showed markings that 

strongly suggested grooving from ice action. This was the only case 

of the kind found. Whatever may have been the mode of transport 

of these stones all the way from the Blue Eidge, it is clear that they 

have been subjected to much attrition from transportation in water. 

This may have occurred after their deposition in their present positions. 

Some of the more compact quartzite cobbles, especially when of large 

size, have their surfaces covered with crescent-shaped markings 

resembling the new moon in form. These are often thickly placed, and 

they seem to be without order. They are not markings left by organisms. The marks seem very slightly depressed. They are on solid 

quartzite, differing from the rest of the cobble only slightly in color 

and texture. It is possible that they are imprints produced by the 

impact of the stones against one another. The blow would produce at 

the point of impact a slight shattering or weakening of the cohesion 

of the quartzite, and when the surface had been long exposed to 

weathering a difference in texture would perhaps show itself.

The pre-Cambriaii masses found in the cobble bed occur almost 

always at its base where it joins the eroded surface of the Potomac. 

These masses are comparatively rare. The pre Cambrian bowlders 

never show much abrasion from transport in water. At most they 

have their sharper corners and edges worn off. They are usually of 

large size, ranging from 2 to 5 feet, and in a few cases they reach the 

diameter of even 10 feet. Those of the smaller sizes occur of rocks 

that are of the Suronian type; that is, they are composed of such 

fine-grained rocks as argillite, chlorite-schist, mica-slate, fine-grained
FONTAINE.] EXPOSURES NEAR DUTCH GAP. 39

mica-schist, felsite, etc. Such rocks do not occur on the James Eiver 

nearer than. 50 miles west of Eichmond. The largest masses are composed of reddish, coarse-grained granite or gneiss. This rock crops 

out on Devils Beach, in the river bank, about 2 miles (air-line distance) 

northwest of the canal. The largest of these masses that I have seen 

is lying near the water's edge a few hundred yards above the canal, 

where it has fallen from the bank above. It is about 10 feet long, and 

its surface is only slightly water worn. Several of these large masses 

occur along the river here, and one of them has a mortar-like depression in its surface, which was artificially made, probably by the Indians. 

Two of these artificial depressions occur in the surface of the rock 

where it crops out in Devils Beach. It is stated that in excavating 

the canal one of these masses was found u as large as a haystack' 1 

and that it had to be broken up by blasting.

The action of the tide at many points near Dutch Gap, as well as 

below and above that place, combined with the waves set up by .the 

steamers moving in the river, cuts away and undermines the banks, so 

that the cobblestone stratum is from time to time thrown down. The 

water washes away the finer matter and leaves the larger stones, causing a gradual concentration of the larger fragments near the water's 

edge. In many places there are great accumulations of large stones 

which line the edge of the water. This material presents a strong contrast to the matter now brought down by the stream, which is fine 

sand and mud.

The cobblestone bed above described is apparently Pleistocene in 

age where it contains the pre-Canibrian material, but a stratum of 

similar character, without pre-Cambrian matter is spread over the surface of the Potomac, and this is certainly pre-Eocene. As a rule, along 

the James Biver the Pleistocene cobble bed has resting upon it a fine, 

gray loam. At Bichmond this is used in making bricks, and the 

material of this horizon is very generally employed for this purpose, as 

at Baltimore, near Alexandria, at Petersburg, and at other places. 

This seems to be Pleistocene, and in most places on the James above 

the gap it is the highest material seen in the banks of the stream. At 

the mill in Trents Beach there is a deposit of Eocene capped by 

Lafayette.

In the hills adjoining the river a thick covering of the Lafayette formation, extending an unknown distance, conceals the Potoinac. This 

younger formation certainly extends over the entire space between the 

James and the Appoinattox rivers that forms the Potoinac terrane. 

Bising through this material, however, in some places are small outcrops of the Tertiary marine beds. In the space between Chester and 

Osborues the Lafayette formation is over 100 feet thick, and is composed largely of gravelly, gray sands and stiff, mottled clay, marked 

with gray, yellowish, and brownish colors. It has some beds of fine 

gravel. All the material is loose and easily eroded. The formation is
40 THE POTOMAC FOEMATION IN VIRGINIA. [BULL. 145.

without fossils and seems generally to have been deposited in shallow, 

unquiet waters.

Along the James between Deep Bottom and Richmond the Tertiary 

is cut entirely away and replaced by the upper gravel or cobble bed, 

the brick clay, and the Lafayette, the latter, when present, underlying 

the two former. To do this work the river at some former time must 

have had greater fall between Richmond and Deep Bottom than it has 

now. Immediately below the canal the Potomac sand sinks down until 

it stands near the level of the water and soon disappears, being covered by the cobble bed and brick loam. For about 1£ miles down the 

river to Aikens Landing the cobbles are abundant, and a considerable 

number of pre-Cambrian masses are found, ranging in diameter from 

1 foot to 3 feet. The rock composing most of them has the lithologic 

character of the Huronian, the most common kinds being felsite and 

argillite. The banks are then low and alluvial as far as Deep Bottom, 

with, no exposures of Potomac.

EXPOSURES AT DEEP BOTTOM, ON THE JAMES.

Deep Bottom is 4 miles below Dutch Gap Canal by the river. The 

course of the river after leaving Dutch Gap is at first due east for some 

distance. It then turns due north, making a sharp bend, and returns 

on itself, so as to form a closely compressed loop. Deep Bottom is situated at the apex of the bend, on the north side of the river. The 

locality is nearly 3 miles due east of the canal, and is consequently considerably farther on in the direction in which the Potomac formation 

sinks down. Here the greensand marl, which south of Potomac Creek 

is the lowest Eocene stratum, makes its first appearance on the James 

River, and here the Potomac formation appears for the last time in outcrop. At this place we fortunately have exposed in the same vertical 

section the Potomac, the Eocene, and the upper cobble bed, with its 

superposed brick loam, so that the relations of these deposits can be 

made out. At the upper end of the bank exposed at this place a rather 

bold stream enters the James at a group of huts. This stream results 

from attempts made about eighty years ago to drain a marsh lying at 

some distance from the river. A ditch was cut from this to the river, 

and the fall being considerable and the surface material loose and easily 

eroded, the ditch soon developed into a gully. It is now a deep ravine, 

which cuts down a little way into the Potomac formation, the first 

resistant material which the water has met. The following section was 

obtained in this ravine:

Section in ravine at Deep Bottom, on the James River.

Feet.

1. Recent deposits at the top.............................. 1 to 2

2. Brickloam............................................. 10 to 20

3. Cobblestone bed ....................................... 5 to 10

4. Greensand marl........................................ Ito 5

5. Potomac cobbles ....................................... 0 to 3

6. Potomac sand.......................................... 3
FONTADJE.] EXPOSURES ON THE JAMES RIVER. - 41

No. 1 is composed of loose matter from No. 2, and is only a foot or 

so thick.

No. 2, the brick loam, is here thicker than usual, and along the sides 

of the ravine has weathered and been washed into the peculiar hummocky and ridged shapes so characteristic of exposures of this material. It graduates below into the cobblestone bed, No. 3.

No. 3, the cobblestone bed, is distinguished here by the great number and large size of the stones it contains. A number of them are 

composed of Cambrian quartzite, while others are of quartz. Some 

masses of pre-Cambrian rock of considerable size occur in the bed. It 

is continued in great force for at least 2 miles down the river. Some 

of the Cambrian stones are as much as 18 inches in diameter.

No. 4, the greensand marl, seems to have been here subjected to 

great and irregular erosion. It is the only one of the Tertiary beds 

that remains, and is itself wholly removed in some places. It has very 

much the same character here that it shows generally in Virginia. 

The greater portion of the mass of the bed is a sandy clay containing 

more or less diffused calcareous matter that sometimes acts as a cement 

and forms a firm rock. Imprints of shells abound in some portions, 

and sometimes the shells themselves are found. The matrix is colored 

greenish, with chlorite and some greensand. Elsewhere in the State, 

where uneroded, the greenish sandy matter is from 10 to 15 feet thick, 

and the shells occur in the base of the bed. It is the lowest part of 

the stratum that is preserved here.

No. 5, the stratum of Potomac cobbles, marks the transition from 

the Eocene to the Potomac sand. It is not always present, for in a 

few places the marl rests immediately on the sand. This seems to be 

due to the fact that these cobbles lie on the surface of the Potomac in 

lenticular sheets. Their presence is accounted for by the erosion of 

the upper part of the Potomac sand, which has concentrated the cobbles on the remaining surface. The cobblestones in this layer are all 

well rounded and smooth, and are composed of Cambrian quartzite 

and quartz. No pre-Cambrian material was seen. This is the only 

essential difference between this bed and the upper cobblestone bed, 

if we leave out of the question the greater number and size of the 

stones in the upper stratum. Most of the stones in the lower layer 

have a diameter of less than 4 inches, although a few have the dimensions of 6 to 8 inches.

No. 6, the Potomac sand, has been cut into by the creek only so 

deeply as to expose at most 3 feet, for the sand here is so cemented by 

ferruginous matter as to form a firm sandstone. The material of this 

bed is the usual coarse, pebbly grit, without regular bedding, and 

varies rapidly in structure and texture. It shows current-bedding 

and in some places is full of finely comminuted, carbonaceous matter. 

Not enough of it is displayed to show any clay layers.

The section above given was obtained in the ravine about 200 yards 

above the point where the stream enters the river. The exposure of
42 ,THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

material in the river bank is much less extensive than in the ravine. 

For some 200 yards the Potomac is cut down so low that its top is 

barely visible above the water. In most of this interval the upper 

cobble bed rests on the Potomac sand, but in one or two places the 

Eocene marl rests upon it. Although so little of the formation can 

be seen here, yet it shows some characteristic features. The banks 

are undermined by the tide and a great deal of matter is thrown down 

from the upper cobble bed and from the brick clay. This covers up 

the Potomac material and adds much to the obscurity of the exposures. Professor Eogers in his reports speaks of the occurrence of the 

Potomac at this locality, but he does not give details. He merely 

mentions the juxtaposition of the formation to the marl, stating 

that this is the last appearance of the formation on the James. lie 

does not seem to have been aware of the existence of the ravine. 

Immediately below the mouth of the vStreain that enters from the 

ravine the river bank is low. The upper part of it is formed of the 

upper cobble bed, which contains many large, rounded cobbles and a 

number of masses of pre-Cainbrian rock. Beneath this is a peculiar 

material, exposed to the height of 3 or 4 feet above the water. It is 

so much covered up by loose sand and cobbles which have fallen from 

above that it is shown only within a very limited horizontal distance. 

This material seems to be debris torn off from some underlying strata 

and redeposited without sorting. Fragment? of the original beds, of 

various sizes and still retaining the structure of the parent strata, 

stand in various positions buried in finer matter formed by the comminution of similar material. The parent strata were of two kinds: 

(1) The most abundant material is a rather fine-grained sandy clay of 

a gray color. A large amount of loose sand has been eroded from 

strata composed of material of this kind. This serves as a matrix 

which incloses all the larger fragments. In this sand, as has been 

stated, there are a number of fragments of the bed, ranging in size 

from a few inches in diameter to several feet. (2) The other kind of 

material is a tough, highly plastic clay of dark-gray or chocolate 

color, being, in fact, the clay so common in the Potomac formation. 

This sometimes becomes rather sandy and occurs only in isolated 

lumps and balls embedded in the sandy matrix. The Potomac was 

not seen in place at this spot, but it is clear that these materials are 

the ruins of Potomac strata that do not differ essentially in character 

from those exposed in the vicinity of Dutch Gap. The beds from 

which this debris was derived seem to have been quite fossiliferons, 

for impressions of leaves are found both in the masses of sandy clay 

and in those of purer clay. The most distinct imprints are found in 

the pure clay, for the sandy matter is a good deal crushed and is quite 

friable.

The fossils are nearly all augiosperms of the same general character 

as those found near Brooke, in White House Bluff, and at Baltimore.
PONTAINE.] EXPOSURES ON THE JAMES RIVER. 43

This occurrence of angiosperrns in predominating numbers is noteworthy, for while this class of plants occurs in the strata about Dutch 

Gap, they are there very rare. The idea is suggested that the beds 

whose ruins are here seeiv are somewhat higher in the Potoinac formation than those found in exposures farther up the river. The 

assemblage of plants found at Beep Bottom certainly has a more 

modern aspect than that of those at Dutch Gap.

The occurrence of-this material gives us a good illustration of what 

is quite commonly seen in the Fredericksburg area, but is very rare in 

that of Petersburg. This feature is the production of a deposit of 

heterogeneous matter formed by the redeposition and consolidation of 

de"bris torn off from some older bed. This takes place with no sorting 

of the matter by water action, and the material seems to have been 

transported but a little distance. Sometimes, indeed, it buries the outcrops of the parent bed. This sort of material may be called redeposited matter.

D6bris fallen from above conceals the rock in place for about 200 

feet down the stream from the spot just described. The first material 

seen in place in descending the river is the marl bed, showing a thickness of 3 to 5 feet, with the upper cobble bed resting on it. Fallen 

debris conceals for some distance the base of the marl; then for the 

 space of a few yards the marl is cut away and the Potomac is exposed, 

showing a thickness of 1 or 2 feet. It is here composed almost wholly 

of dark clay, the remainder being sand of the usual kind. The clay 

shows some tendency to lamination, and is full of undeterminable bits 

of plants. The upper cobble bed rests on this formation here, and the 

lower one seems to be absent. This is the last exposure of the Potoinac 

seen on the James Kiver. From this point for a little over 100 yards 

down the river the low bank is composed of the marl at base with the 

upper cobble bed at the top. Then comes the broad mouth of a creek, 

where the river returns upon itself. Below the mouth of the creek the 

left bank of the river for about 2 miles is in the form of a bluff from 

20 to 30 feet high. This is continually being undermined and thrown 

down by the action of the water. As the finer matter has been washed 

away by the river, there is a great accumulation of large stones near 

the water's edge. Among these we may see large blocks of preCambrian rock, cobbles and subangular masses of Cambrian quartzite, 

cobbles and pebbles of quartz, with some stones of other kinds of Paleozoic rock. There are, for example, some few partly rounded stones of 

a hard, reddish-brown sandstone, which is strikingly like tine red 

member of the Medina formation. One of these 2 feet in diameter was 

seen. Nearly all the largest rounded stones are of Cambrian quartzite. 

Some subangular masses of the latter 24 inches in diameter, and a 

good many measuring'18 inches, may be found. On some of these the 

casts of the borings of Scolitlms Unearis are very distinct.

Many blocks of different kinds of pre-Cambrian rock like those
44 THE POTOMAC FORMATION IN VIRGINIA. [BOLL. 145.

found near Dutch Gap occur here. They are mostly 2 to 3 feet in 

diameter, but some masses of the coarse reddish granite 4 to 5 feet 

through were seen. The stones of all kinds, and especially those of 

Paleozoic material, are surprisingly large to be found so far from their 

parent rocks.

The marl bed continues to be visible at low water for several hundred 

yards and then disappears in the remainder of the river bluff. This is 

composed wholly of the upper cobblestone bed an.d the overlying brick 

clay or its representative. This condition of things continues until we 

enter the horseshoe bend around Curls Neck. Here for some distance 

the banks are low and no exposures occur. In the southeastern portion 

of this bend, however, there are high bluffs composed wholly of the 

Pleistocene formation. In this series of high banks the coarse preCambrian and Paleozoic material disappears and the gravel, when 

found, is composed of comparatively small pebbles. Here, as elsewhere, 

there seems to be a close connection between the occurrence of the 

upper cobble bed and the existence of outcrops of the Potomac in the 

vicinity. From this locality on to City Point nothing but Pleistocene 

deposits appear.

Although outcrops of the Potomac do not occur farther down the 

river than at Deep Bottom, there is, as was stated before, good reason 

to think that the formation is continued beneath the surface at least 

as far as Fort Monroe. The artesian well formerly bored at this place 

was mentioned as giving evidence on this point, and we may now 

examine the character of this evidence.

ARTESIAN WELL AT FORT MONROE.

Fort Monroe is, as was stated before, about 60 miles southeast of the 

last exposures of the Potomac formation on the Appoinattox and James 

rivers, and it is situated nearly in the direction of the apparent dip of 

the strata. An account of the character of the material taken out at 

different depths from this well, and conclusions concerning it, are given 

by Professor Rogers. 1 He says in a letter to the editor of The Virginias, speaking of the examination that he had made: "Ihave thence 

obtained what I think is proof of the presence of Cretaceous between 

this Eocene and what has the appearance of Jurasso-Cretaceous in the 

lower part of the borings."

I can find, however, nowhere in the article any evidence of the existence of Cretaceous strata considered8 as distinct from Jurasso Cretaceous beds, which are Potomac; and in his notes on the well borings 

he makes no reference to the existence of such strata. Indeed, the 

above-quoted sentence is all that he says concerning the existence of 

Cretaceous (or Potomac) beds at Fort Monroe. The entire well section 

is given by Professor Rogers,2 and I will quote only so much of it as is

'Geology of the Virgiuias, pp. 733-736. 2 Loc. cit.
FONTAINE.] ARTESIAN WELL AT FORT MONROE. 45

necessary to show the change from supposed Eocene beds to supposed 

Potomac ones, beginning at what he held to be the base of the Miocene 

and proceeding downward:

Partial well section at Fort Monroe.

Feet below surface. 

Sandy clay, brownish......................................... 583

Very sandy clay, about 5 per cent of coarse sand, with sharks'

teeth ....................................................... 590

Brownish-gray sandy clay .................................... 604

The same, with less sand...................................... 628

Greenish-gray, sandy ......................................... 640

Lightest gray clay ............................................ 670

Brownish-gray sandy clay; less sand than that at 604 feet; same

indeed, as 628 feet ........................................... 699

Brownish-gray sandy clay, same as 604 feet.................... 784

Gray sandy clay .............................................. 815

Very coarse sand.............................................. 835

Conglomerate of clay, sand, and pebbles ...................... 853

Rather fine clay with a little sand............................. 863

The same..................................................... 865

Coarse sandy clay, brownish or reddish blotches............... 870

Clay embedding fragments of granite ......................... 877

Gray clay with occasional fragments of 'coarse sand............. 885

The same ..................................................... 890

Clay and sand in layers, with some coarse pebbles and reddish

blotches.................................................... 900

Reddish mottled clay with coarse pebbles ..................... 901

The same, with coarse sand.................................... 902

The same ..................................................... 903

Total depth of boring below the parade ground................ 907

According to Professor Bogers the top of the Eocene is to be placed 

at the depth of 583 feet. He does not say where he thinks the Potoinac 

begins, but in my opinion there is little doubt that this formation is 

entered not far below the level of 835 feet, for there is at this point a 

decided change from the gray sandy clay which characterizes tlie 

Eocene, and the succeeding strata show the peculiar features of composition and structure that are found only in the Potomac. The conglomerate of clay, sand, and pebbles found at the depth of 853 feet 

seems to be the mixture of clay balls, sand, and pebbles so common in 

the Potomac. At 870 feet coarse sandy clay with brownish and reddish blotches occurs, and this is a very common feature in the Fredericksburg area. The coarse pebbles, the fragments in clay, the reddish 

mottled clay with pebbles, etc., are features that occur in the Potomac, 

and in it only.

It is to be noted that the well did not pass through the deposits having the character of the Potomac. If we take the top of the Potomac 

as situated at the level of 835 feet, then the formation has declined at 

the average rate of 14 feet to the mile in the interval between Deep 

Bottom and Fort Monroe.
46 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145. 

EXPOSURE AT CHAFFINS BLUFF.

The first important outcrop of Potornac seen above Dutch Gap occurs 

in Chaffins Bluff. This is situated on the left bank of the river. It 

begins about 1 mile below Drurys Bluff and extends about 1£ miles 

down the river. The bank in this bluff is abrupt, and has quite uniformly a height of from 20 to 30 feet. The lower portion of the bank is 

composed of Potomac sand. This sometimes rises to the height of 20 

feet, and very commonly shows as much as 10 or 15 feet. It has the 

same general nature as in the exposure near Osbornes. The amount of 

diffused white clay everywhere seems to be large, so that the grit is 

generally light colored to white. Very few; layers of colored clay are 

found, and no plant impressions were seen. Clay balls, however, occur, 

frequently of large size, and a great many small cobbles are found in 

pockets and lenticular layers. The Potomac sand is capped by a heavy 

deposit of large cobbles packed in light-colored sandy matter, the whole 

seeming to be formed of the ruins of the Potomac.

 EXPOSURE AT DRURYS BLUFF.

Drurys Bluff gives the most extensive vertical exposure of Potomac 

to be seen on the James River. In its highest part it shows about 80 

feet of material, most of it exhibited in an almost perpendicular cliff. 

The structure and composition of the material are very complex, and no 

two vertical sections, taken even but a few yards apart, would be alike. 

It is mostly a mass of coarse grit with many stones dispersed through 

it, and having toward the base a number of large clay balls. In some 

portions the cobbles become aggregated into lenticular masses of large 

stones. About midway up the cliff there is an exceedingly variable bed 

of dark-chocolate clay that swells up and thins out suddenly in the most 

irregular way, and toward the lower end of the bluff graduates into a 

fine, light-colored sand. The variations in the character of the cliff, 

indeed, are such as to defy description within reason able limits. Toward 

the top of the cliff there is a heavy bed of large cobbles, which seems to 

form the uppermost part of the Potomac.

At the upper end of the bluff the current-bedded, coarse, gray grit of 

the formation composes the lower portion of it. This rises to the maximum height of 10 to 15 feet, but it has no sharply defined upper limit, 

and its height varies greatly. This material has embedded in it very 

large clay balls, some of them 4 or 5 feet through, containing fragments 

of plants, the species not being recognizable. The clay is dark colored 

and plastic. The grit is full of large stones, composed of both preOambrian matter and Cambrian quartzite, with much quartz. The preCambrian stones are not so well rounded as those of Cambrian quartzite. 

We may recognize fragments of granite, felsite, mica-schist, gneiss, etc. 

Sometimes they are as much as a foot in diameter. The stones become 

predominant at the height of 6 to 8 feet above the water line, forming
KONTAINE.] EXPOSURES ON JAMES RIVER. 47

a very well defined cobble bed. In some parts occasional masses of a 

gray sandstone, up to 2 feet in diameter, occur. - This sandstone looks 

like that of the older Mesozoic or Khetic that is found in the Eichmond 

coal field. In this portion many cobbles of the dark-gray Cambrian 

quartzite occur, some of them 18 to 24 inches in diameter. On some of 

these stones may be seen the peculiar crescent-shaped markings before 

described. Over this very coarse mass we find the chocolate clay above 

mentioned. This varies in color from dark-chocolate to brownish-gray. 

It is mostly a rather fine, sandy clay, with interlocking seams of purer 

sand. Like the cobble bed just described, it does not extend the whole 

length of the cliff', but disappears toward the lower end. It ranges in 

thickness from 6 to 30 feet, swelling out suddenly to the latter thickness 

just before disappearing farther down the cliff.

A few yards below the upper end of the bluff a second cobble bed 

comes in. This in its contour is much better defined than the lower one 

and it has a lenticular form. It contains many large, rounded stones 

of quartz and Cambrian quartzite,.but not the large clay masses and 

pre-Cambrian material seen in the lower cobble bed. The highest portion of the bluff here is composed of incoherent sandy clay of yellowishbrown or yellowish-gray color. This seems to lie horizontal, and it may 

be Pleistocene. At the lower end of the cliff a third and heavy bed of 

cobbles makes its appearance in the base of this material. The abovedescribed features appear, with the exceptions noted, in the upper end 

of the bluff or cliff. It would require too much space to describe all 

the variations in the exposure, for each few yards would demand a different account. The bluff extends about 300 yards down the river. At 

its lower end the height is very much diminished, since here the banks 

of the river are lowered by the entrance of a small run. At this point 

the clay balls and cobbles found in the basal portion of the bluff at its 

upper end have disappeared, and we have instead about 6 feet of rather 

uniform grit. About the middle of this there is a lenticular layer of 

clay in thin laminae parted by fine sand, the maximum thickness of the 

whole being about 12 inches. Some small fragments of coniferous twigs 

occur in it.

There is an abrupt change in this stream in the character of the 

material exposed in the river bank, for on the opposite side of the run, 

within 10 feet of the Potomac bed just described, a grayish, incoherent 

loam is exposed, which has the character of the Pleistocene. It is cer 

tainly not Potomac. From this point this material is exposed in the 

banks for over a mile down the river, showing a maximum thickness of 

about 40 feet. This abrupt change suggests the idea of a fault, but it 

might be accounted for by supposing that at the time of the deposition 

of the material the surface of the Potomac was deeply eroded, so as to 

leave an abrupt hill of it at this point. Near Brook e an equally abrupt 

change is found from the Potomac to the Eocene, but there it is 

clear that the Eocene is deposited in ravines cut in the Potomac.
48 THE POTOMAC FORMATION IN VIRGINIA. [BULL.I«. 

EXPOSURES AT WILTON FARM.

The Wilton farm lies on the north side of the river, opposite Drurys 

Bluff. The Potomac is exposed on the river front of this farm for at 

least a mile. The outcrops end a little above Drurys Bluff. The top 

of the formation is very uneven, and rarely more than 5 or 6 feet of the 

Potomac sand is shown. This latter forms nearly all of the material. 

The upper part of the bluff is formed of heavy masses of cobbles. The 

proportion of clay in layers in these exposures is unusually large. A 

bed of pale chocolate-colored clay, situated near the level of high tide, 

runs for a long distance and shows great- irregularity in thickness. 

Where it first appears at the upper end of the exposures it shows its 

maximum thickness, which is 10 feet. It looks much like the clay bed 

at the eastern entrance to Trents Reach. Only obscure traces of 

plants have been found in the clay of these exposures.

The cobble bed resting on the sand here seems to be. in part, at least, 

Potomac in age. It contains in some places large clay balls of the kind 

common in the Potomac sand. Some of these are 2 feet in diameter. 

Clay balls are never seen in cobble beds of Pleistocene age. The clay 

balls are associated with large Cambrian cobbles, and sometimes are 

full of undeterminable fragments of plants. This cobble bed is on an 

average about 10 feet thick. Besides the clay balls and cobbles of 

Cambrian quartzite, we find in it occasionally some rounded masses of a 

sandstone similar to that found in Drurys Bluff. This sandstone may 

not come from the older Mesozoic or Ehetic of the Richmond coal 

field, but may be a harder and more consolidated form of the Potomac. 

Some fragments of it 2 feet in diameter were seen; no pre-Cainbrian 

bowlders, however, were visible in the cobble bed; and this is confirmatory of the supposition that the deposit belongs to the Potomac 

rather than to the Pleistocene.

Above the Wilton farm, as far as the immediate vicinity of Richmond, 

the exposures of Potomac are unimportant, the Pleistocene forming 

most of the material seen in the river banks.

EXPOSURES AT RICHMOND.

A little below Rocke'tts, and especially near the wharves of the Chesapeake and Ohio Railroad, there are good exposures of the Potomac. 

These are given chiefly by the cuts on the railroad made to approach 

the river. While the top of the formation is shown here, its base is 

found in the river near by, for the dredges generally reach the granite 

at no great depth. It does not seem that the formation at this locality 

can be thicker than 30 or 40 feet at most.

The general character of the formation at Richmond is similar to 

that seen in the exposures occurring down the river. It is mainly a 

pebbly grit rock, without regular bedding, showing an arrangement in 

interlocking, lenticular layers that are affected by current-bedding
PONTAINE.I EXPOSURES AT RICHMOND. 49

and are exceedingly irregular in composition and structure. In this 

material, toward the top, cobbles abound. Not enough of the rock is 

shown to indicate whether or not clay layers and balls exist, but there 

is little doubt that they do.

The lowest rock visible is the coarse current-bedded sandstone so 

common in the Potornac formation. This shows at its top in one place 

a thin layer of chocolate clay that has some fragments of Dioonites. 

The sand has many blotches and irregular markings of reddish and 

brownish colors and many small stones scattered irregularly in it.

 In. the cuts made for the approach of the railroad to the wharves the 

top of the sand is exposed, and also the Pleistocene cobble bed that 

usually rests upon it. The Potomac sand here shows strong currentbedding, and is very coarse and heterogeneous in composition. In 

places in its top it has lenticular pockets of large cobbles of Cambrian 

quartzite and of quartz. These are of the same geologic age as the 

sand. Some of the stones are 8 to 10 inches in diameter.

The Pleistocene cobble bed is shown here ia great force, and it is 

remarkable for the number of pre-Cainbrian masses that it contains in 

its base, where we find large stones composed of granite, gneiss, chloriteschist, argillite, schistose felsite, and other crystalline rocks. These 

occur embedded in numbers of rounded stones of Cambrian quartzite 

and quartz, the pre-Carnbrian masses being, as usual, only partially 

rounded. Some of the masses of argillite and felsite are decomposed to 

clay, showing in some places a core of the original rock. They are often 

3 or 4 feet in diameter. The granite gives the largest stones, some of 

them being over 4 feet in diameter. The Cambrian stones in this bed 

are larger than those in the cobble bed of the Potomac, some having a 

diameter of IS inches. The upper cobble bed is covered by a clay, the 

brick loam, that is extensively used at this spot for the manufacture of 

bricks.

Another exposure of the Potomac occurs in the bed of Shockoe 

Creek opposite the shops of the Chesapeake and Ohio Eailroad. This 

is important, because the Eocene and Miocene 1 strata rest upon it here. 

Only about 4 feet are shown. It is composed wholly of coarse, currentbedded sand of the common kind, with large cobbles in the top. These, 

as usual, are composed of quartz and Cambrian quartzite. Some of this 

latter material is now quite friable and easily crushed to sand. Near 

the Exchange Hotel, which is situated on a hill about 80 feet above the 

level of the creek where the above-mentioned exposure occurs, a well 

was sunk through, the Tertiary down to the Potomac sand. It was 

begun in the yellow Miocene marine bed, commonly called the yellow 

marl, and was carried to the depth of 84 feet, penetrating 7 feet into the 

Potomac sand. After the lowest Eocene stratum, which is the equivalent of the greensand marl, had been passed through, the top of the

1 The Miocene deposits of thia region have been designated the Chesapeake formation on the maps 

of the United States Geological Survey.

Bull. 145 4
50 THE POTOMAC FORMATION IN VIRGINIA. [BULL. U5.

Potomac sand was struck. It was the usual coarse, heterogeneous 

grit. This was full of rounded stones and cobbles of the same character 

as those shown on the surface in the bed on Shockoe Creek. These cobbles were unusually large. One of them, which had an elliptical shape, 

was 10 inches in diameter. This well gives important information, 

since it penetrates all the formations in place. It shows that the 

interval in time between the deposition of the Potomac and of the 

Eocene must have been a long one and that important topographical 

changes took place in it: First the Potomac sand and cobbles were 

deposited; then the region was raised and subjected to great erosion, 

by which large cobbles were accumulated over the worn surface; then 

the surface was depressed sufficiently to let the sea iuvade the area 

occupied by the Potomac. It is made clear, also, that no beds corresponding to the upper Potomac, or the variegated clay of Maryland, 

and no marine Cretaceous exist here.

THE TERTIARY AT RICHMOND.

As the Tertiary in Virginia is the formation following next above the 

Potomac, it will be useful to note briefly the character and succession 

of the strata which compose it. The beds at Eichmond may be taken 

as very nearly typical of the formation everywhere in the State where 

it is fully represented. The exceptional features will be noted in the 

proper places. The exposures of the upper part of this formation 

made here are more complete than at any other point in the State. 

These exposures are mostly artificial and are made in the various cuttings formed in the improvement of the city. The lower portions of 

the formation are well exposed in the faces of the hills that border 

Shockoe Creek above the point where the Potomac appears. Here, 

however, the basal beds are shown only in weathered outcrops, and 

the weathering alters a good deal the color and general, character of 

the material. Unfortunately these strata are for the most part not to 

be seen in any other condition, for the artificial cuttings rarely penetrate to the lower beds of the Eocene.

At Richmond the Tertiary beds have an exceptional character in 

being much thinner than they are farther east. This is evidently due 

in part to the greater erosion to which they have here been subjected 

and in part to the nearness of the locality to the former shore of the 

Tertiary sea. The Tertiary strata at Eichmond are, at their maximum, 

about 90 feet thick. At Fort Mouroe, as shown in the section obtained 

from the artesian well, they are about 700 feet thick. The excavations 

on Twenty-first street and on Marshall street show well the character 

of the Middle and Upper Tertiary beds. These strata, having much 

iron in a combined state diffused through them, are apt to change 

greatly in color on exposure, the iron passing to limonite from the condition of a sulphuret, a silicate, or a carbonate, its forms in the fresh 

beds. The excavations show that the greater portion of the Tertiary
FONTAINK.] THE TERTIARY AT RICHMOND. 51

is essentially a sandy clay. This when fresh has a gray color, which in 

different portions may be dark-colored, greenish, or light to almost 

white. The amount of sand varies considerably, some of the material 

being quite sandy, while other portions are nearly pure clay. On 

weathering, different shades of color are produced. Some of the clay 

that is rich in -diatom, shells and almost free from iron bleaches to a 

nearly pure Avhite; other portions, according to the amount and condition 

of the iron, become light-yellowish, yellow, or yellowish-brown, etc.

Except near the top of the formation there is no evidence of unconformity in the beds, and nowhere, except at the top, is there visible 

any regular separation into distinct strata. The material, when fresh, 

seems to be an unbedded mass of gray, sandy clay. Weathering brings 

out in different layers some difference in color, so that in old surfaces 

we might use the colors to subdivide the mass. A close inspection 

shows that the differences in color are accompanied by differences in 

composition, such as in the amount of sand, of vegetable matter, of 

pyrite, of diatoms, etc., but.the portions showing these different characters graduate into one another and do not form distinctly defined, 

separate strata. For example, that portion of the weathered clay that 

has much diatomaceous matter is generally conspicuous by its whiteness. 

The portion above this, when weathered, often has a lead-gray or 

smoke-gray color from diffused vegetable matter, but the diatomaceous 

portion shades off into the overlying lead-gray matter.

Beginning at the base, we find a stratum 10 to 12 feet thick that is 

the representative of the greensand marl. This, when fresh, is grayish 

in color, often with a.shade of blue or green. At Richmond it can not 

be called a mar! 3 for although the main portion of it is the usually 

sandy, somewhat calcareous clay, it lacks the abundant shells and pro- 

" portionally large amount of greensand found in those localities where 

it assumes the character of a marl. It has, however, imprints of shells, 

and in some portions a few shells. At some places large numbers of 

the bones and teeth of fish, and possibly of other animals, occur. So 

far as I know, the fossils have never been examined by any expert. 

Sharks' teeth are not uncommon in some localities.

This lowest bed is followed by from 20 to 30 feet of very fine material 

that does not show any distinct lamination and bedding. It is a mixture of very fine §aud and clay, becoming in places nearly pure, plastic 

clay. About 20 feet above the base of the Tertiary diatom shells 

appear in this mass, and when they abound the material weathers nearly 

white; otherwise it becomes light ash-gray. The diatoms extend up to 

an unknown height, and are most abundant in the material now being 

described. There is no well-defined bed to which the diatoms are confined, but they are found sparingly much higher than this portion. 

These shells are not uniformly diffused through that portion or horizon 

of the Tertiary which is richest in them. Tliey occur in nests and 

pockets. A particular spot may abound in them, and another near by,
52 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

on the same horizon, may have few and even none. It is, then, very 

misleading to speak of a bed of diatoms so many feet thick as existing 

at Eichmond.

Above the mass of clay last described, which we may call the diatomaceous clay, we find about 40 feet of gray sandy clay, varying to 

nearly pure clay, which does not essentially differ in lithologic character from the material which underlies it. This, like the matter beneath 

it, is not subdivided into distinct strata. We may, however, detect 

differences in composition and in contents in passing upward, and it 

may be subdivided accordingly, but the subdivisions graduate into 

one another. Much of this material, when fresh, has a dark-bluish 

or greenish color, especially if wet. This is the mass of sandy clay 

and pure clay exposed in the cuts made for grading Twenty-first street 

and Marshall street. The following -characters may be given for the 

different portions of this material.

The lowest part is exposed on Twenty-first street. Here, at the base, 

are about 3 feet of a dark-gray, tenacious, rather sandy clay. (The colors 

given for this and all the succeeding beds are for the wet material; 

when dry the color is lighter.) This graduates above into a dark ashgray sandy clay, which is about 6 feet thick on an average. It fluctuates considerably, however, in thickness and in degree of sandiness. 

It contains, scattered irregularly through -it, bits of lignite, sharks' 

teeth, vertebrae and other bones of fish, and probably bones of other 

animals, impressions of leaves of angiosperms, pebbles of dark quartz, 

etc.; all indicating some agitation in the waters, for the fossils seem 

to be waterworn. Some of the imprints of leaves in this stratum are 

very well preserved. It is possible that this stratum may mark the 

last of the Eocene beds. There is no break or want of conformity in 

the gray mass that would enable one to locate the horizon of change 

from Eocene to Miocene. The fossils may perhaps give the means 

Of defining the limits of these formations. They have not yet been 

studied, however, and at present all that can be said is that the lower 

portion of the Tertiary deposits at Eichmond is certainly Eocene and 

the upper is as certainly Miocene,

The next material in ascending order that may be regarded as perhaps distinct in character is an unlaminated mass of dark-gray to 

somewhat olive-gray, quite sandy clay. This, in its upper portion, 

has some impressions of shells, and graduates into the next layer. Its 

thickness is variable, reaching a maximum of 15 feet. On weathering 

it assumes a rusty color in blotches. Above this comes a dark-gray 

clay, which is in the main tough and tenacious, being a nearly pure 

clay where examined. Its thickness varies greatly, since it graduates 

into the next stratum, and this thickens, sometimes at the expense of 

the stratum now being described. Its average thickness may be put 

at 7 feet. It contains many impressions of shells. The calcareous
THE TERTIARY AT RICHMOND. . 53

matter of these has been taken out by the sulphuric acid formed by 

the oxidation of pyrite, which seems to have abounded in the bed. 

The upper portion for 2 or 3 feet becomes rather more sandy than 

most of the stratum, and has a yellowish-green tinge. This stratum 

is probably the widely extended Miocene bed called generally tlie 

" blue marl." The absence of shells seems to be a local peculiarity, 

for if this be the blue marl it is noteworthy as a rule for the great 

number and the perfect preservation of the shells that it contains. 

The number of imprints of shells at Eichmond is very great, but no 

shells were seen. This bed is certainly Miocene.

In the upper part of the material last described there seems to have 

been some change in the conditions of deposition, perhaps a renewal of 

agitation in the waters, for we find here again pebbles and fragments 

of dark quartz and impressions of augiosperm leaves, etc. The leaf 

impressions here are more numerous and better preserved than at the 

lower horizon before mentioned, but on neither horizon are they common. The next distinguishable portion is a layer of sandy clay that is 

generally dark-gray in color but in many places with a decided greenish 

tinge. The thickness of this varies greatly, for its top has suffered

PlG. 1. Erosion horizon in the Tertiary at Richmond.

much from erosion, which has left it with abrupt changes, such as are 

indicated in the accompanying cut (fig. 1), in which a represents the 

material now under consideration, and b the yellow, sandy, and fossiliferous bed which overlies it. One of the fragments of dark quartz 

mentioned above was unusually large to be embedded in such fine clay. 

It was impregnated with pyrite, and had the dimensions 5 by 4 by 1£ 

inches.

At the top of the mass last described is the first stratigraphic break 

that was seen in the Tertiary at Eichmond. Below this the material 

forms practically one mass. Over it lies a bed of yellow, somewhat 

argillaceous sand that is full of large imprints of Pectens, Cytherea, and 

other Miocene shells. This stratum may apparently be correlated with 

the bed called elsewhere the " yellow marl," which is quite commonly 

the highest visible marine Tertiary stratum in the State, all above that 

generally, if ever present, having been removed and replaced by Lafayette or Pleistocene deposits. This is certainly the highest Tertiary 

stratum seen at Eichrnond. This bed is composed mainly of a coarse 

sand with enough clay to make it cohere when moist. In some places 

the amount of clay is considerable, and it is so highly colored by iron
54 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 148.'

that it looks like a handsome yellow oclier. The number of impressions 

of shells is large, but no shells were seen. This material has suffered 

greatly from erosion, and it has evidently lost much of its former thickness. It shows a maximum thickness of 8 feet, its top being undulating 

and quite uneven.

Upon the yellow stratum a heavy bed of large cobbles and large 

stones rests. This stratum supplies the surface cobbles and gravel that 

abound in the vicinity of Kichmond. It is the stratum so generally 

found near the large rivers, and which lias the character of the Pleistocene. It is quite variable in thickness, but 8 feet may be taken as the 

average. It is of the usual character, containing cobbles of quartz and 

Cambrian quartzite with some pre Cambrian masses. Over this comes, 

in most places, a layer of stiff, red clay, which has in the city an average 

thickness of about 2 feet. This clay is spread over the surface for a 

considerable distance west of Eichmond, being found -over the preCambrian arid the older Mesozoic of the Eichmond coal field. It 

extends far up the James Eiver, accompanying the surface cobbles and 

varying much in thickness.

The following approximate section, in descending order, represents 

all the formations seen at Eichmond. It was made out from observations taken at different points:

Approximate section at Eichmond.

Feet.

1. Stiff red clay...... ........ ..._.................'........... 2

2. Cobblestone bed........................................... 8

3. Yellowibssiliferous sand................................ .... 8

4. Dark-gray sandy clay, often greenish.................... 2 to 5

5. Tenacious dark-gray clay, with many imprints of shells 

blue marl ............................................... 7

6. Dark-gray and olive-gray clay, usually sandy.............. 15

7. Dark ash-gray sandy clay, with fossils..................... 6

8. Dark-gray tenacious clay .......I...........-.....--.-.---- 3

9. Gray clay with diatoms.................................... 30

10. Greenish-gray fossiliferous s'andy clay.............."........ 12

11. Potomac sand, probably 25 feet thick, 10. feet seen.......... 25

12. Heavy-bedded gneiss and granite.

The Miocene extends from No. 3 to probably some horizon in No. 0. 

From the top of No. 4 to the base of No. 10 there is no marked lithologic change and no visible stratigraphic break.

EXPOSURES NEAR HANOVER JUNCTION.

From Eichmond to the vicinity of Hanover Junction there are no 

exposures of Potomac. It is probable, however, that the formation 

exists beneath the surface, for along the eastern margin of the older 

Mesozoic and the pre-Cambrian, where they are overlapped by the 

Tertiary and the Lafayette, a large amount of quartz and Cambrian 

quartzite cobbles, often with red clay, may be found on the surface.
FONTAISB.] EXPOSURES NEAR HANOVER JUNCTION. 55

On Falling Creek, at Crowe's mill, about 2 miles north of Ashland, 

where the road from. Ashland to Ellet's Bridge crosses the creek, a 

great amount of this coarse matter occurs in the face of the hill descending to the creek; and this seems to form a continuous stratum 

under the surface, extending to Ashland and beyond. Kecently a well 

was sunk at Ashland down to the gneiss. It passed through a thin 

layer of the Lafayette and a thick mass of cobbles resting on the older 

Mesozoic. This cobble bed is associated with a few feet of matter 

which is probably Potomac. No careful examination of the well boring 

was made to determine the existence of the Potomac formation, so 

nothing positive can be said on this point.

In the vicinity of Ellet's mill the upper beds of the older Mesozoic 

have some lithologic features like those of the Potomac, but they contain Khetic plants and are penetrated by dikes of eruptive diabase. 

The best and most instructive exposure of Potomac in the vicinity of

Hanover Junction is found near Morris Ford, on the North Anna Eiver. 

In general, exposures of the formation in the Hanover area are very 

rare and poor, for the Potomac has lost much from erosion, only a small 

remnant being left, and a covering of Lafayette usually hides even this 

remnant.

EXPOSURES NEAK MORRIS FORD.

The exposure near Morris Ford is situated on the North Anna Eiver, 

about 1 mile above its confluence with the South Anna. It is found in 

the banks of a small stream at its entrance into the river. The lowest 

portion seen, with its base not exposed, is an incoherent, rather coarse 

sand of grayish or brownish-gray color. About 8 feet of this material 

is shown. It is not much like the normal Potomac sand at and below 

Richmond, since it is quite uniform in composition, and is nearly free 

from stones and kaolin-like clay. Above it lies a sand, about 4 feet 

thick, which, when weathered, is yellowish-gray or yellowish-brown in 

color. The upper part of it, when fresh, has a greenish tinge. These 

two layers are not separated by any distinct stratification plane, but 

they form essentially one bed. These sands appear to be much affected 

by current-bedding, and have no regular stratification.

Next, in ascending order, we find a cobble bed. of variable thickness, 

the average being about 2 feet. Most of the cobbles are of quartz. 

They are packed closely in a sand like that which underlies them. 

Most of the stones have a diameter under 4 inches, but some of them 

are 5 or 6 inches through. In this bed we find some rounded masses 

of a fine-grained sandstone that appears to come from the older Mesozoic, which lies next to the Potomac on the west in this vicinity. The 

largest of them are 18 inches-in diameter. The cobble bed rests upon 

the eroded surface of the sand and follows its undulations. It must 

have a wide extent in this neighborhood, for it is found in a similar 

position on the opposite side of the river at Wickham's, about 2 miles
56 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

distant. Here the Potoinac was not seen, but the cobbles were found 

in excavating greensaud marl. On penetrating the marl they were 

found at its 'base. On top of the cobble bed there is a stratum, 4 feet 

thick, of a tine, argillaceous sand, which in the weathered state, in 

which alone it was seen, has a yellowish-gray color. Above this is a 

stratum, 2 feet thick and much weathered, of yellowish, sandy clay, 

packed full of Eocene shells. Above this we have 4 feet of sandy clay 

like that containing the shells, but which is nearly free from them. 

These three layers above the cobble bed belong to the greensand marl. 

The yellowish color comes from weathering, for the iron present is oxidized. Near by, a pit was at one time opened in excavating the marl, 

and in this the material has the usual greenish and grayish colors.

* EXPOSURES AT HANOVER JUNCTION.

Hanover Junction is the junction of the Chesapeake and Ohio and the 

Richmond and Fredericksburg railroads. It is 2 miles south of the 

bridge by which the last-named railroad crosses the North Anna Eiver. 

The Lafayette formation constitutes the surface material of the region 

around the junction, but the cuts on the Richmond, Fredericksburg and 

Potomac Railroad between the junction and the river expose this formation as well as the Potomac and the older Mesozoic. As we have here 

the only known visible contact of the Potomac with the older Mesozoic, 

these exposures are especially important. For several hundred yards 

from the junction the Lafayette alone is visible. This is mostly a sandy 

clay, mottled yellow or brownish and light gray. Occasionally seams 

of small gravel occur in it. It is sometimes reddish or brownish, marbled and seamed with white. In the base of this, and resting on the 

surface of the Potomac, we find sometimes large pre-Cambrian stones 

of such material as gneiss, mica-schist, etc. They are 2 feet and more 

in diameter. These are angular or subangular, With them occur often

large cobbles of quartz. This coarse material seems to belong to the 

Lafayette rather than to the Potomac. About three-fourths of a mile 

from the junction there is an exposure of the Potomac showing a thickness of 6 or 8 feet. This is of the usual kind, it is a coarse gray grit, 

without regular bedding, composed of lenticular, interlocking layers. 

The top contains cobbles of quartz and Cambrian quartzito, some of 

them as much as 8 or 10 inches in diameter. The base of this formation is not visible. A little more than a mile from the junction other 

exposures of Potomac occur in the cuts, showing at most only 4 or 5 

feet of the material. They are found at intervals over the space of several hundred yards. A sandy gray loam, which looks like Pleistocene, 

covers the formation here. The material of the Potomac in these exposures is similar to that shown in-the spot first described. A large 

amount of cobbles and coarse gravel of the usual kinds occurs in it, 

especially toward the top. In several places toward the bottom of the 

material balls and masses of dark-chocolate clay occur embedded in it.
FONTAINE.] EXPOSURES AT HANOVER JUNCTION, 57

Some of them are over 2 feet in diameter and contain unrecognizable 

fragments of plants. At the base of this material are exposed about 

2 feet of a crumbling, reddish-brown sandstone of the older Mesozoic 

or Ehetic. This older Mesozoic is fairly well stratified and has a uniform composition. It contrasts strongly with the incoherent gray sand 

and cobbles of the Potoinac that rest upon it. The top of the older 

Mesozoic where the Potoinac rests upon it is worn extensively, showing 

that a considerable interval in time separates the two formations.

Still farther on, and near the North Anna, this older Mesozoic shows 

considerable outcrops. The brown sandstone is here thinly laminated. 

On the opposite side of the river there are extensive exposures of the 

older formation in the cuts along the railroad, and in a number of 

places there are many large, partly rounded pre-Cambrian stones in 

it, but no cobbles of Cambrian quartzite. This formation is here totally 

different from the Potomac.

The Potoinac is not seen in place beyond the North Anna Eiver until 

we reach the Massaponax Eiver, but Cambrian quartzite and quartz 

cobbles may be found on .the surface along the Telegraph road to a 

short distance beyond Mount Carmel Church, as far as 4 miles north 

of the river. These cobbles may be seen over the pre-Carnbrian to the 

distance of at least 7 miles west of Hanover Junction. They probably 

came originally from the Potomac, and their presence on the surface 

indicates that the Potomac once covered the surface here. They may 

be traced over the ground, often in great numbers, covering the eastern margin of the older Mesozoic west of Ashland and thence southward over the Richmond coal field as far as the Appomattox Eiver, in 

the vicinity of the Clover Hill coal pits, the southern extremity of the 

older Mesozoic. They may extend still farther south in the same line, 

but examinations have not been made in this direction. They are 

found, also, over the granite and gneiss between the older Mesozoic of 

the coal field and the Potomac and Tertiary terraues. They abound in 

the vicinity of the James Eiver. From the North Anna to the Massaponax, in the belt where Potomac outcrops might be expected, a thick 

covering of Lafayette conceals everything except in a few places on 

the streams where the Eocene greensand is shown.

EXPOSURE AT MASSAPONAX RIVER.

The road from Port Eoyal to Fredericksburg crosses the Massaponax 

near its mouth, 5 miles south' of Fredericksburg. At the crossing a 

much-weathered outcrop is shown, about 8 feet thick. The base is not 

visible, and hence its junction with the Potomac can not be seen at this 

point. The material is an argillaceous sand of a dull brownish-gray 

color. It has a number of imprints of shells, but no shells were seen. 

Many small pebbles under half an inch in diameter occur in it, dispersed 

in the sand. This is an unusual feature, seen nowhere else. About 

300 yards north of this spot and on the same road, on the opposite side
58 THE POTOMAC FOKMATION IN VIRGINIA. [BULL.HS.

of the river, there is a small exposure of Potomac sandstone occurring 

in the bed of a run that crosses the road.

It may be stated in this connection that the Potomac sandy matter 

in the southern portion of the Fredericksburg area to and beyond 

Stafford Court-House is in places rather firmly consolidated, and may 

be spoken of as a sandstone. Indeed, in a number of places this 

material was formerly largely used for building purposes. The exposure mentioned above is on a higher level than the top of the Eocene 

near the river, and this illustrates the uneven character of the surface 

of the Potomac when the Tertiary was laid down upon it. Only about 

2 feet of the sandstone can be seen. It is a coarse gray sandstone, 

similar to the material so common on the James and the Appomattox 

rivers. The top is much worn, and it is evident that a good deal of the 

formation has been removed by erosion. It is streaked and blotched in 

an irregular manner with yellowish-brown colors. No regular strati fication can be seen, but the matter, shows current-bedding. Some small 

balls of gray clay occur embedded in it, the largest being 4 or 5 inches 

in diameter. There is but little kaolin in the rock here, most of the 

material being a rather coarse sand. It is proper to state that while, 

for the sake of brevity, we may call the light-gray and white clay of 

the Potomac kaolin, it is not in fact a true kaolin, but rather a lightgray to white potter's clay.

EXPOSURES ON THE RAPPAHANNOCK BELOW FREDERICKSBURG.

Exposures occur on the Rappahaimock River extending from a point 

about 2 miles below the town down to the mouth of the Massaponax, 

(> miles below. The banks show often from 10 to 15 feet of the 

Potomac, which is generally a coarse sandstone of the usual character 

and full of small stones. It has on its surface sandy matter and cobbles belonging to the Pleistocene. Near the mouth of the Massaponax 

the sandstone was formerly quarried. Whefe'it is last seen it rises 10 

to 12 feet above the water in the form of a coarse, pebbly grit. Two 

or three hundred yards below this point the Eocene greensaud marl 

suddenly appears in the right bank of the river, showing a thickness 

of about 15 feet. This indicates that the marl rests immediately on 

the Potomac sandstone. As we descend the river below the last exposure of the Potomac the surface cobbles become rapidly fewer and 

smaller.

The material of the Potomac sandstone exposed in the river banks 

is mostly quartz in coarse grains. Little kaolin is present. No" clay 

balls occur, and very little clay in seams was visible. Small quartz 

pebbles up to the size of 3 inches are numerous, either dispersed 

through the rock or aggregated in irregular nests ancl pockets. No 

dip and strike can be made out, for the sandstone has the usual structure of interlocking, irregular, lenticular layers produced by currentbedding.
WTATNE.) EXPOSURES ABOUT FREbERlCKSfcURG. 59 

EXPOSURES AT FBEDEmCKSBURG.

Exposures at and near Fredericksburg are numerous and quite variable in character. The changes are so frequent and take place at such 

short intervals that each exposure would require a different description. The banks of the ri\7 er extending from the lower end of the 

town to Falrnouth, 1 mile above it, are composed of the Potomac formation, a maximum thickness of about 30 feet being shown. On the north 

side, east of the .Richmond, Fredericksburg and Potomac Railroad, the 

formation is usually so much concealed by the de"bris eroded from 

itself and by the Tertiary'that exposures are rare and poor.

West of- Frederick burg, on Hazel Run and on the line of the narrowgauge railroad to Orange Court-House, good outcrops and exposures 

may be found extending as far as two miles from the town. The preCainbrian terraue here borders the formation on the west. The rock 

next to the Potomac is a heavy-bedded granitoid gneiss, which by

its decomposition may have furnished much of the material for the 

Potomac, but apparently not all of it.

In the vicinity of Fredericksburg, south of the river, the Potomac 

is largely covered by the Lafayette, Pleistocene, and other formations. 

These formations are here com posed of very fine, loamy sands and clays 

that occur in fairly well stratified beds, some of which are well laminated. They are much variegated in color, often mottled with deep tints 

of red, yellow, and brown, the colors being disposed very irregularly 

in masses of gray. The Lafayette shows no fossils. The Potomac here, 

as farther south, is composed almost entirely of sand and sandstone. 

This is a mixture of grains of sand and of kaolin. The proportion of 

these two components varies much, as does the fineness of the material. 

Usually the sand predominates. This is the case in the strata that are 

consolidated enough to be called sandstone. Where there is a large 

proportion of the white clay or kaolin the material is incoherent and 

fine. As a rule, little or no mica is present, and this is surprising, 

since the pre-Cambrian rocks to the west, from which much of the 

material probably was derived, are rich in mica. Small quartz pebbles are very common scattered through the sandstones. Even those 

selected for building purposes often contain pebbles up to 1 or 2 inches 

in diameter. These pebbles show no' tendency to aggregation in beds 

and layers. Dispersed stones, however, as large as 5 or G inches in 

diameter may sometimes be seen. Sometimes much larger stones are 

found. They are well rounded and are composed of various materials. 

In the vicinity of Fredericksburg it is a significant fact that there 

are no Cambrian quartzite cobbles among these stones. These larger 

cobbles occur in nests and pockets in the sandstone or sand. They are 

composed of quartz, of pre-Cambrian rock, and sometimes of Potomac 

sandstone. They are often accompanied by clay balls and seem to 

mark erosion horizons that is, periods when, in the formation of the
60 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145:

Potomac, not only deposition ceased, but some of the deposited material 

was torn up. Such horizons may occur anywhere in the formation, but 

they are most numerous toward the top. Very commonly the erosion 

horizons are marked by redeposited clay alone. The layer of sandy 

shale that yielded so many fossils at Fredericksburg rests on one of 

these cobble beds, and at several other localities plant-bearing shale 

may be seen resting on such beds of cobbles.

The most remarkable material found in the Potomac sandstone is 

the uptorn clay derived from some previously deposited stratum. This 

is redeposited in the sandstone in two ways. It may occur in the form 

of single rounded masses that range in diameter from a small fraction 

of an inch to 5 or 6 feet. The largest masses usually occur singly and 

remote from one another; the smaller ones are often found intimately 

mixed with the sand and forming a large proportion of the rock. In 

the other mode of occurrence the clay in masses, small flakes, and 

fine particles is aggregated into nests and pockets in the sandstone, 

and being cemented together forms masses of more or less pure clay, 

surrounded by the ordinary forms of the sandstone. A mere inspection Often does not suffice to distinguish such redeposited clays, as 

we may call them, from the original clay beds. As, however, such 

deposits often contain plant remains, I have frequently had occasion 

to explore them by excavating the material. On digging into one of 

these masses its nature is soon made known. The clay is often mixed 

confusedly with sand, the latter having been caught up by it in the 

process of deposition. It possesses no lamination and no cleavage, 

but is crushed, contorted, and full of slickeusides, the component particles being agglutinated together in all possible attitudes. The plants 

are generally very fragmentary and stand in all. positions in the mass, 

so that usually it is impossible to get out good specimens. Sometimes, 

however, a large fragment of clay retaining' the cleavage and the character of the original bed may be included, and this may be surrounded 

by a chaotic mixture that gives no good .impressions of plants, while 

in the unchanged mass good fossils may be found. Such features were 

for a long time very perplexing, but in time I learned that in these 

deposits no considerable amount of good fossils could be obtained, 

- The clays of this region show a most surprisingly close resemblance 

to those of the Petersburg area. We find the same two kinds containing fossils and in the same proportions. The fossiliferous clay is generally plastic, and when wet is either dark-gray to black or chocolate 

colored. The latter contains generally more and better preserved 

fossils than the former, which contains all' the lignite found in the 

Potomac. There is a peculiar irregularity shown in the occureuce of 

the more firmly consolidated and indurated varieties of the sandslone. 

Sandstones of firm texture do not occur in extended and distinct beds. 

They are found in the form of lenticular masses and pockets of varying 

size inclosed in more friable and kaolinic matters. The fossiliferous
FONTAINE.] EXPOSURES NEAR FREDERICKSBURG. 61

clays are generally uniform in color. There is, however, in the Fredericksburg area a large amount of a very different kind of clay. This, 

for the sake of distinction, we may call the mottled saudy clay. It is 

an extremely variable material in composition and in color. Usually 

it is a mixture of very fine sand and clay that passes on the one hand 

into pure, plastic clay, and on the other into the typical sandstone or 

sand. It is without lamination and on drying breaks up into angular 

blocks of an indurated material that looks like rnudstoue, and is 

exceedingly irregular in its occurrence. It may occur anywhere, but 

it does not form definite and continuous strata. It may replace a 

large portion of the normal sandstone in one spot and a few hundred 

feet away may disappear wholly, graduating into the typical sandstone. The fossiliferous clays rarely graduate into the normal rock 

which embeds them. The mottled sandy clay is sometimes uniformly 

gray (generally ash-gray) in color, and sometimes uniformly red (brick

red to purple-red), but most commonly the red and gray colors are 

mixed in irregular streaks and blotches, often many together, even in 

hand specimens. The arrangement of these variegated streaks may 

be compared to that seen in a piece of mottled castile soap.

The above-described materials are the most noteworthy components 

of the Potoniac formation in the area now under consideration. The 

sandstone graduating into incoherent sand is, as was said before, 

the typical material. It is difficult to give any general account of 

the structure and other physical characters of this rock. Most of thesandstone about Fredericksburg is light-colored to white, the color 

being lightest when the amount of kaolin is large. But the planes 

that bound the lenticular layers composing the mass and the curving 

current-bedding planes that cross these layers obliquely are often 

stained with yellowish or yellowish-brown colors that make them very 

distinct. Sometimes irregular patches and seams of the same color are 

found in the mass of the rock. Owing to the varying nature of the 

.interlocking layers, the irregular erosion of the surface, and the general 

lack of division into separate and distinct strata, no dip or strike can 

be made out, although, as before stated, there is a general depression 

of the formation to the southeast. The sandy material seems to be 

usually coarser and more firmly consolidated toward the base than 

toward the summit of the formation, while in the upper portions there 

is more of the diffused kaolin and the fossiliferous clay in layers.

Perhaps the best way to give an idea of the character of the formation will be to describe the material shown along a. line run from east 

to west at right angles to the length of the belt of Potomac deposits, 

beginning in the railroad cut by means of which the Richmond, Fredericksburg and Potomac Bailroad makes its approach to the river on 

the northeast side, continuing on the opposite side of the river over the 

plant locality, and passing west to the margin of the pre-Cambrian 

along the line of the narrow-gauge railroad.
62 THE POTOMAC FORMATION IN "VIRGINIA. [BULL. 145.

Opposite the town of Fredericksburg the lowest material seen is 

an incoherent, light-gray sand with a large amount of kaolin. This 

extends down to the level of the water, showing a thickness, of over 30 

feet. This material, in outcrop, has very slight coherence, for it is 

little more than a loose sand. It is not uniformly white and homogeneous in composition, but contains undulating patches of yellowish, 

brown marked out in the form of lenticular layers, which occur irregularly in the gray mass, as is shown in fig. 2, a a being the colored 

patches. The sand contains pockets and lenticular layers of cobblestone which resemble in position and shape the patches of color. The 

stones have a diameter of 3 or 4 inches or less. Bounded stones are 

also scattered irregularly in the mass of the sand. On the top of the 

sand rests a deposit which is commonly present in the vicinityof the 

river and which contains some material brought from a considerable 

distance. This layer is here about 2 feet thick, but in other places it is 

much thicker. It follows the undulating top of the sand, which seems 

to have been considerably eroded before this layer was deposited. . It 

is a confused mixture of saudy matter with quartz cobbles and masses 

of pre'Cambrian rock. Some of the pre-Oarnbrian masses are 4 feet in 

diameter, the largest of them being composed of heavy-bedded gneiss.

Near the steamboat wharf in Frederick sburg and at 

the boathouse of the 

Fredericksburg 

Boating Club, there 

was an important exposure of the Potomac material which

. 2. Lenticular patches of color at Fredericksburg.

yielded many fossil plants. This outcrop has been totally changed 

in the excavations made to obtain the plants. When this exposure 

was first seen it had the form of a projecting ledge of argillaceous, 

flaggy, sandstone, 4 or 5 feet thick, resting on a bed of cobblestones and 

covered by the Pleistocene drift. From the cobblestone bed to the level 

of the water there is a thickness of about 15 feet of material that is 

here very imperfectly exposed, but whose nature is disclosed about 

50 yards farther up the river both by the digging of a-well and by 

exposures in the banks of the river. It is the mottled sandy clay 

with many brick-red markings. Although the well was dug only 50 

yards distant from the plant locality and passed through the horizon 

yielding the fossils near by, none were found in it and it may be stated 

that at no. other spot were plant fossils found on the same level, at 

least in the immediate vicinity of the locality now under consideration. 

The material that afforded the plants was a very fine-grained, argillaceous flagstone. This has been almost wholly removed in the search 

made for fossils and in excavation by the city authorities to procure 

road stone. The deposit was found to be a lenticular cake of matter
FONTAINE.] EXPOSURES NEAR FREDERICKSBURG. 63

that thinned out on all sides, the underlying cobble beet rising by 

thickening and cutting it out. 'The entire layer had a length of about 

20 feet and a width of about 15 feet. The upper portion of the flags 

for the thickness of about 2 feet was full of comminuted vegetable 

matter, showing rarely identifiable fossils. The greatest variety of 

plants and the best-preserved ones occurred near the top of the cobble 

bed, but unfortunately the plane of contact of the flags aod cobble bed 

served in places as a waterway through which a slow seepage took 

place, and by this means many fine fossils were destroyed. The plants 

diminished much in numbers and in perfection of preservation as the 

layer was followed up, and they finally disappeared. It can not be 

donb.ted that the outcrop when first found was much richer in fossils. 

It was discovered by Professer Uhler, and it had been partly removed 

before I saw it. The fossiliferous stratum became more purely argillaceous toward the top of the cobble bed, and was here, in fact, a sort 

of claystone. It had the composition and induration of shale, but was 

not evenly fissile. On the contrary, it Avas lumpy and very irregular 

in its splitting, and this made it very difficult to get out good impressions. This argillaceous matter showed no tendency to graduate into 

the cobble bed. On raising the bottom layers of it the stones of the 

cobble bed would often be raised at the same time, sticking to the bottom of the clay as if at one time that material had been dumped upon 

the stones.

When last examined the locality presented an open cut about 30 feet 

long and 20 feet wide, showing in its walls mostly the material of the 

cobble bed. The cobblestone bed, as shown in the cut, is divided into 

several layers by lenticular sheets of argillaceous gray sand. These 

separating layers do not differ essentially from the normal Potomac 

sand. They thin out rapidly and give place to cobbles. In the portion of the rear wall of the cut at the right, as one faces west, there 

are none of these layers, and the cobbles form a stratum showing a 

thickness of 5 feet, with the base not exposed. A careful examination 

of the cobbles was made in order to determine, if possible, the identity 

of their parent rock. There is no Cambrian material present. It may 

be stated here that none of the streams in this part of the State, the 

Rappahaunock inclndedj rise west of the Blue Kidge so as to touch 

the Cambrian as now exposed. Nearly all the material is pre-Cambrian, 

but, contrary to expectation, almost none of it came from the belt of 

massive gneiss that forms the western boundary of the Potomac area 

in this region. This gneiss and the associated micaceous rocks form a 

belt at least 10 miles wide. West of this gneiss there is a widejband 

of rocks less fully crystalline, that embrace such rocks as mica-slate, 

argillite, hydromica-schist, chlorite-schist, etc. It is from this last belt 

that most of the pre-Cambrian material of the cobble bed apparently 

comes. Some of the cobbles are so much decomposed that it is not 

easy to determine the character of their parent rocks, for they are 

in the condition of clay which is of a gray color. Quartz cobbles are
64 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

the most numerous. The rock seems to be similar to the quartz that 

is so common in veins, pockets, and masses in the pre-Cambrian argillites and mica-slates. The largest quartz cobble seen had the dimensions 6 by 8 inches, but most of them were under 5 inches in diameter. 

The next most abundant cobbles are composed of a schistose, argillaceous material that is now in the condition of a gray clay. In some 

cases the original rock seems to have been a siliceous, schistose argillite. Some few cases occur in which the original appears to have been 

a jaspery schist, such as is found occasionally in the chlorite-schist of 

the Blue Ridge region. This may occur also in the second belt of preCambrian rocks described above. The cobbles of siliceous schist are 

usually large, the largest one seen having the dimensions 7 by 9 inches; 

but the cobbles of the softer and more argillaceous schist are generally 

4 to 5 inches in diameter.

Cobbles of varieties of fine-grained, siliceous gneiss are common. 

This rock is similar to the gneiss that forms bands in the imperfectly 

crystalline, argillaceous schists, and is quite different from the granitoid gneiss forming the western boundary of the Potomac. The largest 

stones of this kind of rock are 6 or 7 inches in diameter. One cobble 

only was seen that probably came from the granitoid gneiss. This was 

composed of.a reddish, coarse-grained, granitoid gneiss, such as occurs 

in some places west of the Fredericksburg Potomac. This measured 

4£ by 5 by 7 inches. A number of rounded masses of gray sandstone 

occur with the pre-Cambrian cobbles. This sandstone seems to be a 

variety of the Potomac, sandstone that is more compact and indurated 

than usual. The largest of these measured 10 by 12 inches. Bounded 

lumps of the mottled sandy clay occur also. These range in diameter 

from 4 to 12 inches. The cobblestone bed now under consideration is 

remarkable for the great proportion of large stones that it contains. 

Most of the cobbles range in diameter from 3 to 5 inches, but many are 

6 inches in diameter. In some portions of the bed the large stones are 

so thick as to touch one another and to appear piled up.

The highest exposed strata at the plant locality stand 10 to 12 feet 

below the level of the plain on which Fredericksburg is built.

Westward, in continuation of the line of section, no exposures are 

seen for about 300 yards. The first are met with in the cut on the narrow-gauge railroad near the depot of that road. The interval forms a 

portion of the plain on which the town is built. The greater portion 

of the depth of the cut (the maximum is over 20 feet) is made through 

a loose sand of brownish color which belongs to the Pleistocene and 

appears to be the equivalent of the brick loam. In the base of the cut 

4 or £ feet of a tenacious clay is exposed, which is generally concealed 

by the loose sand that comes down from above. It was exposed for a 

few yards only, and this is not sufficient to enable one to speak of its 

character in general. At the place seen the clay is .plastic, unbedded, 

bluish-gray in color, and mottled with yellowish-brown. Possibly this 

is a variety of the mottled sandy clay, here less sandy than usual. In
FONTAINE.] EXPOSURES NEAR FREDERICKSBURG. 65

its upper portion it contains a number of cobbles which look as if they 

had been dropped singly on the surface of the clay. Some of them are 

4 inches in diameter. Between this clay and the Pleistocene sand 2 

feet of distinctly laminated sandy clay occurs. This is, in the main, 

yellowish in color, but shows a mottling with gray. These two layers 

are of uncertain age.

Upon the layers last described there rests a considerable thickness 

of the Pleistocene sand already mentioned. This sand is noteworthy 

for showing in its base some very large masses of far-traveled preCambrian rock, which it is difficult to account for except by supposing ice transport.' They are composed of gneiss, mica-schist, chloriteschist, etc. These stones are quite commonly found in the vicinity of 

the Bappahannock, at the base of the finer Pleistocene deposits, as, 

for example, over the fossiliferous strata near the wharf. They are 

also visible in the railroad cut northeast of the river, described as the 

beginning of the line of section. The pre-Cambrian blocks in the cut 

are generally but little waterworn. The largest of them, composed of 

mica-schist and gneiss, are 5 to 6 feet in diameter.

It would seem that in post-Potomac times the Potomac formation 

had been cut away to a considerable depth upon the plain on which 

the town of Fredericksburg is built, for the formation makes up the 

greater portion of the mass of the hills that overlook the plain. Among 

these is Marye's Hill.

The next exposures are found on Hazel Bun at the foot of Marye's 

Hill and near the Federal cemetery. A mill is situated on this run 

where the ^narrow-gauge railroad crosses it. Just back of the mill 

there is a considerable bluff composed of nearly white Potomac sand. 

This forms that portion of Marye's Hill which overlooks the run. The 

bluff shows that the Potomac sand extends nearly to the top of the 

hill and that the formation stands here at a much higher level than in 

the plain just described.

The base of the bluff for 20 or 25 feet is hidden by the abundant 

talus thrown down from its upper portion. A small quarry, however, 

was opened near by on a level with the lower portion of the concealed 

part. The material shown in the quarry is probably of the same nature 

as that composing the hidden part of the bluff. The principal rock 

seen in the quarry is a very irregular and coarse pebbly sandstone of 

the normal kind. This contains irregular layers and pockets of indurated clay. This clay is somewhat variable. Some of it is rather firm, 

and this is bluish-gray in color. Portions, however, are crumbling and 

light-gray in color. A part that is the continuation of the same mass is 

quite different, being well laminated and buff colored, but the greater 

portion is without lamination. In the coarse sandstone that carries 

this clay large cobblestones of quartz and balls of clay are embedded.

The cliff shows exposed in its upper portion about 20 feet of material with a nearly vertical face. This is composed of an incoherent 

Bull. 145 5
66 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

imbedded mixture of grains of sand and kaolin, the proportion of the 

latter being large, giving the material a white color. Numerous quartz 

pebbles of small size are scattered irregularly through the mass, but in 

some places they show a tendency to aggregation into beds. These 

pebbles are mostly under 2 inches in diameter. On the top of the 

sandstone is a bed of the Pleistocene cobbles with some pre-Oambrian 

stones.

Hazel Bun flows about 50 yards in front of this bluff. On the opposite side of the run the exposures show a totally different material, 

although they are not 100 yards distant from the bluff and quarry. 

Here tbe mottled sandy clay extends from the lowest point visible on 

the run nearly up to the level of the railroad, a height of at least 25 

feet. .This would indicate that at this spot the material corresponding 

to the lower part of the bluff is all mottled sandy clay. The top of 

this mass is not well exposed, but there is an exposure near the railroad which shows that the mottled sandy clay passes above into the 

ordinary pebbly sand of the Potomac. The top as seen here has a 

number of cobblestones in it, which were accumulated by the wear of 

its surface. From this point on a hillside along the Telegraph road 

]20 feet of post-Potomac formations are exposed. The Potomac was 

greatly worn before this latter material was laid down upon it. The 

cobblestones found in it occur in lenticular pockets, as is usual in such 

deposits of Potomac age. These are here from 1 foot to 2 feet thick. 

Most of the stones are of quartz, but some are composed of a highly 

feldspathic gneiss and some of a bluish, cherty argillite. Most of the 

stones a^e large and many of them well rounded. One subaugular 

mass of the cherty rock had the dimensions of 6 by 8 inches. The 

gneiss forming the western boundary of the Potomac furnishes some of 

the stones. The Potomac sandstone also forms some of the rounded 

masses. One of these showed the dimensions of 10 by 12 inches.

To indicate more clearly the nature of the deposits lying above the 

Potomac. formation in this region I will give the section of them 

obtained in ascending from the top of the Potomac along the Telegraph 

road from Hazel Run. It must be taken as only an approximate one:

Section near Hazel Eun.

Feet. 

No. 1. Ground partly concealed. The space is in part occupied

by a sandy incoherent and fine clay, which is in color

yellowish-gray mottled with light-gray............... 20

No. 2. Incoherent fine, reddish-hrown sandy clay, mottled with

light gray ........................................... 20

No. 3. Yellowish-brown sandy clay, mottled and seamed with

gray and containing blotches of pint ................ 5

No. 4. Gray sandy clay, loose and friable, seamed and blotched

with yellowish-brown................................ 15

No. 5. Yellowish to brownish-yellow, massive, argillaceous sand,

with some nests of cobbles........................... 40

No. 6. Thinly laminated mottled clay, gray with seams and

blotches of deep rich red arid pink ................... 20

Total thickness......................................... 120
FONTAINE.] EXPOSURES NEAR FREDERICKSBURG. 67

On the top of this rests the usual surface cobbles and d6bris from 

the Pleistocene which are here 8 to 10 feet thick.

The formations here contrast strongly with the Potomac in many 

points. The deposits are divided into distinct beds composed often of 

well-sorted material. They have none of the clay balls, large cobbles, 

and other characteristic features of the Potomac, and are more highly 

colored.

No. 6 in the above section is strikingly like some of the beds exposed 

in the higher lands about Petersburg. It is in places a plastic clay, 

marbled and seamed with very rich and deep shades of red, pink, and 

purple. When plastic it is very thinly laminated, so that in drying it 

splits into ilakes no thicker than cardboard.

In the hill about 250 yards west of the mill, and at about the same 

distance north of the exposure just described, a small pit was at one 

time opened to obtain stone for the use of the Federal cemetery. This 

shows Potomac material at a horizon somewhat higher than the top of 

the bluff at the mill. The rock exposed is very heterogeneous in composition and changes rapidly. The principal substance is a very coarse 

grit, which in some 

spots becomes very firm 

and hard. These portions seem to have been 

impregnated with silica 

deposited from solution, 

and contain some bits 

of silicified wood. Numerous balls of finegrained gray clay as 

well as quartz pebbles Fio. 3. Elliptical streaks of color near Frederickaburg.

are embedded in the grit. Many of the clay balls are only a small 

fraction of an inch in diameter. These are intimately mixed with the 

grit, and in places are so numerous as to comprise most of the rock. 

Some of them, however, are as much as 12 inches in diameter. Thin 

partings and seams of pale-reddish clay occur in the grit, having a 

lenticular shape. This clay contains some fragments of plants similar 

to those found at Fredericksburg, but they are very imperfectly preserved. A thin layer of the same clay occurs in the top of the exposed 

material. The stones of quartz are mostly small, but one 5 inches in 

diameter was seen.

Continuing the line of section west along the narrow-gauge railroad 

we find for about 400 yards no exposures. After this interval there is 

a cut about 30 feet deep on the road, made wholly in the Potomac sand. 

The bottom of this cut is 10 feet above the exposure last noted on the 

Telegraph road, at the base of the described section. The greater portion of the mass exposed in the cut is a rather coarse, incoherent sand. 

The predominant color is light-gray to nearly white, but tins is seamed 

and splotched in many places with yellowish-brown. This latter color
68 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

FIG. 4. Irregular stratification near Fredericksburg.

is often disposed in films and streaks that seem to mark the planes of 

deposition. The stratification is much confused. The streaks of color 

are sometimes horizontal, but run in wavy lines. They sometimes mark 

out elliptical layers, the individual films showing minute undulations, 

as indicated in fig. 3.

Balls of gray clay, some of them 12 inches in diameter, are scattered 

sparingly in the mass and are sharply distinct from the sandy material 

in which they are embedded. This clay is plastic. The sandy matter

varies much; most of 

it is a coarse, loose 

sand with some diffused clay. These 

parts occur in lenticular layers, or short 

confused beds that 

interlock irregularly. 

But, while the more 

central portions of 

these layers may be 

coarse and sandy, the adjacent surfaces of contiguous layers are 

often formed of fine, argillaceous material that sometimes approaches 

a clay in nature. Some of these partings are as much as 4 to 6 inches 

in thickness. They appear and disappear in the most irregular manner. Some of them that have a color darkef than the usual tint contain finely comminuted vegetable matter, many of the larger bits of 

which are fragments of coniferous twigs.

The larger division planes above described incline to the southwest. 

They are shown in 

fig. 4. The lenticular beds of sandy 

matter often show

very distinct currentbedding, as may be 

seen in fig. 5. Small 

stones composed of 

quartz, up to the 

diameter of 2 inches, 

occur sparingly. 

The usual veneering 

of cobbles comes on the top of the Potomac here and at all the expo 

sures along the railroad.

EXPOSURE AT ALUM SPRING.

Alum Spring is situated about 200 yards west of the cut described 

above. The spring is found on the north side of the railroad and only a 

few yards from it. The water of the spring trickles from the under side 

of an overhanging mass of Potomac sandstone and gets its alum and

FIG. 5. Current-bedding near Frederickaburg.
PONTAINE.] EXPOSURE AT ALUM SPRING. 69

copperas from decomposing pyrite in the rock, which yields sulphuric 

acid. There is a deep cut on the railroad at this point made through 

material on the same horizon as that overhanging the spring. The cut 

is about 40 feet deep, and makes a good exposure of the Potomac. The 

bottom of this cut is 10 feet higher than the lowest exposure made in 

the cut last described. Thirty feet of its depth is composed of Potoinac 

material. The top of the Potomac here has some large, rounded stones, 

but nearly all the material from the top to near the bottom is a dark 

gray, rather sandy, claystone, much like the fossiliferous material near 

the steamboat wharf at Fredericksburg. This material on weathering 

breaks up into flakes and flat fragments. Its dark color is due to 

comminuted and diffused vegetable matter. Toward the bottom of 

the cut the material becomes in places coarser and more indurated, for 

we find irregular masses, lumps, and nests of the normal sandstone 

enveloped by the more argillaceous material, the harder portions taking

the form of interlocking lenticular layers.

Some large pieces of lignite occur in the coarser portions of the 

layers, being apparently formed of the limbs of coniferous trees. This 

lignite, like nearly all that found everywhere in the Potomac, has the 

conchoidal fracture of jet, but shows often longitudinally the structure 

of the original wood. It seems here to have a large amount of pyrite 

associated with it, so that it rapidly disintegrates on exposure. A great 

deal of it has been thus removed from this rock and from that exposed 

at the spring, for more or less cylindrical holes running into the sandstone are common. They are hollows left after the removal of the limb 

or twig. The sandstone that contains this lignite often shows that 

induration from deposited silica noted as seen in the quarry on the 

hillside near Hazel Euu. It seems to be connected with the presence 

of vegetable matter in the rock, as the parts immediately surrounding 

the lignite are those most commonly silicified.

The more argillaceous matter, making up most of the cut, shows a 

confused and complicated structure, which seems to be due to currentbedding. The apparent bedding planes, as in the other cut, present 

the outline of lenticular layers which interlock and run into one another in the most intricate manner. The divisional planes are not 

simple curves, but contain undulations on a small scale. Fig. 6 gives 

a representation of what may be seen in the upper part of the cut, a 

being the surface cobble bed; &, c, the Lafayette; e, the cobbles in the 

top of the Potomac, and d the current-bedded argillaceous portion of 

the Potomac. Only the chief points of the structure are given, for the 

details are too numerous and complicated to be represented fully in 

any figure.

The ledge of rock that overhangs the spring is composed in its basal 

portion of material somewhat lower than any shown in the cut, the top 

of the ledge being about on a level with the base of the cut. This 

ledge shows that the indurated coarse sand with lignite exposed in
70 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 14§.

the bottom of the cut is continued downward until it attains the thickness of at least 20 feet.' The rock overhanging the spring shows some 

noteworthy features. The lowest rock seen at the spring is a soft, gray 

claystone. It is not exposed anywhere except under the overhanging 

ledge, and it is owing to the ease with which this is eroded that the 

much harder rock which rests upon it is caused to project and overhang, for the layer of claystone wastes away rapidly and undermines 

the hard ledge. This layer is not fully disclosed. The rock resting 

upon it is a thick mass of coarse, pebbly sandstone, which has been so 

much undermined that the under surface of the hard mass stands from 

3 to 4 feet above the face of the claystone. The under surface then 

slopes down to meet the face of the claystone, so that for a considerable space it is fully exposed, and gives an excellent opportunity for 

the study of its composition in detail. The ledge of hard sandstone 

shows a long front of this kind at least 50 yards. The inclination of

FIG. 6. Current-bedding outlining lenses near Frederickaburg.

its lower face looks much like the dip of a stratum, but it is very 

doubtful whether this is the case. Probably the slope of the lower 

face is caused by the thickening of the mass of coarse matter. It must 

be borne in mind that in Potomac outcrops the surfaces of apparent 

strata are more often determined by the alternations of hard and soft 

lenticular layers, and the way in which they weather, than by the bedding planes. The slope of the lower face of this ledge, as nearly as 

could be made out, is 10° to 12° ENE.

The lower portion of the ledge is a coarse, indurated sandstone, very 

heterogeneous in composition and in texture. The induration seems 

to be caused by the deposition of silica from solution. In certain 

layers or bands there are many large scattered pebbles. The largest 

of these pebbles are of quartz and are 4 to 5 inches in diameter. In 

some places small balls of gray clay are associated with them. The
FONTAINE.] EXPOSURES NEAR ALUM SPRING. 71

rock shows on its under face many imprints and molds of the limbs 

of trees. In most cases the lignite has weathered out. Some of the 

imprints are 5 or 6 feet long and as many inches wide. A considerable 

amount of sulphate of alumina and some sulphate of iron form on the 

under surface of this rock.

The thickness of the hard, pebbly, lower portion of the ledge is 

about 10 feet, and that of the entire ledge about 20 feet. For the upper 

10 feet it is similar to the material exposed in the lower part of the 

railroad cut near by, but in its lower part it is more argillaceous and 

not so pebbly.

West of Alum Spring for 400 yards there are no exposures of the 

Potomac. Then at the second milepost from Fredericksburg we find 

a slight cut made mainly in post-Potomac formations. They are here 

of the same general character as on the Telegraph road, but show 

material only about 15 feet in thickness. It is capped by a bed of large 

cobblestones, varying from 4 to 7 feet in thickness, which has a parting

of brownish-gray sand. This latter is very variable in thickness, but 

is nowhere more than 3 feet thick. The stones in the cobble bed are 

all quartz and are well rounded. This cobble bed seems to have once 

extended over a considerable belt of the pre-Cambriau to the west, for 

we find a large number of cobbles strewn over its surface along the 

western margin of the Potomac.

The next exposure of rock in place occurs about 200 yards west of 

this last cut. Here there is a cut through greatly decomposed micaschist and highly micaceous gneiss, the latter forming irregular bands 

in the former. On the eroded top of this pre-Cambrian rock a layer of 

cobblestones occurs, packed in yellowish sand. Beneath this is a thin 

stratum of yellowish sand that is filmed and streaked with gray, sandy 

clay. The mica-schist soon passes on the west into heavy-bedded 

gneiss that has many of the features of granite. This is the material 

used at Fredericksburg for building purposes and is called granite.

The above-described features may be seen in the line of section 

chosen, but they are by no means always to be found at adjacent 

points on the north and south, even if taken close to the line followed. 

A few points may be selected to show these variations.

On the side of the river opposite Fredericksburg, a little below 

the locality yielding the plants, a deep ravine is cut by a run that 

enters the river at the ferry landing. As this is quite near the plant 

locality, the ravine was examined with the expectation that at least the 

flags carrying the fossils on the Fredericksburg^side would be found. 

The exposure, however, showed nothing of the kind. A thick mass of 

the heterogeneous grit, full of stones and showing no trace of regular 

bedding, formed the entire thickness of the matter exposed. This 

vertical interval included the horizon of the plant-bearing beds, no sign 

of which was found. G-oing up the bank of the river on the same (the 

Stafford) side exposures may be seen near the wagon bridge over the
72 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

river. Although only about 300 yards distant from the ferry, the Potomac is here quite different in character. The upper part of the matter 

exposed is largely composed of the mottled sandy clay. This is combined with the typical sand or grit and graduates into it, cutting it out 

in some places to the depth of 20 feet.

On the Fredericksburg side the changes are even more sudden, for at 

the end of the railroad bridge, about 50 yards above the plant locality, 

the entire thickness, exposed for 40 feet, is composed of mottled sandy 

clay. Here this has much less than the usual number of red blotches 

that characterize it. A large portion of the mass is dull-gray. It 

graduates into a mixture of fine sand and light-gray clay which has 

little indication of arrangement in beds or layers. There is in the river 

bank on each side of the stream a constant change from material of the 

sandy mottled clay type to that of the normal sand. All of the material 

above the town as far up as Falrnouth has a large admixture of lightcolored clay with the sand, which latter is usually fine-grained. Small 

stones are common. There is no regular bedding, but the structure is 

massive or composed of iuterlockinglayers marked with current-bedding.

EXPOSURES BETWEEN FREDERICKSBURG AND FALMOUTH.

There are one or two points between Fredericksburg and Falmbuth 

that deserve a more detailed description.

About halfway between the wagon bridge leading from Fredericksburg into Stafford County and the village of Falmouth, which is in 

Stafford County, the bank of the river on the Stafford side is about 40 

feet high and very precipitous. It is composed wholly of Potomac 

material. This is mostly a fine sand, rather more colored with iron and 

more argillaceous than usual. A considerable portion of the mass has 

the character of the mottled sandy clay. This is irregularly mixed 

with gray sandy portions. The latter are blotched with yellow, yellowish-brown, and brick-red streaks. Small stones occur scattered in 

some parts. All the material is incoherent and falls readily to a loose 

sand. The usual structure of interlocking lenticular forms is found 

here. At the top a crust of coarse, grayish sandstone, blotched with 

brown, occurs. This is about 3 feet thick, and contains small scattered 

pebbles of quartz and small balls of reddish sandy clay. This last 

occurs on the road which interrupts the exposure. Surface cobbles of 

large size appear here that seem to have once occurred in materials 

resting on the sandstone above described.

The next stratum plainly exposed above is shown on the side ot 

the road opposite the river. This is a layer of fine-grained sandy 

clay 4 feet thick. In some places it has a rude lamination, forming 

an imperfect shale. Its color is dark-gray, and it contains many 

fragments of plants, nearly all unrecognizable.

Above this stratum in the hill come about 30 feet of yellowish-gray 

argillaceous sandstone and sand. This is mostly very incoherent and
FONTAINE.) EXPOSUEES NEAR FALMOUTH. 73

is often marked with brownish blotches. This passes above into 

grayish argillaceous sandstone (thickness not known), variegated 

with red splotches. Upon this a stiff red clay rests, which seems to be 

Pleistocene in age.

About 100 yards above this spot a small stream enters the river and 

forms a bluff that gives a good exposure of material corresponding in 

horizon to the upper portion of the river bank described above. This, 

as will be seen, is very different from that shown in the river bank, and 

illustrates well the rapid changes that the Potomac exhibits here. 

Beginning with the lowest portion exposed on the small stream, we find 

the following details. At the base there is a fine-grained, argillaceous, 

light-gray sandstone, uniform in color, composition, and texture. Of 

this a thickness of about 2 feet was seen, the base riot being exposed, 

for some 8 or 10 feet of the lower part of the bluff is covered by talus. 

This layer is quite distinctly separated from that next above. The second layer from the bottom is a well-characterized variety of the mottled 

sandy clay. It is about 5 feet thick, and is a firm, rather sandy, clay 

stone. The color is mottled gray and the stone is seamed with a brightred or brick-red color. It breaks up into angular prismatic blocks in 

the outcrop, and looks like sun-baked brick. This graduates above 

into the next kind of material, forming one bed with it. On the top of 

the last we find a soft and crumbling sandy clay, with gray and deep 

brick-red colors. This is about 6 feet thick. It is very prone to 

crumble, and falls to a loose gritty earth. At the top it has some 

yellowish and brownish colors, and here it graduates into a crumbling, 

soft, sandy shale, which is about 8 feet thick. This last shows a confused mixture of colors such as reddish-brown, yellowish, and bluishgray arranged in irregular spots and patches. The colors are not 

sharply distinct or defined. There is a good deal of diffused limonite 

in this portion.

In this and in all other cases of descriptions of outcrops of the Potomac, although for convenience of description the mass is divided into 

different layers or beds, it is not to be understood that there is any subdivision of the material into different beds separated by regular planes 

of stratification. Such a condition of things was nowhere found. The 

different parts of the outcrop shade into one another and pass very 

irregularly into different kinds of rock. The descriptions apply to 

those portions where the characters given predominate. In some cases 

the transition from one to another kind of matter is so gradual, or so 

irregular, that no subdivision can be attempted.

EXPOSURES NEAR FALMOUTH.

At Falmouth, the Eappahannock bends sharply west, and about 1 

mile above the village passes out of the Potomac terraue. The bed of 

the river at Fal mouth is composed of gneiss, over which the water runs 

in rapids. The Potomac rests directly on this gneiss, being partly
74 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

exposed in the river bank below a church which stands on the top of 

the bluff. Near the water level the Potomac is poorly exposed, but the 

material seems to be of the same nature as that composing the bank 

farther up, which is well disclosed.

The lowest Potomac strata seen in the bluff are the usual light-gray 

to white sandstones and sand. The material, in the main, is a mixture 

of coarse grains of quartz and kaolin. The amount of the latter in 

some places is large, but varies often and suddenly. We may find the 

rock in one spot nearly all coarse sand, while near by on the same 

horizon kaolin may predominate. This results from the arrangement 

of the material here, as usual, in interlocking, lenticular masses, the 

lenses varying often in composition and in texture. At this locality 

the thickness of the typical light-colored sand and sandstone is about 

40 feet. Some beds and nests of cobbles occur in it, but they are not 

common. The^top of the sandstone shows very distinct current-bedding and a lenticular structure. In this top there are nests and pockets 

of large cobblestones, some of them 18 inches in diameter.

On the top of the Potomac at this point is a layer of what seems to 

be Pleistocene cobbles, the top of the sandstone being much worn. 

The cobbles are quartz, and with them, are found large stones from the 

pre-Cambrian of such material as gneiss and mica-schist. The masses 

have a maximum diameter of 6 feet. A portion of the pre-Cambrian 

material of this Pleistocene stratum must come from points farther west 

than the gneiss belt, for here and elsewhere we sometimes find masses 

of chlorite-schist, felsite, and other rocks, which are the most abundant 

ones on the head waters of the Eappahannock. These, however, may 

occur nearer, for the belt of rocks of Huronian type that runs west of 

the gneiss belt contains material of this kind.

A little back from the river, at a higher level than the cobble bed 

last described, we find resting on the Potomac sand a mass of mottled 

sandy clay. The Pleistocene cobble bed appears to occupy a terrace 

cut in this clay. This material has the consistency of a clay stone. It 

is about 20 feet thick, quite sandy, and has a predominant yellowish 

color, which is mottled with gray and brick-red. This is the highest 

bed of the Potomac exposed at the church. A little back of the^ church, 

up the hill on the Telegraph road, the section may be carried to a considerably higher level in the formation. The exposures are not sufficient to permit a connected section to be given. For about 50 feet 

above the level of the mottled sandy clay last described, the material 

is imperfectly disclosed. It seems to be all a fine-grained, argillaceous 

sand, quite incoherent and with a predominant light-gray color. This, 

however, is mottled and seamed irregularly with yellowish, yellowishbrown, and reddish colors. The matter seems to be without regular 

bedding. About 50 feet above the mottled claystone there is a very 

good exposure of material resting on that last described. This latter 

is shown on the road to the thickness of about 20 feet. It is a very 

fine mixture of sand and kaolin, light-gray to white in color, and
EXPOSURES NEAR FALMOUtH. . 75

massive in structure. Above this there is about 20 feet of incoherent 

sandy matter, gray, with blotches and seams of yellow, brown, etc.

On the Fredericksburg side of the river thePotomac, in the vicinity 

of the river above Falmouth, has been cut away nearly or quite down 

to the gneiss. Hence exposures along the river banks are wanting 

until the western margin is reached. Here, in the base of the hills 

which bound the plain on which the town of Fredericksburg is built, 

we may find the formation again displayed.

The road leading up the river to the western margin of the Potomac 

leaves the river and ascends a hill. This point is about 1 mile above 

Falmouth. The hill is capped by the Lafayette formation, which here, 

in the upper portion, is noteworthy for the rich purple, yellow, and red 

tints that are irregularly distributed in the dominant gray color of the 

rock, making it highly variegated. This material is mostly a sandy 

clay of fine texture, ranging into fine sand, all incoherent and not 

distinctly bedded.

At the foot of the hill the Potomac shows a thin remnant of the 

normal sand resting on the gneiss and covered by the Lafayette formation. The top shows indications of great erosion prior to the deposition 

of the Lafayette. Three or four feet of a gray, coarse, crumbling sand 

is shown here. This contains in its top cobblestones of quartz and subangular masses of pre-Oambrian rock, mostly gneiss and mica-schist. 

Some of them are as much as 3 feet in diameter.

EXPOSURES ON POTOMAO RUN.

The next portion of the Fredericksburg area examined in detail is 

the vicinity of Potomac Run, for between that run and Fredericksburg 

exposures are very rare and imperfect. Potomac Eun crosses the 

Telegraph road 6 miles north of Fredericksburg, and flows across the 

Potomac terrane at right angles to its length that is, from west to east. 

In consequence of this, the course of this run will give a line of section 

across the formation similar in direction to that followed at Fredericksburg. On Potomac Eun exposures of sufficient extent to give details 

worth noting were found at four points. These are as follows: (1) On 

the eastern margin of the Potomac area, along the road leading from 

Fredericksburg to Brooke, where this road crosses Potomac Eun near 

the old colonial church known as Potomac Church; (2) at the crossing 

of the Eichmond, Fredericksburg and Potomac Eailroad over Potomac 

Eun; (3) on the Telegraph road where it crosses Potomac Eun; (4) at 

the western margin of the area near the same run on the Woodchopping road. I will give some account of each of these localities, beginning with the most eastern.

EXPOSURE AT POTOMAC CHURCH.

Up from the run, along the road near the site of the old Potomac 

Church, there is an imperfect exposure of all the material from the top 

of the Potomac formation to the Eocene. The locality is important,
76 THE POTOMAC FORMATION IN VIRGINIA. [BULL. U5.

because it shows the superposition of the Eocene on the Potomac where 

the latter appears last iu this quarter, and also because we see here for 

the first time, in proceeding from south to north in Yirginia, that the 

richly fossiliferous Eocene bed known as the Greensand does not rest 

directly on top of the Potomac, but is separated from it by a considerable thickness of nonfossiliferous Eocene material. Prof. William B. 

Eogers, in his Virginia reports, mentions this point as one where the 

Potomac disappears under the Tertiary, but he gives no description of 

it and no account of the Tertiary superposed upon it. The exposure 

is not sufficient to give a close connection between all the consecutive 

beds. The following is an approximate section:

Section on Potomac Run, near Potomac Church.

Feet.

1. Very fine, loose, slightly argillaceous, yellow sand, with

imprints of shells sparingly shown................. 20

2. Greenish-yellow and yellow, fine argillaceous sand, 

with many imprints of shells, especially in its lower 

portion=greensand marl .......................... 10

3. Brownish-yellow, sandy clay,spotted with -white..... 4

4. Dark-gray clay, mottled with brown................. 5

5. Gray, argillaceous sand, with small pebbles........... 3

6. Brownish, sandy clay, speckled and mottled with gray

and white.......................................... 3

7. A thick mass, only partially exposed, of fine argillaceous and variegated incoherent sand.............. 40

(To this point the Eocene extends.)

8. Potomac cobblestone bed............................. 3 to 4

9. Friable, reddish-brown sand.......................... 2|

10. Dark-gray, coarse, somewhat argillaceous sand, the

Potomac sand, seen................................ 2 to 3

The base of the last stratum was not seen. It graduates below into 

the normal, coarse, light-gray grit, which is only partially visible. This 

extends to the level of the creek, through an interval of about 10 feet.

No. 1 is most probably Eocene. It is a very fine, loose yellow sand, 

with much argillaceous matter. It has very few imprints of shells.

No. 2 is the greensaud marl. Its yellow color is due to oxidation, 

owing to the long exposure of the outcrop. The stratum overlying it 

(No. 1) probably owes its color to the same cause. The number of 

imprints of shells in No. 2 is very large, but no shells were visible, perhaps on account of the weathering of the material. The lowest 3 or 4 

feet alone show a remnant of the original green color. It should be 

noted that the Eocene material on this horizon does not always have, 

the character of a marl, for even when fresh it is sometimes mostly a 

mixture of sand and clay, with very little calcareous matter and very 

few; shells.

The beds from No. 3 to No. 7, both included, are not seen at any point 

south of this locality. They contain, as far as' seen, no trace of fossils, 

and by their character indicate a somewhat unquiet condition of the 

waters.
FONTAINE.] EXPOSURE AT POTOMAC CHURCH. 77

No. 3 is very argillaceous and almost plastic. It is mottled and 

specked with white. The white is due to particles of white sand, as 

fine as flour, which seem to be fragments of some bed of fine sand. 

These are embedded in the brownish clay. This peculiar inclosure of 

particles of fine sand was seen at other localities farther north where 

this horizon is exposed. The sand increases in amount toward the base 

of the bed and is there disposed in streaks and irregular pockets, the 

particles being here reduced to a powder and more intimately mixed 

with the clay.

No. 4 is a dark gray, rather sandy, clay, mottled with brown, the 

latter being disposed in small streaks and splotches. This material is 

stiff and much like some of the Potomac mottled sandy clay. It has 

a number of small shot-like pebbles scattered through it.

No. 5 is a whitish sandy matter which is incoherent and full of small 

pebbles.

No. 6 is like No, 3, but more sandy.

No. 7 is seen only in part, the upper portion being concealed. The 

middle and lower portions have a predominant ash-gray color, with 

yellow and brownish colors disposed throughout the bed in pockets aud 

irregular nests.

No. 8 is somewhat sharply separated from No. 7 and clearly forms 

the uppermost portion of the Potomae. The cobbles are disposed in 

lenticular pockets and nests^ as is so commonly the case at the top of 

the Potomac. They are of quartz, are well rounded, and range in size 

from 6 inches downward. Mingled with the cobbles are partly rounded 

masses of quartz, the largest of which are 12 inches in diameter. 

There are also some subangular masses of pre-Cainbrian rock ranging 

from 12 to 18 inches in diameter. All of the stones are packed in 

light-gray sand.

EXPOSURE AT THE RAILROAD BRIDGE ON POTOMAC RUN.

The railroad bridge is about half a mile west of the locality just 

described. In this interval the top of the Potomac rises so as to expose 

about 90 feet of the formation at the bridge. This difference of height 

in its top here and at the church may be due to the declination of the 

strata eastward or to their greater erosion toward the east. The 

strata, however, would be more eroded, it might be supposed, toward 

the west, where the entire formation rises higher. If not due to erosion, 

the amount of declination would be about 140 feet to the mile. Where 

the railroad bridge is thrown across the run it has cut a narrow gorge 

in the sandstone. The walls of the gorge are used as abutments for 

the bridge, which stands 70 feet above the bottom of the gorge. The 

approach to the bridge on the south side is made by a long, deep cut, 

which gives a good exposure of the Potomac above the horizon exposed 

in the gorge. The walls of the latter give the continuation downward 

of the sandstone seen in the bottom of the cut. This is firm and
78 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

durable enough to stand with a nearly precipitous face. Some of it has 

been used in making the piers of the bridge, and although coarse and 

irregular in composition it has proved quite durable. The lower portion of the walls of the gorge is hidden by talus. The hidden portion 

is about 12 feet thick. The mass above this to the level of the railroad 

bed, about 58 feet, is composed of thick layers of hard and coarse 

sandstone, alternating with more argillaceous and incoherent ones. 

Many of the harder layers are very coarse and pebbly. The alternating layers vary considerably in thickness, ranging from 3 to 6 feet. 

Some of the coarser sandstone layers contained rolled subangular 

masses of gray clay. One fragment embedded in the solid sandstone 

showed the dimensions 2 by 5 by 7 inches. Numerous small quartz 

pebbles show themselves on the exposed faces of these layers. The 

predominant color is gray, but this is often stained rusty-brown or 

yellowish in patches and irregular layers.

The material exposed in the railroad cut illustrates well the great 

irregularity of the Potomac deposits, making it plain that no single 

section of an exposure can represent even adjacent parts. Here we see 

well displayed a characteristic feature in the Potomac sand and sandstone. Any given layer, if traced horizontally, will soon pass into something quite different. This cut is about 80 yards long, its northern end 

beginning close to the bridge. The lowest material seen in the base of 

the cut has a thickness of about 6 feet. This, in some places, is a very 

coarse, pebbly, current-bedded, light-gray sandstone, with much kaolin 

in some parts. This material, traced horizontally, soon graduates into 

a crumbling, gray, sandy clay stone, and this again passes back into the 

coarse sandstone. There are several such changes in this mass. In 

some places this sandstone has its top cemented by limonite into a ferruginous crust. On the top of this basal portion comes an incoherent, 

coarse gray sand averaging 5 feet in thickness. This is full of small 

stones that increase in number and size toward the top. This layer 

also changes much from place to place. Its top is undulating, as if it 

had been eroded before the next layer was deposited upon it.

The next layer in ascending order is a cobblestone bed of extremely 

variable character. The matrix of this bed is a coarse sand, showing 

strong current-bedding. The cobbles are irregularly scattered in it, or 

disposed in lenticular layers. This bed varies much in thickness, as 

well as in the size and the number of the cobbles. Its top is undulating, 

as if from erosion. In some places it thins down to 2 or 3 feet, and in 

one place it thickens to 8 feet. Throughout it there are a number of 

clay balls, formed of both pinkish and gray clay. They are mostly small. 

With the clay balls quartz cobbles are mingled, and the two components in some portions make up a considerable part of the layer. The 

largest clay balls come on top of the cobble bed. Most of them are 3 or 

4 inches in diameter, but a few reach 12 inches. These largest ones are 

composed of the pinkish clay.
FONTAINE.] EXPOSURES ON POTOMAC EUN. 79

Throughout most of the length of the cut the layer next above the 

cobble bed is a stratum of pale pinkish clay with some obscure fragments of plants. Where these are shown the bed is about 2 feet thick. 

At the southern end of the cut this layer has been removed by erosion 

or broken up into clay balls and masses that are isolated and embedded 

in coarse gravel. At this point an isolated cake of such clay 5 or 6 

feet long, with a maximum thickness of 2 feet, was seen almost buried 

in coarse, gravelly sand, thus illustrating the mode of formation of the 

clay balls. If the clay stratum be traced toward the northern end of 

the cut, it will be seen to change its character totally. Where the 

cobblestone bed swells out to 8 feet the clay stratum, over it is barely 

12 inches thick, and is rather sandy. A little north of this point the 

clay graduates into a fine-grained, laminated, sandy shale, 4 feet thick. 

This has some comminuted carbonaceous matter dispersed through it, 

and it is current-bedded. In one place here the top has a good deal of

limonite. Limonite impregnations occur, however, at different horizons 

and in no definite way. Still farther north the laminated sandy clay 

graduates into a massive sandstone of the normal kind and it becomes 

8 feet thick. Here the lowest portion of the bed, for 6 inches, is a mass 

of broken bits of lignite and carbonaceous matter resembling charcoal 

mingled with coarse sand.

Over the variable material last described there comes a pockety 

layer of incoherent fine-grained argillaceous sand of a pale greenishgray color. Owing to the undulations of the surface on which it rests, 

its thickness varies considerably. It is much mottled with brownish 

and yellowish colors. Its thickness ranges from 4 to 6 feet. It is 

almost certainly a member of the Eocene. This lowest Eocene bed is 

covered by a stratum of small gravel 3 to 4 feet thick.

Going up the run from this point 2£ miles we find Potoinac debris 

occupying the surface with Potomac sand and sandstone forming the 

rock in place, but the exposures are very poor. The materials seem to 

be of the*same nature as those exposed at the railroad bridge. The 

first tolerably good exposures are seen on the Telegraph road where it 

crosses the Potomac Eun.

EXPOSURES ON THE TELEGRAPH ROAD AT POTOMAC RUN.

The Potomac material is, if possible, more heterogeneous here than 

at any of the previously described localities. The changes are so 

numerous and irregular that it is almost impossible to give a complete 

account of the locality. At this place the road descends a steep hill 

to the lowland bordering the run. On the slope of the hill and on the 

side of the road a considerable gully is washed out and this gives fairly 

good exposures from the bottom to the top of the hill. At the foot of 

the hill a small stream runs alongside of the road, cutting rather deeply 

into the material, and this continues the exposures for some distance 

farther along the road.
80 THE POTOMAC FORMATION IN VIRGINIA. [BOLL. 145.

Besides the outcrops on the road, additional but slight and imperfect 

exposures may be found by following a farm road that leaves the Telegraph road at the base of the hill. This road, after a short interval, 

turns to skirt the base of a small wooded hill, distant from the Telegraph road about 200 yards. Here in the edge of the woods, on the 

hillside, an important plant-bearing stratum is exposed.

The exposures on the Telegraph road will be described, beginning at 

the northern end and ending at tne top of the hill. As a good many 

fossil plants were found in the face of the hill, excavations were made 

and much more was learned concerning the details of composition and 

structure than would have been ascertained from a mere inspection of 

the surface. The lowest material seen at the northern limit is normally 

a coarse siliceous grit or sandstone. This has a light-gray color, mottled and irregularly banded with yellowish and brownish colors in. 

places. Traced horizontally, this is seen to be a greatly variable 

material and it has lost much from erosion. At the northern end 

where first seen it is a coarse pebbly grit, showing a maximum thickness of about 8 feet. It contains here clay balls and some scattered 

quartz pebbles, the largest being 3 by 4£ inches in diameter. The 

stratum graduates horizontally into a highly argillaceous flag or shale, 

which is in places a clay stone. This, when wet, forms a dark bluishgray tenacious material. This shale portion, where it first appears, is 

only 5 feet thick so far as seen, but owing to the undulating character 

of its top, due to erosion, it swells out suddenly, so that a little south 

of the spot where it first appeared it is at least 20 feet thick. At this 

place, however, the entire mass is not of the same kind of material, for 

the upper portion contains some layers of fine-grained gray flags, 

stained irregularly with yellow and brown. The claystone here when 

dry is mostly of a light-gray color. The upper portion shows the marks 

of extensive and irregular erosion. The sandstone layers that form 

the upper portion are sometimes cut away so as to leave isolated frag- 

. ments abutting against the overlying clay.

A little south of this point all the gray shale has disappeared, having graduated into a coarse incoherent gray sand. Then for a short 

distance up to the point where the small stream leaves the road the 

ground is concealed. Up to this point the varying material just 

described forms the only Potomac matter shown. This is covered by a 

peculiar mottled clay, which seems to be made up of fragments of the 

variegated Potomac strata seen in place farther up the hill. It is 

sometimes found dipping down deeply into the Potomac as if deposited 

in trenches cut out in its surface, and it envelops isolated portions of 

the top of the Potomac. It varies in thickness from 5 to 20 feet, and 

in its base small stones were found. It is largely composed of agglutinated fragments of gray, purple, yellowish, and reddish shale. These 

fragments are generally cemented very thoroughly together, but often 

the individual fragments may be detected. Between it and the Potomac
KONTAINE.] EXPOSURES NEAR POTOMAC RUN. Oi

there is a decided break, marked by much erosion, and we find here 

numerous cobblestones of quartz and fragments of pre-Cambrian rock, 

some of the latter being 12 to 24 inches in diameter.

The first Potomac material seen on the road after the concealed 

portion occurs at the lower end of the gully mentioned, the succeeding 

beds being visible in order as we ascend along this gully. The exposures are now continuous to the top of the hill. The lowest stratum is 

a gray claystone, passing into an argillaceous sandstone. It is variegated with yellowish and reddish streaks and spots. This material 

seems to be of the same nature as that shown near Fredericksburg and 

called the mottled sandy clay.

A little farther south and up the gully the claystone is overlain by a 

thinly laminated, highly argillaceous shale and claysfcoue, tender and 

crumbling in texture. The first-described material graduates into this, 

and there is no separation into distinct beds. A thickness of about 1.5 

feet of all these materials is shown. The colors vary much in shade 

and in distribution. In general the color is gray, mixed irregularly 

with brick-red or yellowish-red, but for a considerable space, taken 

horizontally, it is largely red mixed Avith some greenish spots and 

streaks. The portions colored red generally form nests and pockets in 

the gray. Farther south the patches of brick-red are replaced by those 

of yellowish-brown, and still farther south nearly all the material is 

colored greenish-gray, with a very little yellowish-brown. The claystone, however, when followed horizontally to the south is seen to graduate into a fine-grained, thinly laminated, gray sandstone and sandy 

shale that on weathering breaks up into uneven flakes. It has here in 

its upper portion a cobblestone layer, the cobbles being disposed in 

lenticular patches. The stones arc embedded in coarse sand marked 

by current bedding. This is the character of the cobblestone layer 

where first seen in the gully. It is here only 12 inches thick, having 

suffered from erosion. When followed horizontally this splits up, and 

the cobbles are diffused in the finer material, occupying a thickness of 

2 or 3 feet. Besting on the cobbles and often mingled with them we 

find fragments, some of large size, of a pale-chocolate clay and shale 

which have been evidently torn off from some preexisting stratum not 

now visible. These shale fragments contain some well-preserved 

impressions of plants. On the cobblestone bed or on its horizon there 

is a stratum a few inches thick of a pale reddish-brown, indurated clay. 

This graduates horizontally, in the space of a few feet, into a sandy, 

gray shale. This layer in the portion colored reddish-brown contains 

some beautifully preserved plant impressions.

It will be noticed that here, as at Fredericksburg, the fossiliferous 

layer rests on cobbles. Besides the impressions of leaves a number of 

fragments of stems of small size, now in the form of lignite, occur in it.

The material showing the plant fossils has the peculiar structure 

which is not uncommon in the Potomac formation, and \vhich has been 

Bull. 145 G
82 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

already spoken of as shown in uptorn and redeposited clay. This 

structure is visible in several places in the deposit which contains plant 

remains. The fossiliferous stratum is composed of material that .was 

derived from some previously formed bed. The matter thus torn off 

was deposited largely in the form of fragments that vary in size from 

considerable masses down to the finest particles. With the distinct 

fragments more or less of fine mud was formed. All this material, 

being still soft and plastic, was readily cemented together on being 

redeposited and a mass was produced that in composition much resembled the original bed from which it was derived. Without the careful 

examination called for to follow up the plant impressions, such a bed 

appears to be one formed in the usual way, i. e., by gradual accumulation of fine sediment. When, however, one carefully follows up such a 

bed, excavating it and splitting up ,the material to secure the plant 

impressions, he soon finds that it has not the simple structure of fine, 

argillaceous matter slowly deposited as mud. A material accumulated 

as fine mud by gradual deposition, if it does not show lamination, has 

an even grain and generally has cleavage. It can be split with readiness and regularity, especially if leaves are spread out in it. These 

leaves will be found disposed in regular planes parallel with the deposition planes. Such fossils in the Potomac formation are often well 

preserved.

In the stratum now under consideration all of the matter is redeposited, and most of the fossils are poor. In some cases, however, the 

redeposited matter of the Potomac seems to be laid down on the horizon 

of the destroyed bed, and then remnants of this bed may remain buried 

in the debris. The remnants have sometimes well-preserved fossils. 

In some cases, too, the redeposited masses are quite large, and retain' 

the structure and character of the original bed. These also often give 

good fossils. This is the case with one of the clay masses on the Telegraph road, and from this good fossils were obtained. The greater 

part, however, of the plant material here is made up of rather small 

recemented particles. The plants in the remnants of the original bed 

often, but not always, differ in species from those found in the recemented small particles; and this is trne, not only of beds at the locality 

on the Telegraph road, but of others elsewhere, especially of those at 

Brooke. The fossils included in the recemented matter are, of course, 

somewhat later in deposition than those preserved in the original 

stratum, but there is apparently no great difference in their age. Geologically speaking, erosion seems to have quickly followed the formation of the parent stratum, the interval not being long enough to permit 

any material change in the plant remains. The chief difference is in 

the perfection of preservation of the fossils entombed under the two 

conditions. Those of the recemented clays are fragmentary, distorted, 

and bent so as to lie in different planes. They were caught up in every 

possible position as the fragments were being brought together, and 

were inclosed between them as they were cemented together. The
FONTAINB.J EXPOSURES NEAR POTOMAC RUN. 83

recemented clay betrays its character in various ways. More or less 

sand is caught up in the cementation of. the fragments. The clay is 

without cleavage, and breaks along the junction planes in a very irregular way. It shows slickensides with more or less contortion and 

crushing. It is very difficult to get good fossils from such material.

The portion of the layer on the Telegraph road that shows good 

fossils is only about 12 inches thick in its thickest part. It contains 

one or two large fragments of the original stratum from which the clay 

was derived. These contain some well-preserved ferns and conifers. 

This thin layer seems to be in the form of a lenticular pocket. To the 

south it splits up and disappears at first by being interstratified with 

flaggy sandstone and then by graduating into this material. At the 

point where the clay disappears higher strata are shown. These are 

composed of a mixture of gray shale and coarse current-bedded sandstone. This mixed material shows a maximum thickness of about 

2 feet. It contains scattered small pebbles and fragments, both rolled 

and angular, of the plant-bearing clay. Next above this GOine about 

8 feet of current-bedded, thinly laminated, gray, crumbling sandstone, 

which contains many balls of gray clay that range in size from 

4 inches downward. Above this comes a thick bedded sandstone, also 

crumbling, and gray in color. This is about 10 feet thick. Farther 

south and higher up is a variable deposit. This is a thinly laminated 

sandstone, with irregular nests and patches of a more argillaceous 

material, which tends to pass to claystone and to reproduce the mottled 

claystone seen lower down. The more argillaceous portions are 

blotched with brownish-yellow and brownish-red colors. Toward its 

top this material passes into a firm, mottled claystone. The mass just 

described is 25 feet thick. Up to this point all the material is Potomac, 

the maximum thickness of the formation exposed being about 80 feet. 

In the top of the last-described layers comes the same younger mottled 

clay that was seen farther down. It is here 12 feet thick and has the 

same character that it possesses at the foot of the hill. In the summit 

of the hill a thick covering of the usual surface cobbles comes on the 

top of this last-described material.

Another plant-bearing bed is found in this vicinity which is quite 

different from, that seen on the road. About 200 yards to the northwest of the spot on the road yielding plants there is an outcrop of shale 

that shows a number of well-preserved plant impressions. This stratum 

is very nearly, if not quite, on the same horizon with the fossiliferous 

.bed that occurs on the road, but it is totally different from that. The 

shale now considered is evidently an undisturbed and original layer. 

It is not associated with any cobbles and is uniform in texture, 

retaining lamination. It is brownish-gray in color and fine-grained, 

and is a lenticular pocket in the sand, showing a thickness of about 

2 feet, the top having suffered from erosion. Nearly all the plants 

occurring in this stratum are small and delicately cut ferns, none of 

which, strange to say, occur on the roadside. Under this shale there
84 THE POTOMAC FORMATION IN VIRGINIA. . [BULL. MS.

is about 15 feet of fine-grained gray and mottled claystone, that in some 

places graduates into a finely laminated, gray, sandy shale. The 

system of beds shown in the exposures here and on the road seems to 

dip about 4° south-southeast.

About a quarter of a mile southwest of the exposures on the road 

and in the woods the bed of a small stream exposes the top of a layer 

of bluish-gray, sandy shale. This may be seen along the stream for 

about 100 yards, a maximum thickness of about 3 feet being visible. 

The entire thickness is not displayed. This shale or indurated clay is 

remarkable for the great number of flattened trunks of trees, now 

changed to lignite, that lie scattered in it. The trunks and limbs are 

those of coniferous trees that do not seem to have been drifted, but to 

have fallen where they grew. The trunks are not piled up thickly, but 

occur in a scattered manner throughout the entire thickness of the shale 

exposed. The shale or indurated clay containing these trunks is quite 

micaceous, an unusual feature in the fossiliferous clays. It has on the 

top, imperfectly exposed, a stratum of pale reddish-brown shale like 

that containing the plant impressions on and near the road, and it is 

probable that it occurs on the same horizon Avith that. The material 

holding the lignite logs showed no impressions of leaves except a fragment of Scleropteris elliptica. This illustrates a general truth, for it is 

noteworthy that the clays which contain large amounts of lignite very 

rarely show leaf impressions. The material of the lignite trunks is 

similar to that almost universally seen in the trunks embedded in the 

Potomac formation. It exhibits a peculiar, blending of the physical 

characters of jet and lignite in the narrow sense of the latter word. 

It splits freely and shows the grain of the wood well preserved along 

the cleavage planes, but generally the cross fracture is couchoidal 

and shows the pitchy luster of jet. So far as can be decided by the 

unaided eye all the lignite comes from the same kind of coniferous 

tree. This seems to have had a texture like that of the white pine, 

The trees must have been subjected to great pressure, for the trunks

are quite flat. This implies the former presence of a considerable 

thickness of superincumbent matter. The character of the wood, all 

of the same kind, would seem to show that the coniferous trees grew 

in forests composed of the same kind of tree. The size of several of 

the lignite logs indicates that some of the trees were 24 inches in 

diameter.

The cobblestone layer occurring on the road at the base of the fossiliferous shale is very slightly developed there, but at other places on or 

nearly on the same horizon a more extensive cobblestone layer occurs. 

It appears on the Telegraph road on the north side of Potoniac Bun, 

aboub 300 yards north of the exposure on this road previously described. 

Here the bed is from 3 to 4 feet thick and it is a mass of cobbles packed 

closely together. The stones are mainly of quartz and are well rounded. 

Many of them are large, some attaining the diameter of G or 8 inches. 

Some pre-Cambrian fragments occur here, and this is an unusual
FONTAINE.] EXPOSURES NEAR POTOMAC RUN. 85

feature in Potomac cobble beds. The bed rests on a layer of gray 

claystone, and has over it the younger mottled clay.

EXPOSURES ON THE WOODCHOPP1NG ROAD.

The material of the Potomac is exposed for more than a mile west of 

the Telegraph road, and is there of nearly the same character as on 

the road, but is rather more sandy and variable in the western outcrops. The Woodchopping road runs near the western margin of the 

formation and has a course nearly parallel with this margin. On this 

road the formation has risen so much that its base is exposed and a 

considerable thickness of the gneiss foundation on which it rests is to 

be seen. In consequence of the greater height of the Potomac in this 

quarter it has suffered much from .erosion and remains only in. patches.

The gneiss is exposed in ascending the hill, from the run. It is 

exactly like the gneiss on the western margin of the formation at Fredericksburg. It is highly micaceous, very irregular in composition,' and 

deeply decayed. Portions of it have changed almost wholly to clay. 

This clay is quite different from the sandy matter that constitutes most 

of the Potomac, but is more like the mottled sandy clay of that formation. Toward the top of the hill the Potomac is shown, and it there 

reaches the maximum thickness of 40 or 50 feet. It is mostly an incoherent kaolinic sand in the outcrops, but may be more firmly consolidated deeper down where protected from weathering. It is usually 

light-gray in color, but often has irregular inottlings of yellowish 

brown. Some portions are full of small balls, streaks, and partings of 

atgray clay. Great numbers of large cobblestones are scattered about 

on the surface, and some large pre Cambrian fragments are found. 

Some of the latter are 2 feet in diameter and many of the cobblestones 

have a diameter of 8 or 10 inches.

Along the western margin of the Potomac, as at this locality, it is 

common to find this formation only in the tops of the hills, while the 

crystalline rocks form the greater portion of them and occupy the valleys. The amount of erosion indicated is great, for in some places 

farther north the pre-Cambrian rock has been cut into to the depth of 

fully 100 feet, and extensive valleys are formed in it.

EXPOSURES NEAR BROOKE.

The next detailed examination of the geology of the formation was 

made at and near Brooke, on the Eichmond, Fredericksburg and 

Potomac Railroad. This is a portion of the Atlantic Coast Line Bailroad. This station is 9 miles north of Fredericksburg and 5 miles 

northeast of the locality on the Telegraph road, which afforded the 

fossil plants mentioned above. This latter place is near the western 

margin of the formation, while Brooke is on the eastern. In passing 

over the interval between these two localities the Potomac with its 

de"bris is seen to form all the rock of the country, but nothing noteworthy is to be met with.
86 THE POTOMAC FORMATION IN VIRGINIA. [BULL.US.

The examination of the Potomac in the vicinity of Brooke is much 

facilitated by the comparatively recent cuttings made along the railroad between the station and Aquia Creek, a distance of over 3 miles, 

and also by excavations made a little east of the station for the purpose 

of utilizing the greensand marl exposed there. The county road, also, 

near the station, and Accakeek Bun give exposures.

The lowest rock seen in the vicinity of the station is the sandy 

Potomac matter. This is here, and for a considerable distance north 

to beyond Stafford, rather firmly consolidated, so that much of it is a 

sandstone even on the weathered outcrops. In a number of places it 

was formerly largely quarried for building purposes. The sandstone 

is visible in the banks of Accakeek Run. It shows in all about 20 feet. 

It is very massive, and is coarse, hard, and firm in texture. In the 

surface of this rock, as it is exposed on the run, a number of large 

clay balls may be seen, varying in diameter from 6 to 12 inches. A 

little distance away from the run, on the north side, about 20 feet of a 

higher portion of the sandstone may be seen, making the entire thickness of sandstone visible 40 feet. The upper 20 feet, as is very commonly the case, is composed of a more kaolinic and less firmly consolidated material. This is often nearly white and on exposure falls readily 

to a loose sand. Some small quartz pebbles and clay balls occur in it.

Still higher material reaching into the Eocene may be found on the 

south side of the run, along the road from the run. Here may be 

seen the junction of the Eocene and Potomac, the transition from one 

to the other being very clearly shown. This contact of the two is very 

rarely seen, and nowhere else in this area is it so well exposed. It 

may be noted as illustrating the irregular erosion of the surface of the 

Potomac in pre-Eocene times, that the top of this formation on the 

north side of the run is considerably above the Eocene greensand on 

the south side, only 150 yards away. The section was obtained by 

taking the maximum thickness of the material visible on the south 

side of the run, where the Potomac shows only 20 feet.

Section at BrooTce.

EOCENE.

Feet.

1. Yellowish, incoherent sand (surface)........................ 6

2. Greensand marl ............................................ 10

3. Greenish-gray, mottled, argillaceous sand................... 10

4. Mottled, argillaceous sand.................................. 15

Total Eocene ......................................... 41

POTOMAC.

Feet.

5. Cobblestone bed ........................................... 4

6. Potomac sandstone......................................... 20

Total Potomac..................................J..... 24
PONTAINE.] EXPOSURES NEAR BROOKE. 87

No. 5 is the cobblestone bed so common in the top of the Potomac 

sand and making a portion of the formation. The stones here, as 

is commonly the case, are embedded in a coarse, ferruginous sand. 

The proportion of large cobbles is considerable, many of them being 

4 or 5 inches in diameter. All are composed of quartz and are well 

rounded.

No. 4 is apparently the lowest of the Eocene strata. It is without 

fossils and was plainly deposited in unquiet waters. It is an incoherent, sandy shale, fairly well laminated. The predominant color is dingy 

gray, but it is irregularly streaked and spotted with yellowish-brown. 

It is much affected by current bedding, and the bedding planes are 

marked out with this color. This and the next layer above have 

numerous spots and specks of pure white sand embedded in the 

colored material, much like that occurring on the same horizon at 

Potomac church.

The next stratum, No. 3, is an argillaceous sand. It is in the main 

greenish-gray in color, but in some places large masses are colored 

almost uniformly a deep tint of reddish-brown. The greensand 

bed, the one coming next above, rests upon this, but is quite distinct 

from it.

The greensand bed, No. 2, is here considerably weathered, and the 

lower portion of it, for about 4 feet, is a yellowish-brown sandy clay, 

full of the imprints and casts of shells. Over this occurs about 4 feet 

of a greenish-gray sandy clay, with many casts of shells and some 

shells. Cakes and crusts of varying thickness are formed in this portion by the cementing action of calcareous matter, giving an impure 

limestone. The highest portion of the bed is grayish in color, but it 

does not show so many shells.

The higher stratum, No. 1, has lost much from erosion. It is very 

incoherent, and seems to be the same as the highest bed shown at 

Potomac church.

EXPOSURES ON THE RAILROAD NEAR BROOKE.

The exposures next to be noticed occur along the line of the Richmond, Fredericksburg and Potomac Railroad, between the station and 

Aquia Creek. For about 2 miles north of the station to near the 

seventy-second milepost from Richmond nothing but the incoherent 

argillaceous sand corresponding to Nos. 3 and 4 of the preceding section is to be seen in the cuts. This material is mostly gray in color, 

but is often mottled with various colors, due to oxide of iron. It is in 

places greenish-gray. At some points it shows a tendency in weathering to form crusts and sheets of highly ferruginous sandstone in the 

in coherent matter. This is due to the cementing action of iron deposited from solution in the percolating waters. Sometimes nests and
55 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

irregular layers of nearly pure light-colored sand occur in the grayish 

matter.

At the seventy--second milepost from Richmond, which is found about 

three-fourths of a mile south of the railroad crossing over Aquia Creek, 

there is an exposure of the Potomac. This is important because of the 

occurrence at this point of numerous well-preserved fossil plants in 

the clay of the Potomac, and on account of the occurrence of a good 

exposure of the superposition of the Eocene on the Potomac. At this 

point the descent of the railroad grade toward the creek brings the 

roadbed down below the level of the top of the Potomac and the continued descent from this place carries it more and more deeply into the 

formation. On this account the Potomac is continuously visible as far 

as the creek.

A considerable amount of material was removed in search of fossil 

plants, and the careful following up of these disclosed very strikingly 

the peculiar structure found in redeposited clays which has been mentioned in another connection. A very large number of fossil plants 

was found here occurring in the clay pockets and seams embedded in 

the sand and shown in the cut beginning at the seventy-second milepost. The plants are cycads, conifers, ferns, and angiosperms. The 

cycads and ferns are rare, but the conifers and angiosperms are very 

common. A comparatively small number of species is represented by 

great numbers of individuals, while many species have only one or two, 

or at most a very few3 individuals.

The impressions are found both in the confusedly deposited clay 

and in a thin layer apparently deposited at the same time as the irregular clay. This latter has a more uniform composition and structure, 

probably because it was formed of fine particles. Although quite 

irregular in its course, it has a fine grain and good cleavage, showing 

a number of species of angiosperms found nowhere else. These show 

only one or two specimens in some cases, but they fire beautifully 

preserved. The plants in the irregularly redeposited clay are very 

numerous but very fragmentary. They are caught up between the 

fragments of clay as they were cemented together, and hence occur 

in all positions and in different planes in the material, being bent and 

distorted in various ways. They rarely yield good specimens. Some 

of the conifers found in this material do not occur in the more uniform 

layer.

The irregular material composing the Potomac forms the lower part 

of the cuts in the immediate vicinity of the seventy-second milepost, 

and it is capped by marine Eocene containing imprints of shells. 

Fig. 7 gives a generalized section of the two formations as superposed in the cuts next to the milepost. The surface of the Potomac 

was trenched deeply and irregularly previous to the deposition of 

the Eocene, so that at this place the latter cuts out' the former to 

a point below the level of the railroad track, in the form of two
FONTA1NE.] EXPOSURES NEAR BROOKE. 89

narrow tongues. These represent two pre-Eocene ravines filled with 

fossiliferous Eocene deposits. These ravines have a course at right 

angles to the direction of the railroad that is, they are directed a little 

south of east, which is the course of the minor streams of this vicinity 

at the present time. In fig. 7, r r denote the ravines, p p p the irregularly eroded Potoraac, and -e c the Eocene. Below this point toward 

Aquia Creek the Eocene is no longer visible on the surface of the 

Potoinac. The following section is made out from the material shown 

in the cuts next to and at the milepost. The Potoinac deposits found 

in the lower portion of the cuts are so very complex in nature that it 

is difficult to give any adequate account of them without describing

FIG. 7. Pre-Eocene ravines near Brooko.

every few yards of the exposures. The sand and its contents give 

indication of deposition in agitated waters, with pauses in the deposition of the sediment, during which erosion took place; so that there is 

a great deal of redeposited matter, the most noteworthy of which is 

the clay:

Section at the seventy-second milepost.

Eoefc.

1. Greenish-gray sandy clay (seen)...........:.............. 8

2. Yellowish-brown sand.................................... 5

3. Friable, mottled argillaceous sand........................ 12

Total Eocene.

POTOMAC.

4. Cobblestone bed ............

5. Sand and clay...............

6. Gray argillaceous sand.......

7. Ferruginous sand and clay ..

8. Gray sand Avith clay pockets

25

1

H

2

2

7

Total Potoinac (seen).................................. 13i

No. 1 is considerably eroded at top, and the thickness seen is evidently less than that originally possessed by the stratum. The material is massive and very incoherent. It shows no fossils.

No. 2 is predominantly brownish or yellowish brown in color. It, 

also, is a loose, argillaceous sand without lamination. It graduates 

into No. 3, no distinct plane of division existing.
90 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

No. 3 shows casts of Eocene fossils. It is a gray, argillaceous sand, 

showing a tendency to form crusts of ferruginous matter from the 

cementing of the sand by iron. It shows no trace of plant impressions, 

but is evidently a marine.deposit, indicating a decided change from 

the conditions prevailing in the Potomac.

No. 4, the cobblestone bed,- is not uniform in thickness, but is pockety, the cobbles being packed in argillaceous sand. The stones are of 

quartz and well rounded, the largest being 3 to 4 inches in diameter. 

No pre-Cambrian fragments occur with them. It is noteworthy that

in the material above 

this bed no cobbles 

and no clay fragments 

occur.

No. 5 is largely made 

up of fragments of clay 

which are embedded in 

sand. This clay was 

eroded from some pre- FIG. 8. Clay pockets near Brooke.

existing clay bed and redeposited with sand in the form of more or 

less distinct fragments. The clay is bluish-gray in color and is disposed on the same horizon in balls, cakes, and short layers so as to form 

a disconnected stratum of clay in sand.

No. 6 is a very uniform and fine material, graduating irregularly into 

Nos. 5 and 7.

No. 7 has in some places at the bottom an impure iron ore. It contains numerous balls and cakes of clay disposed in one plane, as in No. 

5. It differs from that in having the clay less regularly arranged and 

in being more ferruginous. The material is a mass of sand mixed with 

clay particles.

No. 8 is much the most important and most variable material, since 

it has irregular pockets of clay with many plants. The greater portion of it is a light-gray, 

kaolinic sand, with 

blotches of color from 

iron oxide. It is arranged in lenticular 

masses, which are affected by current bedding. It has also small 

stones scattered through it. In a word, it is the typical Potomac sand, 

the only exceptional features being the fossiliferous clay pockets. The 

clay is in part pale-reddish to chocolate colored. Most of it is more 

sandy than that which commonly contains plant impressions. Much of 

it is ash-gray in color, and this contains the best impressions. Some 

of the clay occurs in balls and cakes. This does not contain fossils. 

It occurs also in pocket-like masses, interrupted layers of lenticular 

form, and in fragments irregularly cemented together and mixed with 

sand. The three last-mentioned forms contain the plants.

'9-. '^M*$j£?. '''^'^^^i\^S^^^^'-

FIG. 9. Clay pockets near Brooke.
FONTAINE.] EXPOSURES NEAE BEOOKE. 91

The accompanying figures (figs. 8, 9, and 10) give some of the shapes 

assumed by the clay pockets formed of redeposited and aggregated 

clay particles. The pockets of clay are sometimes wedge-shaped and 

sometimes they split up-and disappear. Sometimes the pocket-like 

masses are made up of alternating layers of sand and clay. In the 

annexed figures, a a represent the clay layers and pockets; 1) b the 

clay balls; and, in fig. 8, s.s the interstratitied layers of sand. The 

clay portions are rarely composed of uuniixed clay. The aggregated 

particles have usually caught up more or less sand. The clay balls are 

mostly of pale-reddish clay. They contain, if any, only obscure bits 

of plants. The clay of the layers is mostly gray. The layers and 

pockets lie at different angles of slope and at different levels, as if the 

material had been laid down on a shifting surface of sand. Most of 

them are very irregular in thickness and vary much in persistence, 

having a lumpy structure, Tbese portions have no regular structure,

and are clearly composed of fragments torn off from some previously 

deposited clay layer and recemented in. a confused manner. In these 

portions much ^ ^ 

sand is mixed 

with the clay, 

and many imperfect fragments of plants 

are found. They 

are greatly 

contorted and

crumpled. Some of the pockets of clay, such as those represented in 

figs. 8 and 10, seem to have been deposited in hollows and trenches 

scooped out in the surface of the sand during the intervals of erosion.

Going north from the seventy-second milepost, where the abovedescribed materials are exposed, we find several cuts and excavations, 

which mostly show, toward Aquia Creek, little except the basal matter of the Potomac, or No. 8 of the section. In a cut about 100 yards 

north of the milepost the top of No. 8 is capped by from 4 to 6 feet of 

sand mixed with the lumpy, gray, and imperfectly bedded clay. Over 

this comes about 10 feet of very coarse, ferruginous, sandy matter, containing many balls and cakes of gray clay. These are disposed in 

imperfectly outlined layers that are inclined at various angles to the 

horizon. Over the whole there is a few feet of yellowish, mottled, 

sandy matter, which may be Eocene. Most of the clay is gray, but 

scattered masses and balls of pale-reddish, sandy clay occur. The 

original stratum furnishing these must have been very rich in plant 

impressions, for they abound in fragments.

The next cut to the north, which is close to the one last described, 

shows at the bottom 5 feet of current-bedded sand, which is full of 

clay balls. Over this come 4 feet of sand and cakes of clay. The

FIG. 10. Clay pockets near Brooke.
92 THE POTOMAC FORMATION IN VIRGINIA. [BULT, 145.

clay is gray and the cakes are arranged in layers, with pretty continuous clay layers at the top. All this is recemented clay. Over this 

come 4 feet of incoherent sand with clay balls. Above this is a continuous, evenly bedded stratum of gray, sandy clay, evidently an 

original deposit. The highest layer is about 10 feet thick, and is composed of mottled, argillaceous sand. These materials are probably all 

Potornac.

About 100 yards north of this point a perpendicular face of Potomac 

material is shown, a little way off from the railroad. It is about 20 

feet high and 10 feet of the base of. the cliff is hidden by talus. The 

greater portion of the exposed mass is a ferruginous, coarse, currentbedded sand. Irregularly shaped portions of it, however, have much 

kaolin. The coarse sand contains some pebbles and many small balls 

and cakes of clay. The bottom of the exposed mass is more heterogeneous than the rest. It is a peculiar mixture of balls of clay, pebbles 

of quartz, and coarse sand. Two feet of this material may be seen.

About 50 yards north of this point material lower than any shown 

in the last-described exposure may be seen. It, however, may be taken 

as a continuation downward of the section obtained in the 20 feet of 

material last described. The highest portion here corresponds to the 

concealed base of the mass occurring there. The exposure here is 

made in the banks of a small stream and in the stripping made laterally out from the railroad to obtain material to make a " fill." This 

stripping has left a considerable space bare, and has been carried out 

so as to leave, where it stopped, a perpendicular face about 50 yards 

distant from the railroad. In this face a lenticular layer of clay is 

exposed, which is very important on account of the plant impressions 

that it contains. In the description of the plants in the " Potomac or 

Younger Mesozoic Flora" this spot is designated "Bank near Brooke." 1

On the stream, which flows at a level about 10 feet below the top of 

the bare space, about 5 feet of a very coarse, current-bedded sandstone 

is shown, with the base not disclosed. This is full of quartz cobbles, 

and has some large clay masses and many clay balls. Many of the 

cobbles are 4, 6, or 8 inches in diameter. One subangular, prismatic 

quartz mass was seen with the dimensions 3 by 4 by 9 inches. This 

material minus the cobbles extends upward so as to occupy the space 

left bare. In this latter some very large masses of bluish-gray sandy 

clay are embedded. The largest of them were only partially exposed, 

being buried in the coarse sand. They measure sometimes 5 feet or 

more'in diameter.

The material in ascending becomes more friable, passing to a sand 

with more or less clay. In the perpendicular face exposed, the material 

is finer and more argillaceous than that just described, and has the 

usual arrangement of lenticular, interlocking layers. In one portion, 

on the left-hand side, there is a lenticular layer of grayish clay that

1 Mon. TT. S. Geol. Survey, Vol. XV, 1889.
KONTAINB.J SECTION AT AQUIA CREEK. 93

disappears in passing1 to the right-hand side of the bluff. Here, at the 

top of the bluff, which is about 20 feet high, there is a lenticular layer 

of dark-gray, rather sandy clay. This has a maximum thickness of 

about 8 feet. It is composed of clay in irregular layers with some partings of fine sand and of pure clay. The partings of purer clay are full 

of impressions of angiosperm leaves, which nearly all belong to the 

new genus Sapindopsis, most of them belonging to one species. Leaf 

impressions occur also in the dark-gray, sandy clay layers, but they 

are not so abundant as in the partings of purer clay. At this place, 

by taking in the exposures given at the last-described locality, AVC have 

as much as 70 feet of Potomac shown, with no Eocene visible.

EXPOSURE AT AQUIA CREEK.

In the last cuttings made near the point Avhere the railroad crosses 

Aquia Creek, and which are about 200 yards north of the spot last 

described, the* rapid changes in the Potomac are well shown. There 

are here two exposures close together. The one to the south is a short, 

deep, cut; the other, a few yards to the north, is an extensive side cutting. In the short cut the following section is shown.

Section at Aquia CrceJc.

Foot.

1. Gray sandstone, streaked irregularly with yellowish-brown.. 5

2. Gray, incoherent sandstone and sand........................ 6

3. Pebbles packed in ferruginous sand.......................... 4

4. Clay balls mixed with gray sand............................. 1

5. White, kaoliuic sand........................................ 10

The last is a very light-colored mixture of coarse grains of sand with 

much kaolin, and it has a lenticular structure.

In the side cut the basal, kaolinic sand, No. 5, rises up to the height 

of 30 feet and cuts out the variable beds Nos. 1 to 4, shown in the short 

southern cut. The uppermost portion of it here, for the thickness of 

about 4 feet, is full of pebbles, and a stratum of loose gravel, 2 to 3 

feet thick, covers the whole. There are some thin, variable, lenticular 

partings of reddish, sandy clay in this exposed face, and they contain 

a few imperfect fragments of plants. These layers soon give out.

The Potomac continues to be exposed on the north side of the creek, 

and is visible at intervals, but is mostly cut away by the Potomac 

River, which the railroad now approaches. Considerable exposures 

occur at Quantico. The material is almost wholly the irregular lenticular, bedded sand and sandstone so common in the formation. The 

railroad crosses Aquia Creek about 4 miles (by the creek) above its 

mouth. On the north shore of the creek bluffy banks occur in some, 

places, but they are formed of the Pleistocene. This material forms 

the banks of the Potomac Eiver to the vicinity of Quantico, where the 

Potomac formation is seen in place in the river banks. The south side 

of Aquia Creek shows the Potomac sandstone farther down toward 

the mouth than the north side.
94 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

The Eocene greensand marl bed has been worked about three-fourths 

of a mile east of Brooke Station. The level of this bed here, as compared with that shown at the station, indicates that it, and consequently the top of the Potomac formation, sinks about 30 feet to the 

mile in a direction a little south of east. At the opening made in the 

marl its base is not exposed, since the stratum was not cut through. 

The lowest material seen is a greenish-gray sandy clay, full of the 

impressions and casts of shells and containing also many shells. Seven 

feet of this is in sight. Four feet of greenish-gray sandy matter occurs 

on the top of this, but no imprints of shells are visible in this last. 

The uppermost material seen is a loose sandy clay that weathers 

yellowish-gray with bands and streaks of yellow.

At Marlboro, near the mouth of Aquia Creek, there is an imposing 

exposure of Eocene material shown in bluffs in some places over 100 

feet high. The bluffs extend from the mouth of Aquia.Creek to the 

mouth of the Potomac Creek, but nowhere show the Potomac formation. 

Over 100 feet of the Tertiary may be seen in these exposures. This 

condition of things shows great inequality in the erosion of the formations along the Potomac Eiver, for north of the mouth of Aquia Creek 

all this Tertiary is cut away and, as stated above, nothing but Pleistocene is visible in the banks of the river.

Ahout half a mile from Brooke, in a direction a little east of south, 

a gully exposes about 20 feet of the Potomac formation, which belongs 

to the same horizon as that shown at the station. But here none of 

the sandstone or sand occurs. The material is all composed of the 

mottled sandy clay, almost exactly like that shown at Fredericksburg, on Hazel Run, and on the Stafford side of the river, opposite 

Fredericksburg. It occurs in rather regular layers. The material is 

firm and coherent. It is mainly gray in color, but is marked with spots 

and streaks of brick-red, sometimes of purplish-red, the colors being 

bright. Large masses, however, have solid colors, sometimes red and 

Sometimes gray. Ou this rests about 5 feet of yellowish sandy clay.

EXPOSURES NEAR STAFFORD.

Stafford is 10 miles north of Fredericksburg and 3 miles northwest 

of Brooke. In the vicinity of this place, and between it and the 

station, patches of the lowest Eocene bed occur within the Potomac 

terrane. This material, from some cause, is much indurated, and 

more or less impregnated with silica deposited from solution. Portions of it look like a calcareous sandstone. It is owing to this induration and silicification that the material has been preserved from 

erosion. About forty years ago, when the Potomac sandstone was 

largely used as a building stone, the vicinity of Stafford afforded a 

number of quarries of it. Over a considerable area in this neighborhood the lower portions of the Potomac material seem, to be more 

firmly consolidated than usual, and in some places it has a composition
FONTAINE.] EXPOSURES NEAR STAFFORD. 95

and a texture sufficiently uniform to furnish a rough Mud of building 

stone. Most of the quarries were located on Austin Bun, a tributary 

of Aquia Eun, and on the upper part of Aquia Creek, northeast of the 

court-house. The old quarries on Austin Eun are situated to the west 

of the Telegraph road, and are found at intervals over the space of 

about a mile up the run. They are so much overgrown and filled in 

that they now disclose very little of the details of the texture and the 

structure of the rock.

The thickness of about 100 feet of Potomac sandstone may be seen 

along the run. The uppermost part of this shows some reddish and 

mottled sandy clay. The rock along Austin Eun is essentially of the 

same nature as that shown on Aquia Creek, and as the recently opened 

quarries there disclose its character very well, the description given 

for this latter will apply to the rock of the former locality.

The width of the Potomac belt in the vicinity of Stafford is about 5 

miles. There is no essential change in the character of the rock in 

passing from one side of the belt to the other. The exposures near 

Brooke may be taken as good types of the material on the eastern margin, and those on the upper part of Aquia Creek, near the court-house, 

of the rock on the western side.

QUARRIES ON AQUIA CREEK.

Several of the old quarries on the creek have been recently reopened 

in search of stone for railroad culverts and bridge abutments. Two of 

them were being worked when I made an examination of the Potomac 

formation along the creek. These quarries showed the uselessness of 

attempting to make out details of texture and structure from old outcrops. In such variable and easily disintegrated rock many of the 

most peculiar features were disguised or wholly obliterated after long 

exposure.

The two quarries examined are situated on the north side of the 

creek immediately on the water. They are about 1£ miles apart.

THE LOWER QUARRY.

The lower quarry is about 2 miles above the railroad crossing over 

Aquia Creek. At this place the thickness of the Potomac shown, 

measured by aneroid barometer from the lowest excavations to the 

highest, is 140 feet, nearly all of which is a coarse sandstone. The 

hills around show about 40 feet more of similar material. Hence we 

have now visible about 180 feet of the formation. It has lost much 

from erosion, and its surface has been left in a very uneven condition. 

There are two openings or workings here, one about 200 yards farther 

up the creek than the other.

The greater portion of the Potomac at this quarry is a pretty firm, 

coarse, gray grit, generally composed of rather large grains of sand, 

packed in more or less of light-gray clay. This is not true kaolin,
96 THE POTOMAC FORMATION IN VIRGINIA. [nuu,U5.

although it is quite light-colored. The normal color of the fresh rock 

is light-gray, but it often contains irregular patches, spots, and seams 

of colors due to oxide of iron, such as yellowish, brownish, and reddish. These colors often mark the current-bedding planes and the 

outlines of the lenticular layers which generally characterize the rock. 

The rock contains very often scattered pebbles ranging in size from 3 

to 4 inches down. In some spots and in some horizons clay balls are 

common, and occasionally large masses of rolled clay are found. These 

clay balls sometimes are mingled with pebbles. They seem to mark 

erosion horizons. Clay seams and partings, lenticular in form and 

limited in length, may be seen, but they are rare at the quarries 

because these are located where the rock has such qualities as would 

exclude clay layers.

The lower opening shows mostly a coarse grit, stained with much 

ferruginous matter, and lying in heavy, lenticular layers. Most of the 

stone shows scattered, small quartz pebbles, and clay balls occur sparingly. The latter range in diameter from 3 or 4 to 12 inches, being 

mostly of the smaller size. About 25 feet below the top of the quarry 

there is a layer indicating erosion. It is made up mainly of clay balls 

and quartz pebbles, and is from 1 to 2 feet in thickness.

Kear the top of the sandstone there is another erosion horizon. It is 

marked by an irregular layer composed of sandstone containing numerous clay balls and cobbles. The clay is arranged in more or less continuous layers made up of clay b.alls. On this horizon there is a huge 

mass of clay, apparently a rolled portion of the original clay bed. It 

is isolated and embedded in the coarse sandstone. On the exposed 

face it is 6 feet long and 4 feet thick. As it is composed of a tenacious 

and plastic bluish-gray clay, it contrasts strongly with the coarse grit 

that surrounds it. The stratum of mixed cobbles and clay balls in 

which this mass lies is from 1 to 2£ feet thick. On the top of this 

erosion horizon is a layer, 8 to 18 inches thick, of laminated pale-reddish 

sandy clay, which contains a few obscure plant remains. The sandstone 

quarried here has at its top a layer 4 feet thick of the mottled sandy 

clay. It is soft, and in color brick-red, mottled with a little gray. 

Some portions of it are yellowish-brown.

The upper opening was made on a point 140 feet high that overlooks 

the creek. Except at the base, near the level of the water, the material is nearly all sandstone from top to bottom. It is, however, very 

variable, some portions being very siliceous and coarse, and others more 

argillaceous and incoherent. The mass is arranged mainly in thick, 

interlocking, lenticular layers. Here and there a thin, irregular layer 

of clay occurs. One of these clay layers was seen about halfway up 

the bluff. It was 12 inches thick where seen, and pale-reddish in color. 

Some fragments of coniferous twigs were found in it.

The lowest portion of the bluff exposed showed 8 to 10 feet of mottled sandy clay, which graduated into the normal Potoinac sandstone.
FONTAINE.] QUARRIES ON AQUIA CREEK. 97

This clay is mottled and marbled with reddish and brownish-red colors, 

the tints being bright. .

The Potomac material is best exposed at and near the top of the 

bluffy point, for here most of the excavation was made in search of 

stone for the purposes of construction. Portions of the layers here, 

over an area of many square yards and sometimes to the thickness of 

3 feet, are made up of a mixture of clay bal's and sandstone. In some 

places nearly all the material is clay, and in others the balls are scattered in the sandstone. Most of these balls are small, ranging from 

the diameter of 4 inches down to a small fraction of an inch. The 

greater part of the clay is pale-reddish to chocolate in color, and it 

varies from, a sandy clay to a pure plastic clay. Only small, unrecognizable fragments of plants occur in the clay here. Some portions of 

the sandstone, coming above the horizon of the clay,balls, seem to have 

been at one time full of fragments of the limbs of trees. These fragments, from some cause, have been removed, and are represented now 

by cavities. Some little lignite still remains.

Much of the rock in the upper part of the bluff is full of small quartz 

gravel. The largest pebbles do not surpass the diameter of 2 inches. 

 While most of the clay is deposited in the sandstone in distinct balls of 

some size, this is not the case with all of it. It is sometimes mixed 

intimately with the sandstone in minute pellets that are hardly larger 

than the grains of sand. In rare cases the clay occurs in redeposited 

large cakes and masses, embedded in the sandstone. The small stones 

abound on the horizon of the clay masses and balls.

THE UPPER QUARRY.

The upper quarry is situated at Coal Lauding, a point near the head 

of the estuary or " creek" portion of Aquia Bun. In former times 

large quantities of stone were taken out from an island-in the creek at 

this point, and some of this was used in the construction of the old 

Capitol at Washington. The stone taken from the island appears to be 

more uniform and of better quality than that found in the newly opened 

quarry near by, which is one of those originally worked and is situated 

higher up in the hills on the creek, being about 80 feet above the water. 

A large space has been excavated here, leaving an open pit with abrupt 

walls on three sides. These walls disclose pretty well the character of 

the rock, about 20 feet being thus exposed. The lower portion of one 

of these walls is formed by a massive, projecting ledge of sandstone 

which has been undermined, and in consequence of this its under face 

is shown for some distance. If we may take the lower face of this ledge 

as giving the dip of the rock, this amounts to 15 degrees, in a direction 

50 degrees east of south. But in the base of the hill containing the 

quarry, and near the level of the creek, where 30 feet of the lower rock 

is shown, the sandstone seems to dip only from 1° to 2°, in a direction 

Bull. 145 7
98 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

70° east of north. Such apparent dips are probably only the faces of 

particular masses in the sandstone.

The rock of the quarry is sandstone of the usual kind, showing much 

variation in composition and texture. The normal color is gray, often 

very light-gray, but many of the usual irregular markings and blotches 

of yellowish-brown and reddish colors occur. As a rule, the rock is 

pretty firmly consolidated, and is a coarse grit. Toward the top of the 

qfiarry considerable lignite occurs scattered confusedly in the sandstone. This is formed of the trunks and limbs of coniferous trees, 

broken into rather small fragments. These occur in pockets, and stand 

sometimes on end in various positions, as they were thrown down confusedly. Balls and streaks of tenacious gray clay occui; scattered 

irregularly and at different levels in the sandstone. These contain 

obscure fragments of plants.

The more firmly consolidated parts of the sandstone, as is commonly

Ti'u' 1 " .. :." :_v*-ir*r^-\fn,*jrfrt\4*-'

FIG. 11. Erosion horizons at the upper quarry on Aquia Greek.

the case elsewhere, occur in wedge-shaped masses and lenticular layers 

that interlock. The walls of the quarry show at several elevations 

erosion planes or horizons. The most important of these is near the 

top of the sandstone in the quarry. This is covered by 4 or 5 feet of 

a very coarse and current-bedded, highly ferruginous sandstone. This 

erosion horizon is a layer in the sandstone varying in thickness from 

6 to 48 inches. It has much lignite in fragments, a number of large 

balls and masses of clay, and many cobbles of quartz. One of the 

rounded clay masses lying on this horizon is 3£ feet in diameter.

Another horizon or plane of the same general character occurs at a 

lower level, but it has a curving course, so as to embrace, with the adjacent erosion planes, a wedge-shaped mass of the sandstone. This 

horizon also is marked by lignite, clay balls, and cobbles. In one place 

there is a cavity once occupied by the lignitic trunk of a tree which 

must have been 18 inches in diameter. The lignite of this tree has 

mostly wasted away. Other and shorter erosion horizons occur which
PONTAINE.J QUARRIES ON AQUIA CREEK. 99

also have a winding course, and, by their junction with adjoining ones, 

mark out on the walls of the quarry wedge-shaped and lenticular spaces. 

Fig. 11, gives a generalized representation of these erosion planes.

In some parts of the sandstone small clay pellets occur in such numbers as to make up the larger portion of the material. In the upper 

part of the quarry several thin partings of pale-reddish shale occur. 

These show a few recognizable imprints of plants. These layers are 

mere parting films in the sandstone and soon disappear.

The character of the Potomac material here down to the level of the 

creek does not vary essentially. At the foot of the hill in which the 

quarry is situated the sandstone is mostly gray, but has many seams 

and spots of colors from iron oxide. The material is here not so predominantly consolidated as at the quarry. It is composed of friable, 

argillaceous, and fine-grained layers, alternating with coarse and firm 

ones. Some large balls of sandy gray clay are embedded in the sandstone. One was seen having dimensions 5 by 10 by 12 inches.

EXPOSURE ON THE TELEGRAPH ROAD AT AQUIA RUN.

Immediately south of Aquia Bun (which is the upper part of the 

stream known in its tidal portion as Aquia Creek), on the Telegraph 

road, there is a tolerably good exposure of the Potomac. The material 

here differs from the typical sand and sandstone of the Potomac in its 

more eastern exposures, for it has a larger proportion of colored clay, 

and the deposits are finer-grained and more highly colored. The following is an approximate section made in ascending the road from the 

run:

Section of the Potomac formation along the Telegraph road south of Aquia Creek.

Feet.

1. Laminated gray and pale-reddish sandy clay................ 6

2. Greenish-gray mottled sandy clay........................... 15

3. Very variable mottled sandy clay........................... 30

4. Cobblestone bed, partly exposed (seen)..........'........... 2

5. Sandstone (top only seen)..................................

No. 5 shows its top only partially, about 20 feet above the level of 

the run. It is of the usual variable, coarse, gray kind.

The cobblestone bed, No. 47 owing to the waste of its outcrop, was not 

well enough exposed to show its true thickness. It contains quartz 

cobbles and subangular fragments of pre-Oambrian rocks.

No. 3 is a thick mass of crumbling, argillaceous matter that is not 

exposed throughout its whole extent. Toward its base it is dark purplish gray. A little higher up it is more purely gray in color. Portions 

of it are mottled with yellowish-brown and white colors. An interval 

of about 4 feet- is concealed. The rock is then marbled in the same 

manner as the mottled sandy clay of Hazel Kun, near Fredericksburg. 

This portion is 5 feet thick and is quite sandy. It graduates above 

into No. 2.

No. 2 is a sandy clay, predominantly greenish-gray in color. In some
100 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

parts it is mottled with yellowish-brown; in other portions large nests 

and pockets have this tint in solid mass. The material is fine in texture; as a rule, but shows some lenticular masses with a more sandy 

composition and coarser texture than usual, and' these tend to pass into 

the typical Potomac sand. These masses have a light-gray color, 

mottled with yellowish-brown.

No. 1 in its lower portion, which is 3 feet thick, is a well laminated 

fine sandy clay. The color is uniformly gray. The upper 3 feet of the 

bed, however, differs in character, for it is a pale-reddish well laminated 

sandy clay of very fine texture, that has many small fragments of plants. 

It differs in no respect from the reddish clay so common in the Potomac 

group. The fragments of plants, however, are not recognizable.

EXPOSURES AT QUANTICO.

The Potomac sandstone is well exposed in Shipping Point, at the 

entrance of Quantico Creek into the Potomac Eiver. It shows the 

usual features possessed by the sandstone. Current-bedding, clay balls, 

and cobblestones are prominent in it, and the structure of interlocking, 

lenticular layers occurs. From this point as far as Fort Washington 

the Potomac formation is visible on the Potomac Eiver, except in the 

places where it is replaced by the Pleistocene.

The distance by the Telegraph road from Aquia Creek to Dumfries, 

on Quantico Bun, is about 11 miles. Throughout this distance the road 

keeps near the western margin of the formation, whereas the mouth of 

that run is near the eastern margin. The material along the road in 

this interval differs considerably from that exposed along the river, 

being more argillaceous and highly colored and of finer grain. In fact, 

the section given on this road at Aquia Creek would fairly well represent most of the exposed material along the Telegraph road. The formation shows in its upper portion various mottled and incoherent sandy 

clays and highly argillaceous sands. They are imperfectly displayed, 

owing to the great erosion of the outcrops. The lowest material seen 

is the coarse, variable sandstone, which in this quarter is a sand rather 

than a sandstone. The most abundant material, forming the higher 

parts of the exposures, is an incoherent fine sand, with varying amounts 

of clay, sometimes approaching a pure clay. This is mottled and banded 

with various shades of red irregularly arranged in the predominant 

gray color. In this interval the Telegraph road runs parallel with the 

Potomac Kiver and about 5 miles west of it, the Potomac formation 

making all the surface rock shown in this space. The western border 

of the formation is generally located from 1 to 2 miles west of the 

road.

In the vicinity of Dumfries, and south of it, the Potomac formation 

is mainly composed of a very light-gray to white, friable, loose sand with 

much diffused kaolin. About 80 feet of this is shown, and it extends 

for about 2 miles west of the village. It quite commonly contains small
FONTAINE.] EXPOSURES AT QUANTICO. 101

clay balls and scattered small stones. Toward the base some bands of 

purplish clay, and balls of the same material, occur in it. This Potomac 

material west of the village and up to it forms the upper portion of the 

hills.

In approaching the village from the west, and in descending to the 

level of Quantico Run, on which Dumfries is situated, we pass below 

the Potomac formation and come upon the crystalline rocks on which it 

rests. These are exposed in the village and for some distance.(about 50 

feet) up in the hills. They are laid bare by the run, which has cut 

down some distance into them.

This crystalline rock is quite different from that which forms the 

basis and the western margin of the Potomac south of this point, as at 

Fredericksburg and at Potomac Run. At Dumfries the crystalline 

rock is a dark-gray to bluish-gray slate. It is lustrous and has a very 

fine grain, and it contains a great deal of quartz, in nests, pockets, and

irregular veins. It is very much like the teal Paleozoic slates. There

is a great contrast between the nearly white Potomac sand and the 

clay which results from, the decay of this slate. This indicates that 

the sandy portion of the Potomac must have gotten its material from 

some other source than this slate. Slate of similar character bounds 

the Potomac on the west, and serves as its foundation, as far north, 

certainly, as Alexandria.

On the northern edge of Dumfries, mottled sandy clay, like that so 

abundant between that place and Aquia Creek, may be seen resting 

immediately on the slates and showing a thickness of 15 feet or more. 

It replaces the white, crumbling sand seen farther west, and seems to 

be simply a modification of that material, and not a distinct formation. 

A little north of the village the white sand comes in again, and upon 

this, or at least at a higher level, various mottled sandy clays appear 

in force on the road toward Neabsco Run. This sandy material is like 

that seen on the road before reaching Dumfries 5 that is, south and 

west of that village. It is gray, and mottled with reddish colors. la 

some places large patches are colored solidly reddish. Some of it is 

nearly pure clay, and is so tenacious in wet weather as to make it difficult for one to force one's way through it.

Neabsco Run is 5 miles north of Dumfries. TKe Telegraph road 

crosses all streams emptying into the Potomac above the head of tide; 

that is, over the u run" portion of them. The following section was 

taken along the Telegraph road on the south side of the run. 'The 

road here is about half a mile from the western margin of the formation:

Section on Neabsco Bun.

Foot.

1. Mottled greenish-gray sandy clay ........................... 10

2. Mottled gray incoherent sandstone .......................... 15

3. Concealed ground ........................................... 6

4. Coarse gray crumbling sandstone .............................. 10

5. Slate breccia and cobblestones in disintegrated sandstone, 

partially exposed; thickness unknown.
102 THE POTOMAC FOEMATION IN VIRGINIA. [BULL.WS.

The deposit No. 5 is an unusual one on account of the large proportion of slate fragments in it; their sound condition, and their angular 

form. It is visible at several localities in this neighborhood, but 

nowhere else. It occurs about 20 feet above the water of the streams 

and is mainly a mass of angular slate fragments, large, rounded, quartz 

cobbles, and subaugular masses of gneiss, mica-schist, and other distinctly crystallized pre-Cambrian rock. These are all packed in coarse, 

sandy matter. The slate comes from that bordering the Potomac on 

the west. All the outcrops showing this stratum were much worn and 

broken down by erosion. Owing to this it was not possible to determine positively the relations of this material to the Potomac formation. 

It may be of quite recent origin, the material being brought down by 

the present runs. In one place, however, it occurs remote from any 

run. On Neabsco Eun the quartz stones range in diameter from 3 to 

12 inches, the latter size being rare. Some large rounded stones of 

Potomac sandstone are found. The slate fragments are very numerous 

and mostly small. Subangular masses of siliceous argillite, evidently 

coming from the slate belt, may be seen, having the dimensions 2 by 2£ 

feet. Subangular masses of quartz from 2 to 3 feet- in diameter may 

occasionally be found. One Subangular mass of a hornblendic granitoid rock, having the dimensions 2 by 2£ feet, was present.

No. 4, and indeed all the sandy matter from this point to Alexandria 

and beyond, is, in outcrop at least, an incoherent sand. In going north 

from Stafford County there seems to be a change in the consolidation 

of the Potomac sand, firm sandstone being no longer found anywhere. 

The material of No. 4 is a coarse, gray sand, but this is banded with 

yellowish-brown streaks that are formed of sandy clay and contain 

some balls of sandy clay.

No. 2 is a massive sand of gray color, but its contains irregularly 

shaped masses colored yellowish-brown, and toward the top embeds 

some clay balls,

No. 1 is mainly a greenish-gray sandy clay inottled with bright 

tints of reddish and brownish colors. In this material there are 

embedded spots of a yellowish, highly ferruginous sand.

EXPOSURE ON POWELLS BUN.

The Telegraph road crosses Powells Run 1~£ miles south of Neabsco 

Eun. A partial section was made along this road on the south side of 

the run ascending from it. On the north side of the run and about 

20 feet above it there is a slight exposure of the slate breccia with 

quartz cobbles. The finer matrix here is a mottled sandy clay. On 

the south side of the run the material to the height of 50 feet above 

the run is mostly hidden. It seems to be all a mottled sandy clay or 

argillaceous sand. The color of the greater portion is dark gray, but 

this is mottled freely with yellowish-brown, and considerable portions.
KONTAINE.] SECTION ON POWELLS EUN. 103

irregular in shape, are colored solidly yelloAvish-brown. Fifty feet 

above the level of the run the exposures, beginning in a gully, are good 

and nearly continuous to the top of the hill. 

The exposed material is as follows:

Section on Powells Run.

Feet.

1. Variable and variegated sandy clay .......................... 30

2. Cobblestone layer \vith clay masses................ 2 inches to 6

3. Greenish-gray clay .......................................... 1

4. Mottled gray, pebbly sand, thickness unknown.

No. 1 is composed of very variable material. In some places rounded 

pockets and nests of quite pure limonite are-embedded in the mass, 

seemingly formed by concretionary action. The ore is nodular and in 

crusts. About 4 feet above the base the material is a nearly pure 

clay. It is here pretty well laminated, has a dull reddish-brown to 

liver color, and contains poorly preserved fragments of plants. This 

portion is from 2 to 3 feet thick. The material lying over this, farther 

up in No. 1, is an argillaceous sand of uneven or lumpy structure. It is 

mainly gray in color, but large irregular portions of it have a yellowishbrown color. In some spots and layers the colors are reddish-brown 

and pinkish- or greenish-gray. Near the top of the mass there is a 

brick-red claystone, marbled with a little gray. Associated with this, 

and lying immediately under it. there appears what seems to be a local 

layer of gray sandstone 6 feet thick.

No. 2 is composed of balls and masses of greenish-gray clay and of 

cobblestones packed in a coarse sand like that of No. 4. The cobbles 

and clay balls are arranged in lenticular layers that thicken, and then 

thin nearly out. Some of tbe clay masses are over 2 feet in diameter. 

Most of the cobbles are composed of quartz and are well rounded; 

some, however, are formed of a much decayed felsitic rock or of argillite. The original nature of the parent rock could not be made out 

with certainty, as the material was too much decomposed. Some of 

the largest cobbles are from 8 to 10 inches in diameter.

No. 3 is a mere remnant left uneroded of some bed that was once 

much thicker. In some places it has been entirely removed. It is a 

fine-grained, well-laminated, greenish-gray clay. This bed has apparently furnished by its erosion "the clay balls found in No. 2.

No. 4 has its lower portion concealed, and hence its thickness is not 

known. It seems to be the usual coarse and current-bedded rock 

of the lower part of the Potomac. It is seamed and blotched with 

yellowish-brown and reddish colors. This material is full of small 

quartz pebbles that are scattered irregularly in its mass.

The exposures described along the Telegraph road are all near the 

western margin of the Potomac formation. Those next to be noticed 

lie near the eastern margin.
104 THE POTOMAC FORMATION IN VIRGINIA. [B0LL.145.

EXPOSURES ALONG THE RAILROAD FROM POWELLS CREEK TO

COCKPIT POINT.

The Potomac formation is well exposed from Powells Creek to 

Cockpit Point, along the line of the Alexandria and Fredericksbnrg 

Kailroad, and is shown in the cuts. This railroad forms a portion of 

the Atlantic Coast Line Eailroad, and extends from Quantico to Alexandria. It is the only railroad running in the Potomac formation 

between these points. Its location for a short distance south of Powells Creek is near the Potomac River, and then as far as Quantico it 

runs immediately on the banks of that stream.

The Potomac material in the series of exposures made along the 

railroad, in the space between Quantico and Powells Creek, is very 

variable, but a loose, light-gray to white sand predominates. In most 

respects this is identical in nature with the typical Potomac sand. It 

has, however, rather more diffused kaolin than usual, and is quite 

incoherent.

In the cut nearest to Powells Creek the sand shows a maximum 

thickness of about 30 feet. It is white and crumbling. Near the top 

there are some thin, lenticular layers of pale greenish-gray clay, and 

some lenticular beds of pebbles of small size. Some pebbles are mixed 

with the sandy clay even. The upper portion of the sand, as followed 

successively in the cut, shows pretty constantly some lenticular clay 

beds, and sometimes these layers unite and form beds of considerable 

horizontal extent.

A few hundred yards farther south, in another cut, the sand is not so 

white. It now is a gray, sometimes a dark-gray, material. It is also 

more ferruginous, and shows a number of irregularly shaped portions 

that are colored yellowish-brown. A sandy clay layer, lenticular in 

shape, and having a maximum thickness of 2£ feet, now comes in at the 

base of the mass. It has the character of the mottled, sandy clay, 

being pale greenish-gray in color, mottled with yellowish-brown in an 

irregular manner. It is a redeposited and receniented clay, and has 

small pebbles scattered through it. At its top there is a pebble layer 3 

to 4 inches thick. These layers of sandy clay, occurring at both top and 

bottom of the cuts, seem to represent erosion horizons. The sand 

under the lower layer of sandy clay and gravel has occasional clay 

balls embedded in it which attain sometimes the diameter of 8 or 10 

inches. The sandy clay which appears in the upper part of the cut 

here is a lenticular plate with a maximum thickness of 12 inches. It 

is a well-laminated gray clay with obscure impressions and fragments 

of plants.

A little south of this spot, and on the horizon of the lower gravel and 

the sandy clay, we find a lenticular bed of gravel, packed in coarse and 

current-bedded sand. It seems to be a local expansion of the gravel 

layer over the lower, sandy clay. For a long distance, this lower
FONTAINE.] FROM POWELLS CREEK TO COCKPIT POINT. 105

horizon of sandy clay and gravel seems to be one indicating the action 

of tumultuous waters, for the gravel last spoken of passes into a layer of 

sand varying in thickness from 1 foot to 3 feet. This is full of rolled 

fragments and balls of greenish-gray clay and gravel. This condition 

of things continues for several hundred yards.

The sand above this erosion horizon is massive, and contains occasional cakes and balls of gray clay. Some of these have an angular 

shape and are 10 to 18 inches on the exposed face. Some quartz stones 

on the erosion horizon are 5 inches in diameter. The massive sand has 

in many places no trace of structure planes, and it looks like a mass of 

agglutinated sand.

At one place the upper part of the mass of sand shows a plate of 

well-laminated pale-reddish clay, 12 inches thick and 20 feet long. 

This is embedded in the sand and has both ends cut squarely off, as 

is shown in fig. 12. It has some obscure plant impressions.

A little south of this, and in a cut at Cherry Hill Station, the lower 

layer of gravel and recemented clay is capped by a plate of sandstone 

6 to 7 inches thick. This is'covered by a lenticular layer of laminated 

dark-gray clay, 2 to 3 inches thick, that contains a few well-preserved 

plant impressions, the ______ ______ 

most common being coni ferous twigs. For 

three-fourths of a mile 

south of Cherry Hill 

Station, where the railroad first strikes the FIG. 12. Eroded clay layer near Powells Creek.

banks of the Potomac

Eiver, the cuttings are slight and show only from 6 to 10 feet of the 

formation. For half a mile south of the station none of the typical 

sand is visible, but all the Potomac matter in sight is a greenish-gray 

sandy clay, from 4 to 6 feet thick, with the base not visible. The color 

and the composition of this material, so far as seen, appear to be quite 

uniform. In some portions, however, the material is decidedly green, 

and the tint is brighter than any seen elsewhere in the formation. Some 

of the argillaceous matter is laminated, but most of it, in the outcrops 

at least, is lumpy and disintegrates into small angular particles. After 

passing over the space of about half a mile, some brick-red color begins. 

to shows itself in the predominant gray, appearing in very irregular 

streaks and patches, and producing a mottling; This, then, is a variety 

of the mottled sandy clay.. From this point for some distance to the 

south, the cuts are deeper and show more of the formation. The sand, 

so common in the Potomac, appears over the mottled sandy clay, this 

last graduating into it and not forming a distinct stratum. This sand 

is coarse and predominantly gray in color, but it has yellowish and 

yellowish-brown colors and occurs in irregular masses arid pockets. 

Some, of it is laminated locally, although most of it is massive.
106 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

A little farther south, and near Cockpit Point, a cut in the face of a 

small bluff indicates that the greenish sandy clay has disappeared, 

apparently from the thickening of 'the overlying sand. The lowest 

material seen here is the typical current-bedded coarse sand. This 

shows about 10 feet. Its top is eroded and is covered by a layer, 4£ 

feet thick, of very large cobbles, with occasional large masses of subangular quartz in its base. The cobbles are of quartz and many of 

them are 8 inches in diameter. One of the subangular quartz masses 

measured 8 by 12 by 18 inches.

In one place between the cobble bed and the underlying sand there 

is a small remnant left uneroded of a stratum of pale reddish-brown 

clay. This, in its thickest portion, is 2 feet through, and it contains 

some fragments of plants. This and all below it are Potomac, but the 

cobble bed and all above it are probably Pleistocene. Over the cobble 

bed comes.a brownish and yellowish-gray imbedded sand with several 

layers of cobbles in it. This is 4 to 5 feet thick. It is the highest 

stratum exposed here.

Two hundred yards south of this spot is'a high bluff cut through by 

the railroad. As it is situated near Cockpit Point, the exposure there 

may be identified by this name. Cockpit Point, like many other points 

on the river, is a low marshy tongue of laud extending out into the 

river.

EXPOSURES AT COCKPIT POINT.

The main cut at Cockpit Point is the most northern one. It shows 

a considerable vertical height of material. Several cuts farther south 

give continuations of the horizons exposed in the main cut, and show 

very important modifications in the material, illustrating well the 

extreme changeableness of the Potomac strata. In these cuts the 

level of the railroad track is 12 feet above the river and the material 

occupying this interval is not fully exposed. It seems, however, to be 

all composed of the usual current-bedded sand.

The material exposed above the railroad track is so variable that a 

section taken at one point can not be considered as representing the 

condition of things even in the same cut a few yards away. At the 

apex of the curve made in cutting around the principal bluff and in 

the main cut, the following materials are shown, in descending order:

, Section in railroad cut at Cock}nt Point.

Feet.

1. Sandy clay, brownish and imbedded ....................... 3+

2. Brownish-yellow sand and gravel .......^.................. 5

3. Grayish stratified sand .................................... 5

4. Incoherent brownish to yellowish-gray sand ............ 2 to 3

5. Light-gray coarse and massive sand........................ 20

6. Ferruginous sand.......................................... |

7. Greenish sandy clay varying to sand....................... 10

No. 1 forms the top of the nearly vertical face left in the cut. Much 

of it has evidently been removed by erosion. . It is a loose sandy clay
FONTATNK.] SECTION AT COCKPIT POINT. 107

of brownish-gray color,, with distinct stratification planes, has small 

stones scattered though it, and appears to be Pleistocene.

No. 2 is composed mostly of small gravel packed in brownish-yellow 

sand. The material is well stratified, and the stratum is composed of 

alternating layers, some full of stones and others almost free from them. 

In the central part of the stratum there is a conspicuous layer, containing very large cobbles that contrast strikingly in size with the adjacent 

gravel. All of these stones are of quartz, the largest of them attaining the diameter of 12 inches.

No. 3 is well stratified. The greater portion of the material is gray 

in color, but the stratification planes are marked out by yellowishbrown colors so as to divide the bed by colored bands. The material 

of this bed is fine and somewhat argillaceous. It and all the strata 

above seem to be Pleistocene. All those below are certainly Potomac.

No. 4 appears to be the highest bed of the Potomac formation. It 

seems to thicken when traced horizontally, and to partially cut out No. 

5. This is not due to erosion, but to the graduation of the one kind of 

material into the other. No. 4 thickens rapidly southward at the 

expense of No. 5, for it becomes 10 to 15 feet thick and more. It contains at the locality now being described, in its middle, a layer about 1 

foot thick that is full of large cobbles. Some of these cobbles are 12 

inches in diameter, and they are all of quartz, as are all the larger stones 

in this exposure. The brownish and yellowish colors of this bed are 

mingled in a very irregular manner, and the material is much more incoherent than that of No. 5. It has the current-bedded structure and 

interlocking lenticular layers so common in the Potomac formation.

No. 5 is the most important member in the cut. It is for the most 

part composed of sand of the normal type. This is arranged in massive and interlocking wedges and lenticular layers. It is in places a 

sandstone, being firm and coherent. It tends to thin out north and 

south, and to give place to an incoherent sand. The change takes place 

to the south in the space of a few yards. Two or three feet from the 

top of this stratum, there are some lenticular beds of pebbles, arranged 

on the same horizon along the face of the cut, together with clay balls. 

They continue beyond the horizontal limits of No. 5. Embedded in the 

top of No. 5, and extending beyond it, we find a number of clay balls 

and clay masses, from 12 to 36 inches in diameter. These form a definite horizon of clay balls.

In and near the base of No. 5 is a remarkable exhibition of masses 

of rolled clay. We find here clay balls and huge masses of clay 

embedded in the coarse sand. They are generally associated with 

gravel and cobblestones, forming a well-marked erosion horizon. The 

original clay bed from which this clay was eroded seems to have been 

entirely destroyed, for no bed of clay like it is visible. Some of the 

clay balls of smallest size, 1 foot or 2 feet in diameter, are composed of 

pale-reddish clay and contain some indistinct fragments of plants. The 

large rolled masses are sometimes 4 or 5 feet in diameter. They are
108 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

composed mostly of pale reddish-brown clay,, and are in places full of 

fragmentary rootlets. These larger masses are found at the very base 

of No. 5, where it joins No. 6. For the space of 4 or 5 feet above the 

base of No. 5 many smaller clay balls and numerous cobbles occur. 

The cobbles are found scattered, and also, in the lower part of No. 5, 

aggregated into detached lenticular layers.

No. G is a local lenticular plate that serves as a capping to No. 7. It 

soon thins out and disappears. It seems to be merely a portion of the 

top of No. 7 which has been cemented by iron oxide. In. some places 

it has enough iron to make it a sandy ore of that metal.

No. 7 is the lowest material of the Potomac that is fully exposed at 

this place. It is essentially the same material as the greenish-gray 

sandy clay seen toward Cherry Hill, and is on the same horizon with 

that. It is a variety of the mottled sandy clay so common in the 

Potomac. It is here, however, more sandy and variable in color. It 

has a predominant greenish-gray color, but shows much red and reddish 

brown, arranged in irregular spots, blotches, and films. In some cases 

a marbling of the colors occurs, so minute as to be shown even in hand 

specimens, but in the greater portion of the material the colors are 

found in considerable masses. The material of No. 7, when dry, 

appears as a firm sandy claystone, with no structure planes.

The above description applies to the material seen in the apex of the 

main cut. In the next cut a few yards to the south, No. 7 has thickened to 13 feet, the top is more sandy and more firmly consolidated 

than in the main cut, and the larger portion of the bed is a firm sandy 

claystone of a pale greenish color.

No. 6 in this cut, which we may call cut No. 2, is ,2 feet thick, and 

is a firm sandstone, being the only firm rock above No. 7. Here it is 

nearly free from iron. Over No. G come 12 feet of soft crumbling white 

sand, with some clay balls in the lower portion, and some bands of

cobbles. This, the representative of No. 5 of the main cut, shows none 

of the consolidation seen in that, and is much more uniform.

No. 4 is here 10 to 15 feet thick, having increased at the expense of 

the other strata. It extends to the top of the cut, which is lower here 

than in the main or most northern cut. It has the same character as 

in the section given for the main cut; some of the cobbles, however, 

are unusually large, being 24 inches in diameter.

A few yards farther south, and still in cut No. 2, stratum No. 7 is 

more than 20 feet thick, increasing at the expense of the overlying beds. 

Large irregular portions of it are colored bright brick-red or purplish 

red, the colored parts taking the form of nests and pockets in the 

greenish-gray material. Nos. 5'and G have both disappeared, and all 

the material overlying No. 7 is a brownish-gray sand, 20 feet thick. 

In the base of this, resting on No. 7, there is a cobblestone bed with 

clay balls. When examined closely, this brownish sand, as it seems to 

be when seen at a little distance, is found to be a coarse gray sand,
FONTAINE.J SECTION AT COCKPIT POINT. 109

with so many minute films and spots colored brownish-yellow, and so 

intimately mixed with the gray material, that the general effect of the 

mixture is to produce a brownish-gray color. This sand contains 

numerous thin shor,t layers and balls of pale-reddish clay, so arranged 

as to form more or less continuous zones and layers that are made up 

mostly of this clay. The clay is plainly eroded from some older clay 

bed and redeposited here.

In the next cut, 30 yards south of the last, which may be called cut 

No. 3 and which has slight depth, No. 7 may still be traced, but its 

central portion has become indurated to some extent, and changed to 

the whitish sand so common in this portion of the Potomac area. This 

part is 10 to 12 feet thick. It has above and below it two layers with 

mottled colors, each 2 or 3 feet thick and both belonging to the bottom 

bed,' or No. 7. The upper one is a gray clay stone with brown colors in 

irregular patches and spots. The lower one, with its base not visible,

is the usual mottled sandy clay.

Over No. 7 in this cut comes 12 to 20 feet of the brownish-gray sand 

described as seen in cut No. 2. Its character here is unchanged, except 

that it has a number of short seams of small pebbles, and much of the 

transported clay is in the form of numerous small scattered balls.

LIG-NITB DEPOSIT NEAR NEABSCO RUN.

This deposit is situated on the western margin of the Potomac formation about three-fourths of a mile west of the Telegraph road where it 

crosses Neabsco Run. The lignite occurs in seams of dark-gray and 

pale-reddish clay, embedded in the Potomac sand. The clay is very 

tenacious and plastic. It occurs in the original uneroded beds, which 

take the form of lenticular layers lying in coarse gray sand. This sand 

has many irregular patches and seams colored yellowish-brown. It 

rests on a dark-gray to bluish-gray slate, that forms the western boundary of the Potomac formation here. This is similar to the slate seen 

near Dumfries. The sand and clay combined give a thickness of about 

8 feet. The lignite is shown along the channel of a small stream for 

the space of about 40 yards. The clay contains nearly all of the 

lignite and all of the plant impressions found here. The clay layers 

occur very irregularly in the sand, and vary in thickness from 1 to 12 

inches. These layers extend through a vertical thickness of about 4 

feet." In them may be found a number of the flattened trunks of 

coniferous trees, now changed to lignite. This lignite has the same 

character as that found near Potomac Run, uniting the features of 

ordinary lignite and jet. Some of the trees must have been quite large. 

One trunk was seen having a maximum thickness of 7 inches, and a 

width of 42 inches. In the clay a few well-preserved impressions of 

ferns and coniferous leaves were found. The most important fossil 

plant occurring here is the fine cycad, Dioonites buchianus, which is 

so common at Dutch Gap, and which is of special interest, as. it is.
110 THE POTOMAC FORMATION IN VIRGINIA.

characteristic of the lower Cretaceous (Neocomian) strata of the Wernsdorf beds of Europe. This plant is so abundant in some portions of 

the clay that the leaves are matted together. They are preserved well 

enough to retain nearly all- their plant substance. Both here and at 

Dutch Gap, wherever this fossil is very abundant in a stratum, few 

other impressions are found. It is to be noted that this is the first 

place north of the James River where this was seen. As has been 

stated, it Avas much the most abundant plant on the James Eiver. 

While a very,large number of fossils were at Fredericksburg, Potornac 

Run, and near Brooke, no trace of this plant appeared. It was 

therefore surprising to find it in such numbers at this place. It is an 

important connecting link between the floras of the Petersburg and 

Frederick sburg areas.

The higher portion of the sand, at the spot now described, contains 

clay balls, cobbles, and remnants of clay layers left uneroded.

EXPOSURES ON THE COLCHESTER ROAD NEAR NEABSCO RUN.

Immediately after crossing ISTeabsco Run the Colchester road leaves 

the Telegraph road and turns northeast to the village of Colchester. 

This is situated on the north side of Occoquan River. The Telegraph 

road pursues a course more nearly north, and crosses the Occoquan 

2 miles above Colchester at the village of Occoquan. This latter road 

from Neabsco Run to Occoquan runs close to the western margin of 

the Potomac formation and usually a little east of it, consequently the 

Colchester road keeps well within it.

On the Colchester road, about three-fourths of a mile from its point 

of departure from the Telegraph road, there is an exposure of the 

Potomac worthy of mention. The formation may be seen in ascending 

a long hill on the southwest side, and also in descending the same on 

the northeast side.

Beginning on the southwest side the lowest material seen shows a 

thickness of about 20 feet. It is mainly a tolerably well laminated 

sandy clay, of ash-gray color, sometimes dark gray. This is mottled 

with irregular spots, seams, and films of yellowish-brown. In this 

material, the sand in which varies in amount, we find at different 

levels and confusedly distributed irregularly shaped partings and 

seams of a pale reddish-brown clay, which in some places is associated 

with and graduates into an ash-gray clay. These clay layers vary in 

thickness from a few inches to 3 feet. They are mostly lenticular in 

form, and thicken or thin in a very irregular manner. The color varies 

irregularly, for in one portion the layers are pale-reddish while in the 

adjacent parts they are ash-gray. Obscure fragments of leaves of 

plants and many small pieces of lignite occur in these clays, which 

are all redeposited matter. In one of these layers, where it is about 

3£ feet thick and more sandy than usual, there is a flattened log
FONTAINE.] . EXPOSURES NEAR NEABSCO RUN. Ill

of silicified wood. This is coniferous. It is 8 feet long, 2£ feet wide, 

and 6 inches thick, as now visible. It was once much longer.

On the eroded top of the above-described material comes a bed of 

cobbles from 3 to 4 feet thick. The stones are of quartz and some, 

of them are of large size. Above the cobble bed there is a mass of 

material about 50 feet thick. This is not well exposed on this portion 

of the road. It seems to be in the main a more or less argillaceous 

sand, usually colored some shade of gray, but much mottled with 

yellowish and brownish colors. The clay partings are much fewer in 

this than in the lowest material described above.

As seen on the road descending the hill on the northeast side the 

highest material exposed in the top of the hill is a mass about 40 feet 

thick, composed mostly of light-gray sand. Portions are beautifully 

seamed with yellowish-brown films. These mark the current-bedding 

planes and are abundant in the material. Near the top is an erosion 

horizon which is marked by a line of balls of gray and pale-reddish 

clay. Cobblestones also occur here, and uneroded remnants of a seam 

of pale-reddish clay, Clay balls and small stones are scattered through 

the mass of the material below the erosion horizon, and 40 feet from 

the top there are numbers of clay balls and some thin layers of palereddish and gray clay. The exposure is then imperfect to below the 

horizon of the lignitic material mentioned as seen on the southwest 

side of the hill. This lignite does not show itself on the northeast 

side of the hill. On this lower horizon all that was seen was a wellmarked clay-stone of a predominantly gray color that is marbled with 

brick-red.

EXPOSURES AT FREESTONE POINT.

The cut through Freestone Point is the most extensive one on the 

line of the Alexandria and Fredericksburg Eailroad. The cut is made 

through the promontory which projects into the Potornac Eiver between 

Powells and Neabsco creeks. The material exposed in this cut is a 

little north of the northern end of the exposures described before, and 

which extend from Powells Creek to Cockpit Point.' The railroad 

track in the cut descends slightly from south to north, so that somewhat lower strata are exposed in the northern than in the southern end 

of it. The material exposed in the cut is so variable and so confusedly 

mixed that no complete account can be given of it without tedious 

detail. I shall attempt to give only the leading features that are to be 

seen in going from south to north. Most of the matter seen is an incoherent, light-colored, coarse sand, with more or less diffused kaolin. It 

shows numerous alternations in composition, structure, and color.

In the southern end of the cut only 10 to 15 feet of rock can be seen, 

for the excavation here has.its least depth. All the material shown at 

this point is a gray sandy clay, with some layers of fine-grained, argillaceous sand. The matter is divided into tolerably distinct beds, which.
112 THE POTOMAC FORMATION IN VIRGINIA.. [BULL. 145.

seem to be horizontal. It is all much affected by current bedding, and 

is seamed irregularly with yellowish-brown films. In some places the 

more argillaceous layers assume the nature of. the mottled, sandy clay, 

which is a characteristic member of the Potomac formation. This is 

lumpy and massive in structure. Such is the character of the lowest 

portion seen, and this has many small pebbles and ferruginous partings.

Going farther into the cut, this material thickens rapidly, causing the 

top of it to appear to rise. Here the basal portion contains some clay 

balls, and under the mottled sandy clay, a coarse sand begins to appear. 

This latter is mainly dark gray, but contains numerous blotches of 

yellowish-brown. This sand has the usual structure of the coarse 

portions of the Potomac formation, being composed of interlocking 

lenticular layers. It also contains some lenticular beds of cobbles or 

pebbles; the largest being 3 inches in diameter. Many scattered pebbles also occur, most of them being of quartz, but some are composed 

of argillite and gneiss. The former are much decomposed. Some small 

clay balls occur with the pebbles. '

A little farther on in the cut large rolled masses of greenish-gray 

sandy clay occur in the sand. Some of these are as much as 6 feet in 

diameter. This sand, with its contents, does not seem to be a bed distinct from the overlying mottled sandy clay, but graduates into it. It 

thickens considerably in going north, raising the base of the mottled 

sandy clay higher and higher. In one place the basal sand shows several layers of pebbles, which are inclined at various angles to one 

another and have an undulating course. At this point there is, immediately over the sand, a lenticular layer of brownish-purple and greenish-gray clay 4 feet thick. The colors occur mixed together in irregular bands. Here the mottled sandy material that forms the upper part 

of the cut has graduated into a yellowish sand that is mottled with 

gray. This latter material toward the northern end of the cut passes 

through several minor modifications and finally at the northern end becomes a white or light-gray kaolinic sand. This extends from the top 

downward, so as to make up much the larger portion of the exposed 

material.

Tracing the basal pebbly sand farther north in the cut we find it 

graduating into a yellowish, mottled sand, and then into a pale greenish-gray sand. This latter contains small clay balls and pebbles. A 

little farther on it becomes mottled with yellowish and reddish colors. 

The colors are mingled confusedly, showing no particular arrangement.

A little beyond the point last mentioned the cut at its northern end 

is deepest, the material being here 75 feet thick. Here all the material 

exposed, except a stratum at the base, is a light-colored kaolinic sand. 

The basal stratum, which is the continuation of the basal sand described 

above, is here quite different. It is 3 or 4 feet thick, and is a greenishgray tenacious clay, the sand having disappeared. This passes horizontally into a layer 5 feet thick, composed of clay balls mixed with
FONTAINE.] EXPOSURES ON TELEGRAPH ROAD. 113

cobblestones. These are packed in a ferruginous sand. Some of tlie 

clay balls are 18 inches in diameter, and'are composed of greenish-gray 

clay, which appears to have been torn off from the clay layer that 

immediately precedes this bed. The overlying kaolinic sand shows 

occasional seams and blotches of yellow and contains a few clay balls, 

the largest of which are 12 inches in diameter.

TELEGRAPH ROAD FROM NEABSGO RUN TO OCCOQTJAN.

As was stated before, the Telegraph road from Neabsco Bun to the 

village of Occoquan, a distance of 5 miles, runs close to the western 

margin of tlie Potoinac area. The material exposed on it shows no 

features not seen near Neabsco Bun. The exposures are very poor, 

and extensive erosion is indicated in the Potoinac formation. The 

surface is mostly occupied by cobblestones and loose material of recent 

origin, derived from the Potomac formation. A great number of cobbles occur, mostly of quartz. They are often la,rge, a diameter of 8 

inches and more being not uncommon. Besides these, a number of 

large, subangular quartz masses occur. Some of these exceed 12 

inches in diameter. Occasional fragments of pre-Cambrian rock are 

found with these, but none of notable size.

About 2 miles from Neabsco Bun there is a small outcrop of Potoinac 

material containing a number of large balls of gray clay. At this 

point the slate that forms the western margin of the Potoinac is shown. 

It looks much like a metamorphosed form of the Primordial slates. 

From this point to near Occoquan there are frequent exposures of tlie 

Potomac sand on tlie tops of the hills and on tlie higher ground, while 

at the level of the streams and valleys the slates make their appearance.

About 1 mile south of Occoquan a stratum of pale-reddish, laminated clay, with seams of yellow, occurs in the Potomac sand. This, 

however, contains no plant fossils. In the top of the hill overlooking 

the river and in the village a thin remnant of the Potomac may be 

seen. It is composed of gray, argillaceous sand, variegated with 

yellowish-brown and reddish colors. In this vicinity the Potoinac lias 

been almost wholly removed by erosion and is visible only on the 

highest points. The depth to which the depressions have been cut 

since the deposition of this formation indicates for it a great age. In 

tlie immediate vicinity of the village of Occoqnan the thickness of 

fully 200 feet of pre-Oainbrian rock is exposed under the Potoinac, 

being laid bare by erosion in post-Potomac times. These pre-Cambrian 

rocks are no longer wholly slates, but in the lower portions rather 

coarse-grained and highly crystalline mica-schist and highly micaceous 

gneiss may be seen. On the north side of the Occoquan Biver, opposite the village of Occoquan, the high hills that overlook the river seem 

to be composed wholly of mica-schist.

From the village of Occoquan due east down the river to Woodbridge, where the railroad crosses the river, the distance is 2 miles. 

Bull. 145 8
114 THE POTOMAC FORMATION IN VIRGINIA. . [BULL. 145.

The station is on the opposite side of the river from the site of the 

former village of Colchester, where the Colchester road crosses the 

river. At Woodbridge the railroad approaches the bridge by a long 

cut. In this cut the Potomac formation may be seen resting on preCambrian slate similar to that which, at Dumfries and northward, 

forms its western boundary. The bottom of the Potomac formation is 

here about 40 feet above the river. The formation, in the interval of 

2 miles between this station and the village of Occoquan, has declined 

toward the east certainly as much as 160 feet. We may then conclude 

that the dip in this direction is as much as 80 feet per mile. The southern end of the cut has the least depth. The slates crop out here with 

no superposed Potomac. This slate contains some ledges of purplishbrown quartzite. A little farther within the cut, where it is deeper, 

we find about 2£ feet of Potomac on the slate. This is mostly a dark, 

slightly reddish shale, with some remnants of the sand which once 

inclosed it. The shale contains many small fragments of lignite and 

some recognizable leaf impressions. The most abundant of the imprints is that of Dioonltes bucliianus, the plant so common at Neabsco 

Eun. The dip of the shale seems to be to the northeast and to amount 

to 5 degrees. The shale a little farther north is covered by from 8 to 

10 feet of Potomac sand. On the top of the whole there is a bed of 

cobbles and loose, sandy matter, varying from 5 to 10 feet in thickness. 

This bed contains fragments of slate. It extends throughout the cut. 

Large masses of mica-schist and gneiss occur in its base, some of them 

more than 4 feet in diameter. The cobbles are of quartz, many of 

them exceeding 8 inches in diameter. Toward the northern end of the 

cut 4 feet and more, of a yellowish, sandy clay, well stratified, come in 

over the bed of cobbles. The lignitic shale seen at the southern end 

of the cut soon gives out, for it is a local lenticular layer composed of 

redeposited clay. Its place is taken by the overlying sand. In the

deepest part of the cut, near the northern end, the Potomac material 

is nearly all a brownish-gray sand. The lowest portion of this contains subangular masses of quartz, some of them 1 foot in diameter, 

and partly rounded masses of slate, mica-schist, and other pre-Cambrian rocks equally large. A little north of this point the pre-Cambrian 

slate shows itself in the bottom of the cut. The lowest Potomac resting upon this is a lenticular layer, about 2 feet thick, of gray clay full 

of fragments of lignite. Next above this is a stratum 12 inches thick of 

gray, sandy matter, with partings of yellowish clay, and on the top 

of this comes the brownish-gray sand with large clay balls. The basal 

lenticular bed here soon gives place to a stratum composed of a mass 

of small and large clay balls, mingled with some gravel. The brownishgray sand above this contains some scattered lignite and short 

patches of pale-reddish clay. One of the latter contains some good 

impressions of Dioonites. The old Colchester road, between Woodbridge and Keabsco Eun, north of the locality previously described as
FONTAINB.] EXPOSURES CN TELEGRAPH ROAD. 115

occurring on that road, affords at various points imperfect exposures 

of the Potomac. The material shown is mainly a loose, more or less 

argillaceous sand, ranging in color from gray to white. This shows, 

generally, a mottling from the mingling of yellowish-brown with the 

predominant gray.

TELEGRAPH ROAD FROM OCCOQUAN TO TELEGRAPH. STATION.1

The Telegraph road, after crossing the Occoquan Biver at the village 

of that name, ascends more than 200 feet to the general level of the 

country on the north side. The road passes over heavy masses of typical mica-schist until the summit of the hill is nearly reached. On this 

summit there is a thin covering of fragmental rock, which appears to be 

Potomac. Some 2 miles from the summit the road turns more directly 

east. Before making this bend it runs toward the western margin of 

the Potomac, and only remnants of the formation can be seen. There 

are along here marks of extensive erosion in the formation. The surface shows the loose, unbedded, sandy materials with abundant cobbles 

that are common where the Potomac has undergone much erosion. The 

cobbles are often large, and the fragmental rock in place is imperfectly 

exposed. It is of the usual kind, mainly a gray sandy clay or sand, 

often mottled with yellowish-brown. The cobbles occur sometimes in 

heavy beds that seem to be associated with the mottled sands.

The eastward turn of the road takes it in the distance of 2 miles to 

Telegraph Station, a point on the Alexandria and Fredericksburg Kailroad near Pohick Bun, 3 miles north of Woodbridge. In approaching 

this station the road penetrates farther and farther within the Potomac 

area, so that near that place all the surface rock is Potomac. This is 

well displayed in the railroad cuts north and south of the station. There 

is no important change in the character of the material exposed as the 

station is approached. It becomes coarser and more sandy, taking on 

more of the typical character of the Potomac. Some of the cobbles seen 

before the station was reached are 12 inches in diameter. All are of 

quartz.

A number of cuts on the railroad in the vicinity of Telegraph Station 

give good exposures of the Potomac. An examination of them was 

made both to the north and to the south of this place.

EXPOSURES BETWEEN TELEGRAPH STATION (LORTON) AND WOODBRIDGE.

A few yards north of Telegraph Station there is a small cut that 

shows about 20 feet of nearly white sand. This is arranged in the usual 

interlocking lenticular layers, and shows the commonly occurring irregular patches of yellowish-brown color. The upper 10 feet of the material contains balls of both pale-reddish and gray clay. Most of the 

balls are small, the largest attaining the diameter of about 10 inches.

1 This station is now named Lorton.
116 THE POTOMAC FORMATION IN VIRGINIA. [DULL. 115.

Small stone's of quartz also occur. The stones and clay balls are found 

both scattered and showing a tendency to aggregate into layers, since 

portions of the sand that are lenticular or pocket-like in shape contain 

them more thickly than usual. Some small slate .fragments are scattered through tbe mass. A cobblestone bed caps the whole, but this is 

better displayed in the cutting next to be described.

A small side cutting at the station, 30 yards south of the cut above 

described, shows the following features: The basal portion is composed 

of sand of the same nature as that just described. This shows a thickness of about 8 feet, and is capped by a cobblestone bed varying in 

thickness from 1£ to 3 feet. This is composed of layers, lenticular in 

shape, that are full of cobbles. These interlock with others, in which 

the material is mainly sand. In one spot from 1 foot to 2 feet of the 

normal sand occurs above the cobble bed, showing that this latter is a 

member of the Potomac formation. A large number of clay balls occur 

mingled with the cobbles, some of them being 10 inches in diameter. 

Many of the cobbles are from 3 to 5 inches in diameter, the largest not 

being fully rounded. No Cambrian cobbles were seen. Most of the 

stones are composed of quartz, but some of them are formed of crystalline pre-Cambriau rocks, and these are much decayed. Some of 

the pre-Cainbrian material seems to have been argillite, and some 

fragments of gneiss and mica-schist are found. The latter is similar 

to that seen at Occoquan. It is highly probable that the numerous 

surface cobbles that occur in this region came from the erosion of this 

or a similar cobblestone bed in the Potomac.

In another shallow cut, that is found a short distance south of the 

station, only the lower portion of the sand can be seen, and this contains some short lenticular layers of gray clay and a few balls of the 

same, some of them 1 foot in diameter.

Half a mile south of the station there is a long cut. This, where 

deepest, shows as much as 40 feet of the material. Here the lower 20 

feet is formed of current-bedded gray sand, which contains scattered 

clay balls, and some subangular masses of quartz that are sometimes 

8 inches in diameter. Over this comes a stratum of gray sandy clay, 

which at its northern end is single and 2 feet thick. Toward the south 

it splits into several layers that are interstratifled with more sandy 

matter. The whole is here 5 feet thick. This is capped by 8 feet of 

sand similar to the basal matter. In this there are layers of gravel, 

and a little farther south its upper portion is a solid mass of pebbles. 

Over the whole come the usual surface materials.

One mile south of this point the next cut occurs. In the interval no 

exposures are found. This cut is 25 feet deep, and shows the Potomac 

resting on slate of pre-Cainbrian age. This slate does not appear 

throughout the cut, but shows itself locally^ seeming to have had an 

irregular surface at the time of the deposition of the Potomac. Besting 

on the slate come 10 feet of alternations of clay, sandy clay, and fine
FONTAINE.] EXPOSURES NEAR OCCOQUAN. 117

argillaceous sand, all gray in color, but Laving, some thin layers of palereddish, sandy clay. Most of the clay and sandy clay is redeposited 

material, having been torn off from some older layer, then redeposited 

and cemented together, chiefly in flakes and small fragments. It has 

consequently a lumpy nature, and is without lamination and cleavage, 

but some of the clay seems to be remnants of the original layer, in the 

form of cakes and masses buried in the ruins of the original bed. This 

structure Avas disclosed in a search for plant impressions, a number of 

which were found in the yellowish clay. The plants are chiefly conifers 

and ferns, with a large number of specimens of Dioonites buchianus. 

The plant-bearing material has over it a cobblestone bed about 4 feet 

thick. This contains some clay balls, more than 1 foot in diameter. 

The uppermost portion of the cut is formed of brownish, sandy clay, 

about 6 feet thick at its maximum. 

The next cut to the south is only 100 yards distant from that last

described. Although so near, it is made wholly through pre-Cainbrian

slate, and at its maximum is 60 feet deep. The slate cleavage dips 

to the northwest at an angle of 80 degrees. No Potornac appears on 

the top of this material. The great height of this outcrop so near the 

Potomac in the last-described cut indicates that the surface of the 

slate, at the time of the deposition of the Potomac, must have been very 

irregular and deeply worn.

Beyond this cut the metamorphic rocks continue to the south for 

over half a mile. The ground is then, for about one fourth of a mile, 

concealed. We then find the Potomac again exposed. The cuts are 

now of small depth as far as Occoquan Riverj and do not show more 

 than 10 to 15 feet of material.

The first Potomac deposit met with after passing the cut of metamorphic or pre-Cainbrian slate belongs to the mottled sandy-clay type, 

and material of the same nature extends to the Occoquan. This first 

exposure is a brownish sand, graduating into yellowish sand. Frequent specks and spots of gray clay are dispersed through this sand, 

apparently coming from redeposited fragments. This dispersed clay 

gives the brownish sand a mottled nature. Pebbles and clay balls of 

small size occur all through the mass. The clay sometimes lies in tortuous, thin layers, which have no particular position and direction. 

In one part of this cut the base of the material exposed is a stratum 3 

feet thick of greenish-gray clay, mottled with reddish-brown.

The next cut, 200 yards south of the last, shows at most 15 feet of 

typical mottled and marbled sandy clay. It has pockets of large stones 

scattered through it. This brings us to the immediate vicinity of the 

Occoquan. Here the cuts are still in the sandy mottled clay. In the 

long cut by which the approach OIL the north side of the river is made 

to the bridge some portions of the material exposed are brick-red in 

color, without any admixture of gray. In one portion of this cut about 

4 feet of the ordinary gray sand of the Potomac shows itself under the
118 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

mottled sandy clay. The latter at this point has embedded in it several masses of pre-Cambrian gneiss from 2 to 3 feet in diameter. One 

of these masses showed a character not possessed by any pre Cambrian 

rock seen in place in this region. It is a coarse granitoid gneiss, with 

a large amount of coarsely crystallized red feldspar. This material is 

probably Pleistocene.

EXPOSURES IN CUT ON POHICK KUN.

Immediately after crossing Pohick Run the railroad passes northward through a long and moderately deep cut. This is about half a 

mile north of Telegraph Station. At its southern end it shows from 

the bottom for about 10 feet a gray current-bedded sand of the usual 

kind. This contains at its base in one place a mass of clay balls and 

cobblestones 4 feet thick. Many of the stones are from 4 to 5 inches in 

diameter. Some masses of pre Cambrian slate occur in this stratum 

with the cobbles. One of these shows the dimensions 5 by 9 inches on 

the exposed surface. This sand is the predominant and only constant 

material in the cut. It is sometimes very coarse and current-bedded, 

but is full of irregularly shaped layers, varying much in composition, 

that are more argillaceous than the coarse embedding sand. These 

layers sometimes assume the form of true clay. They are dispersed in 

the saud in the form of short lenticular beds. They are sometimes 

so much thickened that they make up a large part of the material 

exposed. Some of them extend horizontally a considerable distance, 

but they vary rapidly in composition and color. The shorter layers 

are mostly composed of a pale-reddish sandy clay. The longer layers 

are in some places dark-gray, but mostly show reddish or reddishbrown colors. The cut in its deepest part does not show more than 

12 or 15 feet of the Potomac material. The clay layers mentioned 

above occur in the middle and lower part of this material. The upper 

portion of it is a coarse, current-bedded grit, full of small stones. All

through the cut cobbles and clay balls are sparingly dispersed through 

the sand.

At the southern entrance of the cut the 10 feet of sand shown has 

on its top masses and partly torn-up layers of clay. Toward the 

northern end of the cut a stratum occurs that shows a great degree 

of variability. Its shape is typical of many occurrences of the clay 

layers in widely separated places. Beginning as a single stratum, it is 

soon divided for a short distance into two layers. This is done by the 

insertion of a lenticular sheet of sand 3 inches thick. The lower member of the divided layer is 7 inches thick and the upper one 10. The 

two unite to the north, forming again one stratum. The material is 

up to this point a reddish clay. The united stratum may be traced for 

some distance, declining to the northeast. In this part of its course 

it shows many and rapid changes. At first it is 4 feet thick, the lower 

2 feet being a very dark-gray tenacious clay (black when wet). The
EXPOSURES ON POHICK RUN. -

upper 2 feet is composed of reddish and yellowish clay. Traced farther 

on to the north, all of the material becomes pale-reddish. In one place 

where the top and bottom differ it thickens to more than 5 feet and 

extends to the bottom of the cut on one side of it, while on the opposite 

side it is only 2 feet thick and rests on a sand which is full of gravel.

At the point mentioned above, where the clay layer is divided by the 

3 inches of sand, we find immediately under it 18 inches of currentbedded gray sand, mottled with yellow. At the base of the cut below 

this last come 3 feet of sand, full of cobbjes, clay balls, and fragments 

of pre-Cambrian slate. Toward the northern end of the cut the variable clay bed rests immediately on the coarse material. Here the 

basal stratum of coarse matter is full of cobbles of quartz and rounded 

stones of pre Cambrian rock, such as gneiss and mica-schist. Some 

of the cobbles are 8 inches in diameter.

A few yards north of this last-mentioned point the pre-Cambrian 

floor is exposed 011 the west side of the cut with the reddish clay resting on it. On the east side, however, the lowest material shown is 

Potoinac, being a gray clay. The pre-Cambrian exposed here is a true 

gneiss. It shows at most only 2 or 3 feet, and is very much decomposed.

All the matter in this cut is capped by a cobble bed, which contains 

in its base subangular quartz masses, some of which are more than 1 

foot in diameter. With these occur masses of gneiss over 3 feet in 

diameter.

The next cut occurs 400 yards north of that just noticed and shows 

a different state of things. This, in its deepest part, displays 35 feet 

of Potoinac material. The floor of crystalline rocks is not exposed. 

None of the clays so abundant in the cut last described are shown 

here, although the horizon must be nearly the same. The material 

visible iu the cut now considered is a, rather coarse current-bedded 

sand, light-gray to nearly white in color. It is mottled and streaked 

with the usual colors from oxide of iron, In the finer parts of the sand 

much kaolin is diffused. The amount of gravel and cobblestone occurring in the material here is so large that the railroad company has used 

the material as ballast. The following are some of the features shown:

In the south end of the cut, on the west side, the lowest material 

exposed is a current-bedded coarse sand 10 feet thick, with the bottom 

not visible. This is full of small stones. Over this comes a lenticular 

sheet of coarse sand with a maximum thickness of 3 feet. This sheet 

soon gives out and is overlain by a light-gray sand 8 feet thick. This 

in some places is full of stones. In other places the stones are not 

diffused, but are collected in irregular nests, pockets, and seams. The 

stones are of quartz, Avith possibly a few of Cambrian quartzite. The 

largest of them, are not fully rounded and have a diameter of 8 or 10 

inches. One subangular fragment of quartz showed the dimensions 5 

by 9 by 12 inches. Nearly opposite the point just described and on the 

east side the lowest portion of the sand visible is, for the thickness of
120 . THE POTOMAC FORMATION IN VIRGINIA. [BULL. us.

2 or 3 feet, packed full of cobbles and contains many clay balls. Many of 

these cobbles are 3 to 4 inches in diameter. Above this bed of cobbles 

we have 8 feet of current-bedded sand full of small stones. On this 

rests the stony layer, 8 feet thick, before mentioned. On the top of this 

come 4 to 5 feet of sand almost free from, stones. At the summit of 

the whole 4 feet of yellowish-brown sand occurs, full of small stones. 

Some of the material of the cobbles seems to be Cambrian quartzite. 

This is the first appearance, going north, of Cambrian material in the 

Fotoinac of the Fredericksburg area.

For rather more than a mile beyond this point, no cuts occur. A 

little south of Long Branch Station a cut 20 feet deep is made in 

the usual current-bedded light-gray sand. This sand extends from 

the bottom of the cut to the height of 12 feet. It is capped by from 

2 to 3 feet of brownish-yellow, mottled, sandy clay. On the eroded 

and undulating top of this a layer of cobblestones 4 to 5 feet thick 

occurs. This is sharply distinct from the underlying Potoniac, and 

seems to be younger than it. The cobblestone layer is remarkable 

for the whiteness and uniformly large size of the cobbles. Most of 

them are from 3 to 5 inches in diameter. Some larger quartz and 

pre-Cambrian masses lie in the bottom of this'bed. It seems to be 

Pleistocene.

At Long Branch Station a cut 15 feet deep is made wholly in a sandy 

clay of dark ash-gray color.,

North of this station to Alexandria the railroad runs all the way in 

the Potomac formation, and a number of cuts 10 to 15 feet deep occur. 

Those nearest to Long Branch Station are occasionally made in lightgray to nearly white sand that contains more or less diffused gravel 

and cobbles5 but most of the cuts here, and all of them as we approach 

Alexandria, occur in highly colored or mottled sandy clays or argillaceous fine sands. They are gray or greenish-gray in color, often having great patches colored brick-red, purplish, yellowish-brown, and 

other colors, or are marbled and mottled on a smaller scale with these 

colors. The proportion of argillaceous matter on the railroad increases 

toward Alexandria, and the material is almost all of the character 

shown in the .mottled sandy clay variety. An account will be given 

further on of some typical localities'in the vicinity of Alexandria.

Beyond the vicinity of Telegraph Station examinations Avere made 

along the Telegraph road to Pohick church. 'At Pohick church the 

road to Accotink, a small hamlet, was taken. From Accotiuk the route 

to White House Landing and White House Bluff was followed. This 

last is found on the eastern margin of the Potomac area.

Northeastward from Telegraph Station no exposures of Potomac 

were seen until beyond the valley of Pohick Bun, on the slope of the 

high hill on which the old colonial Pohick church is situated. The 

top of this hill is rather more than a mile from Telegraph Station. The 

road from the bottom to the top of this hill; overlooking the valley
IFONTAINE.] EXPOSURE NEAR POHICK CHURCH. 121

of Poliick Kun, affords some imperfect exposures of Potomac material. 

In ascending the bill the following features were seen:

SECTION NEAE POHICK CHURCH.-

At the foot of the hill the lowest rock visible was only partially 

exposed. Here a yellowish-brown sand was seen whose thickness was 

not made out. On this rests a bed of ash-gray, sandy clay, 4 feet 

thick. Traced horizontally this last graduates into a laminated, fine, 

argillaceous, gray sand. After passing an interval of a few feet of 

concealed ground, there is an imperfect exposure of gray mottled 

sandy clay. After another concealed interval of 3 or 4 feet, a 

whitish sand with yellowish-brown streaks is imperfectly displayed. 

This contains small pebbles. A little above the last a bed of brownish gray sand 4 feet thick is seen. Over this last, and immediately 

succeeding it, comes 8 feet of thinly laminated, ash-gray, sandy clay. 

This has a slight greenish tinge, and is mottled and streaked with yellowish-brown. On this rests a bed, 5 feet thick, of laminated, darkgray sand, with films of yellowish-brown along the bedding planes. 

Upon this occurs 5 feet of yellow and ferruginous sand, with partings of 

ash-gray clay. Then comes 7 feet of yellowish-brown sand. These 

beds are all consecutive and all seem to be horizontal. The ground 

then, for the succeeding height of 10 feet, is concealed.

Next above these beds, and forming the upper portion of the hill, 

there is, well exposed, a mass, 20 feet thick, of coarse, loose-textured, 

yellowish-brown sand. This is strongly marked by current-bedding 

and shows the common, interlocking, lenticular structure of the Potomac 

sand. A close inspection of this material shows that its yellowishbrown tint is due to the intimate mixture of numerous small films and 

spots of that color with the normal gray of the ordinary gray sand. It 

is much like a portion of the material seen at Cockpit Point. This 

deposit is full of small stones, 3 inches and less in diameter. In its 

base large masses of gray clay occur, embedded in the sand. These 

attain sometimes the diameter of 4 or 5 feet. Throughout the lower 

part of this material the pebbles are disposed in irregular lines and 

layers, but in the upper part they tend to a more regular arrangement 

in beds.

On the top of this bed comes a thick stratum of cobbles, most of 

them large. In the base of the cobble bed large, rounded stones occur. 

Some of these are composed of a gray sandstone that seems to belong 

to the Potomac. A stone of this kind, having the dimensions 8 by 10 

by 12 inches, was seen. With these a number of large, subangnlar 

 masses of quartz are found. One of these showed the dimensions 10 

by 12 by 14 inches. A subangular mass of Cambrian quartzite lies on 

the roadside, a little below the outcrop of the cobble bed, and seems 

to have come from it. This is white in color and measures 2 by 2 by 2£ 

. feet. This was the first considerable mass of Cambrian material seen
122 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

in the Fredericksburg area. Such masses become abundant between 

this point and the Potomac Eiver and on the road to Accotink and 

beyond. They all appear to come out of the base of a bed of very 

large cobblestones which rests on the eroded surface of the Potomac 

and which seems to be Pleistocene in age. The spot now considered 

is more than 3 miles from the nearest point on the Potomac Eiver.

A little beyond the locality showing the exposure last described the 

road to Accotink leaves the Telegraph road. The exposures along the 

latter, beyond this point, do not call for special notice. Those along 

the Accotink road may be disposed of in a few words.

EXPOSURES ON THE ACCOTINK ROAD.

At the distance of about 300 yards the Accotink road crosses the old 

Colchester road. At the point of crossing there is a slight exposure 

of Potomac material. It should be stated that all the material seen in 

ascending the hill, on the highest portion of which this last-mentioned 

exposure occurs, is above that seen in the railroad cuts in the vicinity 

of Telegraph Station, except the lowest that occurs in the hill. The 

railroad runs but a little above the level of Pohick Eun.

At the crossing of the roads last mentioned the following beds were 

seen imperfectly displayed: At base lies a yellowish clay, 4 feet thick. 

This was interlaininated with thin layers and films of clay, having a 

rich purple and peach-blossom color. On. top of this comes a thinly 

laminated purplish-red clay, 2 to 3 feet thick. Above this is a layer 

1 foot to 2 feet thick, composed of cobblestones of quartz and Cambrian 

quartzite, packed in a pinkish and peach-blossom colored, sandy clay. 

Some of the Cambrian cobbles are over 1 foot in diameter. The highest material is a whitish sand, lined and seamed with yellowish brown, 

2£ feet thick. In the top of this are some thin partings of mottled 

clay that weathers a bright purple red.

From this point to Accotink the exposures are very poor. Cobbles 

and subaugular masses of Cambrian quartzite and of quartz are not 

rare on the surface. Some of the Cambrian masses are 2 to 3 feet in 

diameter. Descending to the.level of Accotink Creek and the hamlet 

of Accotink, materials are passed over that are of the same general 

nature as those seen in ascending the hill to Pohick church. The large 

size and abundance of the surface masses and cobbles of Cambrian 

material are especially noteworthy. One mass measuring 2 to 3 feet 

was seen. The masses all evidently come from the base of the uppermost cobble bed before described.

At Accotink there are considerable exposures of Potomac. At the 

base about 30 feet of the normal, light-colored sand may be found, and 

over this 12 to 15 feet of ash-gray sandy clay. A cobblestone bed occurs 

here, and it apparently comes between the two materials just mentioned. 

The exposures are too poor to give connected sections.

On the slope of a long hill from Accotink on the turnpike, for over
FONTAINE.] EXPOSURE AT FREESTONE POINT. 123

a mile, the exposures are still very imperfect, and give merely glimpses 

of the rock in place. At the top of the hill the road to White House 

Lauding leaves the turnpike and turns clue east. This was followed 

to White House, distant 3 miles. This road follows the tongue of high 

laud that ends on the Potomac in the tall banks that form White House 

Bluff. As the road is situated on the crest of this tongue, no exposures 

worth noting were seen on it.

We may now turn to the description of the Potomac and Pleistocene 

formations as they were seen along the Potomac Kiver north of Powells 

Creek, which was the point on the river last described.

EXPOSURE AT FREESTONE POINT.

The Potomac material that is shown in Freestone Point on the river 

is composed, for about 30 feet above the water, of ledges of pretty firm 

sandstone. This material is below any shown in the railroad cut made

through the point. It is capped by soft and crumbling, light-colored 

sandstone and sand, similar to the matter shown in the railroad cut. 

It is much less perfectly exposed here than in the cut. The banks of 

the river are then low and give very imperfect exposures until we reach 

the northern shore of the estuary portion of Occoquan Creek. The 

material that composes them seems to be all Pleistocene. The broad 

promontory known as Masons Neck, lying between the mouth of the 

Occoquan and Gunston Cove, faces the Potomac for a long distance 

with bluffy banks that range in height from 20 to 40 feet. These give 

good exposures. Exposures are also to be found on the opposite or 

Maryland side of the river. Gunston Cove is the estuary of the united 

creeks, Pohick and Accotink.

High Point is a locality marked on the State map as occurring at 

the apex of Masons Neck. It does not justify its name, as the bank 

here is not distinguished by a height surpassing that of the average of 

the shore above and below it, and in this respect it is much inferior to 

Freestone Point.

Professor Eogers in his reports l gives this locality as one of those 

showing his Upper Secondary, but in this statement he was mistaken, 

as the material shown is all Pleistocene and of the same character, 

essentially, as that which occurs at Stony Point fishing shore, but the 

material is a sand strikingly like that of the Potomac, which it was 

at first thought to be. Only after a careful examination was the Pleistocene age of the deposit determined.

EXPOSURE AT STONY POINT FISHING SHORE.

Stony Point fishing shore is about one-third of a mile below High 

Point. Both of these localities are on the Virginia shore. The bluff 

here is about 30 feet high. All the material exposed is Pleistocene. 

The lower 20 feet seen from a little distance seems to be very distinctly 

horizontally beaded. This apparent bedding, however, is due to the

1 Geology of the Virginias, p. 440.
124 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

presence of horizontal plates of highly ferruginous and indurated 

sandstone. These are,caused by the deposit of oxide of iron in particular layers of the sand. This iron has cemented the sand together 

and formed plates and crusts that are more durable than the layers 

with which they are interstratified, so that on the weathering of the 

material these stand out.

The greater portion of the matter composing this lower part of the 

bluff is a loose, gray sand, much stained and streaked with oxide of 

iron. It has some pebbles of quartz diffused through it. Like the 

material in High Point, the sand is so much like that of the Potomac 

formation that it is not easily distinguished from it. There is little 

doubt that this close resemblance is due to the fact that it is Potomac 

debris which, in Pleistocene times, has been eroded and deposited 

with little or no change of lithologic character. This is a feature not 

uncommon in Pleistocene deposits hear their base, where the Potomac 

is but a short distance below. Tt is most probable that the Potomac 

formation is present here not far below the deposit exposed in the 

bank now described. No doubt originally the ferruginous crusts had 

the character of the intervening layers, and it is probable that these 

ferruginous plates exist only in the weathered substance of the face of 

the bluff. The bluff throughout its upper 10 feet is formed of an 

imbedded, yellowish-brown, argillaceous sand that shows the characteristic hummocky surface seen so often in the more argillaceous fine 

sands of the Pleistocene. This is the portion found at so many places 

to afford a clay suited for the making of bricks.

The films of color due to oxide of iron found in the sand occur in 

some places with a waving course. They are sometimes so closely 

placed and so thin that they look as if painted in imitation of the grain 

of wood. Toward Sandy Point, at the mouth of the Occoquan, the 

crusts of ferruginous sand disappear, and the lowest portion of the 

Pleistocene shows large irregular masses that are colored reddish or

pinkish, the colors showing themselves only on weathering. 

EXPOSURE.ABOVE INDIAN HEAD.

On the Maryland side of the river, above Stony Point fishing shore 

and High Point, bluffs occur at intervals, beginning at Indian Head and 

extending for several miles up the stream. Diagonally across from 

High Point, and about 1£ miles up the river, there is a high bluff that 

rises immediately from "the water's edge. This shows the Potomac 

formation with the Eocene resting on it. Although on the Virginia 

side the Potomac formation is wholly removed above water level and, 

replaced by Pleistocene, we find here no less than 70 feet of it displayed. There has been too much slipping down of the higher parts of 

this bluff, and consequent burying of the lower portions, to permit a 

detailed and consecutive section to be given, but enough is shown to 

enable one to determine the general character and thickness of the 

Potomac material here.
KOMTAIME.I EXPOSURE ABOVE INDIAN HEAD. 125

The lowest portion of the bluff is similar to the lower part of the 

Gunstoii Cove Bluff, to be described further on. It is composed of 

tough, lumpy, sandy clay, which, when fresh, is colored green, but 

which, on weathering, assumes in spots and streaks bright red and 

purple colors. This material graduates in ascending into a gray sand 

which, in various places and at different horizons, contains irregular 

layers of imperfectly laminated sandy clay. This latter substance has 

varying amounts of fragmentary lignite, and is sometimes dark in color 

from its diffusion. It is possible that minute search might disclose 

recognizable plant impressions in some of these argillaceous, layers, but 

none were found when the bluff was visited.

The Eocene rests upon the top of the Potomac here. It shows about 

"20 feet of a yellowish-gray, somewhat argillaceous sand, which discloses 

no division into beds or layers. No fossils were seen in it. It is full 

of the burrows of swallows.

On the Eocene rests about 10 feet of yellowish-brown Pleistocene 

sand. This has in its base a number of large quartz cobbles'and partly 

rounded masses of pre-Cambriau rock. This exposure is of great 

importance, since it indicates no diminution in the thickness and no. 

change in the lithologic character of the Potomac formation from which 

it could be inferred that the eastern limit of the group is anywhere 

near at hand. This locality and that of Fort Washington give the 

most eastern exposures of Potomac material in this region, but the 

locality now under consideration shows much more of the formation 

than can be seen at Fort Washington. It has here apparently suffered 

much less from erosion in pre-Eocene times. The great development 

and unchanged character of the Potomac at this spot confirm the 

evidence obtained from the artesian well at Fort Moiiroc, indicating 

the extension of the formation to the eastward under the Tertiary, far 

beyond the last outcrops.

EXPOSURE AT GUNSTON COVE BLUFF..

Gunston Cove Bluff is situated on Gunstoii Cove, opposite White 

House Bluff. The cove, formed by the united estuaries of Pohick 

and Accotink creeks, is about 2 miles wide and makes the northern 

boundary of Masons Neck. Between the bluff now to be described and 

the banks of the Potomac River, in the vicinity of High Point, there is 

a long stretch of 4 or 5 miles in which the banks are low and show only 

a few exposures, and these disclose nothing but Pleistocene.

The bluff now described, although so near to White House, shows 

very different material from that exposed at the last-mentioned place. 

About 80 feet of Potomac is visible, the bluff being composed wholly 

of that formation. It rises in going up the cove, the greatest amount 

of deposits being shown at the upper end. At the lower end, which is 

only about 20 feet high, all that is visible is a tough, massive, lumpy, 

rather sandy, greenish clay, which on exposure weathers so as to
126 THE POTOMAC FORMATION IN. VIRGINIA. [BOLL. 145.

develop bright red and purple colors. These colors occur in irregular 

spots and streaks.. The material is much like the mottled, sandy clay 

seen at Cockpit Point. At the highest part of the bluff, 50 yards up' 

the cove, this mottled sandy clay becomes more sandy, tending to pass 

into a sand. It shows here some indications of division into beds. At 

the same time it is more consolidated, and in places approaches a sandstone in nature. At this spot it has over it about 40 feet of a grayish, 

incoherent, somewhat argillaceous sand, which has the character of the 

ordinary Potomac sand. This has no. apparent bedding, but is divided 

here and there by short and thin lenticular layers of laminated, brownish-gray sandy clay. In the lower part of the mass there are two layers 

of this clay, 12 to 18" inches thick, that are more persistent than the 

rest. These bend around so as to unite, and inclose a lenticular sheet 

of the incoherent sand in which they are embedded. They contain a 

great deal of comminuted vegetable matter, and are in places colored 

brown by this material. The upper part of the bluff, for 20 feet, is 

composed of a massive, somewhat argillaceous sand, light-gray in color, 

that is much bored into by swallows. There are no indications of 

Eocene and Pleistocene in this bluff.

EXPOSURES AT WHITE HOUSE BLUFF.

White House Bluff is caused by the jutting out into the Potomac of 

a promontory that lies just north of Gunston Cove. The bluff lies 

about 2 miles below Mount Vernon. It gives an exposure of the Potomac formation that is very considerable in both height and horizontal 

extent. This exposure is important also because it displays the Potomac 

formation well at one of the most easterly points where it is visible in 

the Fredericksburg area. The features seen here may be taken as 

characteristic of many of the most easterly outcrops in this region. 

The air-line distance from east to west connecting this locality with the

exposures formerly described near Telegraph Station is a little more 

than 4 miles. . '

The bluff at White House begins near the wharf and extends several 

hundred yards around between the river and Grunston Cove. At no 

point is the whole face of the bluff exposed so that a continuous vertical 

section can be given, but this, if obtained at one point, would not 

apply to others, although near by, owing to the great changeableness 

of the Potomac strata when traced horizontally. The only way in 

which any intelligible account of exposures of this kind can be given 

is to note the more prominent features of the material at different 

horizons in the face of the bluff.

The first thing that strikes the observer walking along the beach 

around the point is the close resemblance of the deposit to that shown 

at various points on the James and Appoinattox rivers. It is a repetition of that seen on those rivers. Even the beach between high and 

low water marks is of the same nature. The sloping shore is paved
FONTAIHE.] EXPOSURES AT WHITE HOUSE BLUFF. 127

with cobbles of quartz and Cambrian quartzite, and large blocks of the 

latter material may be seen here and there, but the cobbles here are 

not so large generally. The blocks of quartzite are not so numerous 

as in the other localities named, but they are larger. The cobbles and 

large blocks have fallen down from a stratum at the top of the bluff. 

The large quartzite masses are but little abraded, and although polished 

quite smoothly they have only the edges and angles worn off. They 

are of Cambrian quartzite. It is quite probable that these come from, 

the same stratum that furnished the blocks which are strewn over the 

surface between Accotink and Pohick church. The most abundant 

material is a white, compact quartzite, but some masses of dark-gray 

quartzite are found, which resemble some of the rock exposed in the 

gorge of the river at Harpers Ferry, occuring near the base of the 

Cambrian formation. One large mass of white, congloineritic quartzite 

was seen. This resembles the white member of the Medina formation. 

These large rock masses must have been transported a long distance, 

for nothing east of Harpers Ferry resembles the substance forming 

them. One of the masses of dark-gray quartzite had the dimensions of 

2^ by 4 by 5 feet. One, of white quartzite, measured 2£ by 3 by 4 feet. 

A number of others had nearly as great dimensions. Occasionally 

blocks of gneiss, mica-schist, and other pre-Cambrian rocks may be 

seen, but they are fewer and smaller than the quartzite masses.

The bluff, in its highest part, rises fully 100 feet above the water. It 

declines in height both up and down stream. At the base of the cliff 

the typical light-colored material of the Potomac formation is found. 

It is here sufficiently indurated to be called a sandstone. This shows 

a great tendency to disintegrate, the rate depending upon the amount 

of diffused kaolin, which varies much. In some portions as much as 

one-third of the bulk is kaolin. Much of the rock contains very coarse 

sand, and a portion is pebbly. This part of the cliff shows all the 

features so characteristic of the Potomac. It has an exceedingly variable composition, and structure, with interlocking lenticular beds. The 

more indurated sandstone portions are lenticular layers in the more 

incoherent material. These harder parts seem to resist the action of 

the water very well, and large blocks have been undermined and lie on 

the beach. In some portions of this material particles of quartz of the 

size of a pea make up a large proportion of the deposit. The white 

sandstone and sand above described extend up in the bluff to the height 

of from 20 to 25 feet. At the top of this there is a decided erosion 

horizon, which may be traced wherever this top can be seen. It is 

marked by the presence of clay balls, most of which are small. On 

this horizon, in some places, patches of clay a few inches thick are 

found. These seem to be remnants of some stratum left uneroded. 

Above the basal light colored stratum there usually comes a mass of 

sand, which is not so coherent as the material on which it rests. It has 

so yellowish-gray to'brownish-gray color, due to the diffusion of colored
128 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

clay instead of kaolin throughout the sand. It contrasts pretty strongly 

with the underlying- light-colored sand. It is also much marked by 

current-bedding, and is quite coarse. It closely resembles the mass 

20 feet thick that occurs in the top of the hill at Pohick Church. The 

colors are strongest on the planes of lamination, and the yellowish or 

yellowish-brown lines are very numerous and closely placed. They are 

often wavy in their course. Many small pebbles occur, scattered irregularly in the mass or aggregated in irregular seams and pockets. The 

thickness of this mass is variable, but may be takeu as averaging from 

20 to 25 feet. It is not exposed anywhere sufficiently to show its entire 

thickness in any one place. On the top of this rests an argillaceous 

layer, which has generally the character of a somewhat sandy clay of 

dark-gray to ash-gray color. In some places a more or less pale-reddish 

clay takes the place of the dark-gray clay. This stratum seems to be 

composed of redeposited clay, the fragments of some older bed or beds 

deposited and cemented together by pressure. In the cases where a 

portion of the stratum is composed of pale-reddish clay it appears that 

fragments of two kinds of clay are cemented together, the reddish and 

the gray. This layer may be traced along the face of the bluff for a 

very considerable distance. Its outcrop appears to be nearly horizontal, 

lying about 50 feet above the water, and is usually marked by the exit 

of water from its upper surface. This stratum is from 3 to 5 feet in 

thickness. It does not always appear at the horizon where it might be 

expected. This results either from its not having been originally deposited, or from its having been eroded away after deposition. The former 

is the more probable cause.

This bed is of special interest, since at one locality it contains a 

number of fairly well-preserved fossil plants. .These are mainly angiosperms, similar to those found at the two localities near Brooke. With 

them occur a few ferns and conifers, which in the main are also like 

those near Brooke. There can be no doubt that this stratum is similar 

in age to the deposits near Brooke. The plants occur between the fragments of clay that are cemented together by pressure and in the fragments themselves, in the latter case having been deposited in the original 

clay layer. As the plants caught up in redeposition are not essentially 

different from those preserved in the clay, as originally deposited, it 

follows that no great period of time, geologically speaking, elapsed 

bet/ween the first deposition of the clay and its erosion and redeposition. 

At many points this clay stratum is inaccessible, since it crops out on 

the vertical face of the bluff, so that I could not examine it fully at all 

points to determine the existence in it of fossils. At all places where 

it was examined traces of plants and bits of lignite were found, but in 

all cases except one the plants were too fragmentary to be distinguished.

Above this clay bed comes an incoherent sand, mainly gray in color, 

but it has many irregularly shaped portions that are colored yellowishbrown. This color is distributed in spots, blotches, and films. The
FONTAINE.] EXPOSURES AT WHITE HOUSE BLUFF. 129

bed is fairly well exposed at the locality.yiehiing the angiosperm 

plants. It is here 5 feet thick and has iu its top a short lenticular 

layer of pale-reddish clay, 18 inches thick in the thickest part. This 

is composed of recomputed fragments and shows no plants. Above 

this clay comes 7 feet of brownish-gray sand, with much yellowish 

color irregularly disposed in the mass. This is a continuation of the 

underlying material, the lenticular clay layer not being separate from, 

it, but merely differing somewhat iu color and composition. No 

exposures occur at this point above the beds last described. At this 

height above the river exposures are rare.

A little farther down the river the bluff rises higher, and material 

aboAre that last described is exposed.. The deposits found here at this 

higher horizon may perhaps be taken as representative of other portions of the bluff at this level. Here everything up to the height of 50 

feet above the water is concealed by fallen talus. The lowest stratum 

shown is 18 inches of sandy gray clay, resting on a gray sand mottled

with yellowish-brown. Next above is a bed of recemented sandy 

clay 4 feet thick, which is the fossiliferous layer. The bottom of this, 

for a foot, is a dark-gray sandy clay full of small fragments of plants 

and bits of lignite. Toward-the top the color of the material is lighter 

and more sandy than toward the bottom. Above this comes 4 feet of 

yellowish-brown ferruginous sand. This is seamed by closely placed 

partings of yellowish-brown, that appear to mark the lamination 

planes. These partings are 1 inch or less in thickness. Above the 

last we have 7 feet of gray sand, which also is thinly laminated. This 

has many thin layers' and films, colored dark by finely comminuted 

vegetable matter. This material is quite argillaceous and the stratum 

corresponds to the highest seen at the angiosperm locality. The next 

in order ascending is a bed 6 feet thick of highly ferruginous sand, 

which is in the main yellowish-brown. It has some layers of gray.. 

The highest portion of the bluff' that is exposed shows next in order 

above the last 5 feet of gray sand, much marked with yellowish-brown 

in thin films. Th.ese materials are exposed in an abrupt bluff which 

rises from the top of the slope made by the fallen talus that conceals 

the lower portion-of the cliff. This same abrupt bluff in the upper 

part of the formation is continued for some distance up the river.

About 40 yards above the point last described the bluff' exposes the 

bed just noted with some still higher material, but a considerable 

change has taken place, even in the short interval, in the material 

immediately above the fossiliferous stratum. It all now forms one 

continuous mass of dark-gray coarse sand, showing strong currentbedding. The current-bedding planes are marked with yellowishbrown streaks. On the top of this mass comes a thin layer of gray 

sandy clay, which is mixed with films and streaks of gray clay: Still 

higher and this is material not seen at the locality last described w'e 

have a layer, 8 feet thick, of yellowish-gray sand tinged with brown. 

Bull. 145 9
130 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

FIG. 13. Remnant of clay layers at White House Bluff.

Besting upon this, and forming the top of the bluff, is a bed of large 

cobblestones. This bed, judging from the debris thrown down, once 

formed, everywhere in the bluff, the capping of the Potomac material, 

but it is now generally undermined and destroyed. The cobbles are 

composed.of quartz and of Cambrian quartzite. A large proportion 

is of the latter material. With the cobbles occasionally come- large, 

partly rounded masses of Cambrian quartzite. It is from this stratum 

that the large Cambrian and pre-Cambrian masses lying on the beach 

came. One cobblestone of Cambrian quartzite coming from this bed 

had the dimensions 8 by 10 by 12 inches. This cobble bed is of

uncertain age^ but it 

is most probably Pleistocene.

At a point lower 

down the river and 

200 yards from the last 

described locality the 

highest material visible' in White House 

Bluff may be seen, for 

it lies upon the cobble bed. At this point the cobble bed is 5 feet 

thick. It has at its base some cobbles of Cambrian quartzite 12 inches 

in diameter. Above the cobblestone bed is a layer 5 feet thick composed of pale yellowish-gray sand and containing pretty regular seams 

of small stones. This material is distinctly stratified, and is apparently 

Pleistocene in age.

From this account, which embraces all the different kinds of beds 

visible in White House Bluff, it will be seen that no Tertiary strata 

occur here.

The above is a general description of the strata that occur in succession at this place. We may add to this several features seen at different spots, in order that some of the peculiarities of the beds may be 

noted. The angiosperrn locality should be specially described, as the 

mode of occurrence of the plants is a common one in the Potomac 

strata. The locality of the plants is not easily found, as they occur in 

a limited space at a.spot difficult-of access. This place is a few yards 

below the old wharf at White House. As the fossils are found in the 

fossiliferous clay bed before described, they occur about 50 feet above 

the water. All the material below the plant bed is here concealed by 

talus, and this makes a steep slope up to the stratum yielding the 

plants. A precipitous cliff about 15 feet 'high overhangs the plant 

layer. The fossiliferous clay bed seems to contain material different 

from that found in it at most other places. Usually it is a gray sandy 

clay, but here it seems to vary locally, the gray clay being replaced by 

a- mixture of very light-reddish clay and sand. The clay alone contains the plants, and it seems to be a redeposited material, being in the
FONTAINE.] . EXPOSURES AT WHITE HOUSE BLUFF. 131

form of lumps, which are sometimes isolated in sand. In other cases 

the clay is cemented together, but is mixed with more or less sand. 

This clay mingled with sand is at its maximum about 3£ feet thick, but 

the pure clay can rarely be obtained more than a foot thick. It then 

oftentimes thins out entirely. In the clay with the plants several 

imprints of Estheria 1 were found. These appear to be of the same 

nature as those found near Brooke.

As illustrating the character of the white sand forming the lower 

part of the bluff the following features, seen at different places, may be 

given. Clay balls are occasionally to be seen in it, distributed without 

any particular order. Besides the balls patches of clay occur that are 

plainly uueroded remnants of beds that furnished the material of the 

balls. In one place a patch of reddish clay 5 feet long and about 10 

inches thick was seen embedded in the sand, as shown in fig. 13. This 

was cut off abruptly at both ends and abutted against the sand. The 

clay balls are most commonly reddish in color, but in one place several 

balls composed of a beautiful white clay were seen embedded in the 

sand. This clay is much indurated, is of very fine texture, and is quite 

plastic. Some of these balls are from 12 to 18 inches in diameter. 

They occur 2 or 3 feet below the top of the white sand.

A detailed section of the white sand member was taken at one point, 

but owing to the great chaugeableness of the material it can be 

regarded as holding good only for the spot where it was determined. 

The lowest portion seen here is 5 feet of grayish-white currentbedded sand. This contains a good deal of diffused kaolin and some 

scattered clay balls. Above this comes a lenticular sheet having a 

maximum thickness of 10 inches. This is composed of small clay balls, 

pebbles, and kaolinic sand. It thins out at one end and at the opposite end is concealed. On the top of this rests 2 feet of sand of the 

same kind as that shown at the bottom. Both this and the bottom 

layer have some yellowish wavy lines and films. These are most numerous in the top layer. Above the last we find a layer 1 foot to 2 feet 

thick, composed of a coarse whitish grit full of pebbles 2 inches or less 

in diameter. With the pebbles there are many clay balls, mostly small, 

but measuring in some cases 10 inches in diameter. The clay is mostly 

pale-reddish. Above this we have 2 to 2£ feet of material similar to 

that at the base of the bluff. Then comes a lenticular layer with a 

maximum thickness of a foot. This is composed mainly of balls of gray 

clay cemented together. But in this same layer some balls of palereddish clay occur mixed with the gray. The largest of them are 10 to 

12 inches in diameter. On the top of the last is a layer, 10 inches 

thick, of incoherent gray sand containing many thin films of yellowishbrown. Kextin order ascending is a sheet, 2 to 4 inches in thickness, 

of plastic doye-colored clay. This lies horizontally, thinning out at

1 The imprints of Estheria occurring here and near Brooke were determined as such by Dr. C. A. 

White.
132 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

\

one end and cut squarely off at the other. Above the last come 2 feet 

of light-gray sand full of clay balls, large and small. This forms the 

uppermost portion of the light-gray sand which constitutes the lower 

part of the bluff. The top for a thickness of several inches is an argillaceous sand passing into clay. It should be noted that the amount of 

argillaceous material occurring at this horizon is larger than, is usual 

in the white or light-colored sand of the Potomac.

On the road leading from the wharf to the top of the hill, and just 

above the upper end of the bluff, the strata are partially exposed. 

Descending from the top of the hill we find in succession the following 

materials:

The highest rock seen in place is a brownish-gray and mottled sandy 

clay, showing here and there a good deal of brick-red color. This is 

not continuously exposed, but seems to be about 20 feet thick. The 

next bed under it is a yellowish-brown ferruginous sand, about 4 feet 

thick, containing white seams. It is followed below by a stratum, 4 

feet thick, of a rather sandy liver-colored clay, with spots and patches 

of gray. Some portions of the bed have the pale-reddish color so common in the Potomac clay. This stratum is formed of recemented clay, 

and the differently colored parts are due to fragments that come from 

beds originally different. This seems to be the bed that elsewhere 

shows the angiosperrns. None of these were found at this locality. 

The bed, however, contains a good deal of lignite in small fragments 

and many small bits of leaves. Some of these can be identified. 

Among them we find fragments of the leaflets of Dioonites buchianus 

and a conifer with acicular leaves. Underneath this stratum we find 

a bed of gray sand, which is marked with large spots and patches of 

yellowish brown. It contains a number of flakes of gray clay and 

lumps of the same mixed with the sand in its upper portion. About 3 

feet of this material is shown. The strata are then concealed down to 

the level of the water.

Although the bluff at White House rises to the height of about 100 

feet and is composed almost wholly of Potomac material, yet in the 

space of a few hundred yards it sinks down to the height of even 10 or 

15 feet and finally disappears when followed upstream. Both up and 

down the Potomac River the material forming the bank of the river 

is all Pleistocene. When the bluff is followed around to the mouth 

of Gunston Cove, it is found to rapidly diminish in height, until, 

within 300 yards of the highest portion, there is exposed only about 

10 feet of the deposit. This is a light-gray pebbly loam belonging 

to the Pleistocene. Up the river also the Potomac soon gives place to 

the Pleistocene. The character of this, as exposed at Eed Bank, will 

be noted below. This condition of things shows that in Pleistocene 

times a high bluff of Potomac age must have stood at this place. If 

any Tertiary ever capped it, this must have been removed by erosion. 

The very irregular character of the surface left by erosion in Pleistocene 

times is noteworthy here.
FONTALVE.] EXPOSURE ON THE POTOMAC AT RED BANK. 133

EXPOSURE AT RED BANK.

Eed Bank is the name of a bluff about 20 feet high, exposed at the 

water's edge about half a mile above the upper end of White House 

Bluff. It is formed wholly of Pleistocene, the Potomac having boeii 

removed by erosion. The base of the bluff, .to the height of about 

12 feet, contains a series of horizontal layers of brownish-red sand. 

These are interstratified with layers formed of mixed sand and cobbles. The cobbles are formed of quartz and Cambrian quartzite. Some 

of them are 5 to 6 inches in diameter. The upper 8 feet of the deposit 

is composed of a brownish-yellow sand. This has numerous layers or 

crusts of the sand firmly consolidated by limonite. These layers are 

from 1 inch to 2 inches thick. This material seems to be the equivalent of that which occurs in White House Bluff, above the uppermost 

cobble bed.

The distance by river from White House to Mount Vernon is about 

2 miles. Along the shore to the immediate vicinity of the last-named 

place no Potomac is displayed, and the Pleistocene alone is visible in 

the banks. The banks do not in their highest parts rise more than 40 

feet above the water. The exposures of the Pleistocene begin about 

three-fourths of a mile below Mount Vernon, where the banks first show 

bluff-like faces.

At a point near where these bluffs begin there is a very fair exposure 

of the material in the highest part of the bank. The lower 30 feet of 

the bank is hidden because of the undermining and throwing down of 

the upper part. The exposed material is about 15 feet thick. At its 

base occur masses, 2 feet or more in diameter, of brownish-gray Potomac 

sand embedded in mottled clay. These masses have been torn up and 

redeposited. They are full of small clay balls. Owing to the incoherent 

character of this substance it cannot have been transported far. This 

brownish-gray sand is similar to that which occurs in White House 

Bluff at the top of the white basal sand. The mottled clay that surrounds these masses is about 3 feet thick. It is a brick-red sandy claystone rather than clay marbled with gray. The red color is bright. 

On this rests a cobble bed averaging about 3 feet in thickness. The 

cobbles are packed in mottled clay similar to that on which they rest. 

With the cobbles occur some large, partly rounded masses of Cambrian 

quartzite. They reach the diameters of 3 or 4 or even 5 feet. This 

stratum is evidently the equivalent of the bed of cobbles and large 

stones that occurs at the top of White House Bluff and near Pohick 

church. Above the cobble bed come 10 feet more of mottled clay. 

All of these mottled clays are decidedly sandy, and look much like 

the mottled sandy clay of the Potomac, but they appear to be of 

Pleistocene age. Deposits of similar age occur all the way up to the 

immediate vicinity of Mount Vernon, where the Potomac white sand, 

here mostly a sandstone, shows itself in the lowest portion of the bank
134 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

EXPOSURE AT MOUNT VERNON.

At Mount Vernon the hills that border the river slope back gently 

from it, so as to expose only occasionally a precipitous face immediately 

at the water's edge. At one or two points immediately above and 

below the wharf we find the banks showing abrupt faces ranging in 

height from 6 to 15 feet. These abrupt faces are formed by a lightgray to white sandstone of Potomac age. This appears to be pretty 

firmly consolidated. It has many irregularly shaped portions colored 

yellowish-brown. The hill on which the Mount Vernon mansion stands, 

at its upper end, as Professor Eogers long ago stated, is the last locality 

to show the white sandstone on the river in a coherent form. As will 

be noted later, this light-colored Potomac material is visible at Fort 

Washington^ but it is inconspicuous there, and is in the form of a 

crumbling sand. The greatest thickness of the Potomac sandstone at 

Mount Vernon is shown at the pavilion, just below Mount Vernon 

wharf. Here about 15 feet of it may be seen. This has some thin lay. 

ers of pinkish indurated clay. These are inclined to one another at 

various angles, and are irregularly arranged in the mass. This clay 

occurs both in remnants of the original layers and in recemented fragments. This material contains some fragments of plants, among them 

a fragment of Sapindopsis.

At the wharf the light-gray sand rises about 10 feet above the water, 

and is succeeded by a fine-grained sandy shale and argillaceous sand. 

The shale occurs as subordinate layers and x>artings in the more sandy 

portions. The shale partings, when purely argillaceous, are usually 

only 1 inch or 2 .inches thick. In the lower part of the mass, however, 

they sometimes swell up to 12 inches. , In some of the shale or clay 

partings poorly'preserved fragments of plants occur, but they are not 

numerous. The flaggy, fine-grained sandy shale or sandstone is 

mostly dark gray in color, but it often has a yellowish-brown color, 

arranged in streaks and films. The thickness of the material seen was 

about 20 feet. This substance is strikingly like the shaly beds that 

occur in Baltimore, Md., and which there lie just above the white sand. 

These Baltimore beds are rich in angiosperm plants, and also contain 

many other kinds. This deposit differs widely lithologically from the 

light-colored coarse sand or sandstone that always forms the lowest 

of the Potomac materials. The occurrence of such material here and 

at Baltimore and the frequent appearance of redeposited clay of similar kind near the top of the light-colored 'sand suggest the idea that 

there was once an extensive formation of flaggy argillaceous matter 

lying upon the sand. A precisely similar deposit occurs at the locality 

in the woods near Potomac Run, on the Telegraph road, but this lies 

within the sandy member.

Above this horizon only small and detached outcrops could be seen. 

They show that, at the height of about 70 feet above the water, several
FONTAINE.] EXPOSUKE AT FORT WASHINGTON. 135

feet of pale-reddish and gray sandy clay occur. The highest part of 

the hill seems to be formed of reddish clays that are apparently Pleistocene in age.

EXPOSURE AT FORT WASHINGTON.

Fort Washington is on the Maryland side of the Potomac River, 

diagonally above Mount Yernon, and about 4 miles distant from the 

latter in a-northeasterly direction. There is here a pretty high bluff a 

little below the wharf. The following approximate section was obtained 

at this point:

Section qt Fort Washington, Md.

Feet.

1. Sandy clay, with cobbles at base ............................ 5

2. Gray sandy clay, with shell imprintsl . ....................... 15

3. Variegated clays (upper Potomac)............................ 30

No. 1 is Pleistocene of the usual kind. It is composed of sand and 

clay in the upper portion and has at its base a cobblestone bed of variable thickness. Five feet may be taken as the average thickness of this 

stratum.

No. 2. The base of this stands at nearly the same level, showing that 

the top of the variegated clay is eroded down to a quite uniform plane. 

This is marked by a crust of impure sandy iron ore, which is not continuous. At the junction of No. 2 with the variegated clays there is much 

gypsum; and this is often well crystallized, sometimes forming rosettes 

of crystals. No. 2, taken on an average, is an argillaceous, gray, often 

greenish-gray, sand, showing, in the weathered portions, crusts and 

concretions of ferruginous matter, which is often a fair iron ore. There 

is no apparent stratification in the old outcrops, and the material is 

loose in texture and very incoherent. ' Impressions of shells and a little 

imperfectly formed lignite occur in it in some places. The lower portion, extending up for about 12 feet from the base, is dark gray, with a 

slight greenish tinge, and is full of streaks and spots of brown pulverulent oxide of iron. This part is decidedly more argillaceous than the 

upper portion and contains imprints of shells. The upper part of No. 2, 

for the thickness of from 3 to 5 feet, is a light greenish sandy clay 

that weathers to an olive color. This appears to be without shell 

imprints.

No. 3 is the variegated clay, or iron-ore clay, so abundant in Maryland. It appears here for the first time on the Potomac River, not 

showing itself at White House Bluff, G-unstone Cove, or other points 

farther down the river. This group of beds, so far as investigations 

yet go, is not to be separated from the Potomac as seen in Virginia, 

but it is probably an upper member of the same formation. The upper 

most portion, for the thickness of about 5 feet, is mostly light-gray in 

color, with a lumpy and nncleavable structure. It shows many small

'These.are of Eocene fossils near tLe top, and, according to Dr. W. B.'Clark, marine Cretaceous in 

tho lower beds. This marine Cretaceous Las been designated the Severn formation on the maps of the 

United States Geological Survey.
136 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 115.

streaks and spots of bright-red color. These make this clay look much 

like the variegated Lafayette matter to be seen at Petersburg. The 

material below the portion last described is in the main lead-gray or 

dark-gray in color. It is clear that this was the original.color of all 

this lower deposit, so long as it was fresh and unweathered, but 

owing to the oxidation of the iron in some portions on the weathering 

of the mass bright-red and purplish-red colors are developed. These 

form numerous spots and streaks in the gray clay. Sometimes large 

masses are colored in this way. The amount of red color seems to be 

in inverse proportion to the amount of comminuted and diffused lignite. 

This latter appears to have prevented the oxidation of the iron.. In 

some places the clay seems to be composed of spots of dark-gray and 

pinkish clay mixed in about equal proportions. The clay contains a 

good deal of iron ore scattered irregularly through it in the form of 

lumps and crusts. It is mostly impure, and was evidently formed by 

percolating waters taking iron in solution from, the clays and concen-- 

trating it by concretionary action. The same clays, between Washington and Baltimore, have loag been worked for the iron they contain. 

Sometimes the iron concretions strikingly resemble fragments of lignite 

in shape and structure, and it is probable that they are formed by the 

petrifaction of that material by a pseudomorphic process. A good 

deal of diffused lignite is found, mostly in the form of small fragments. 

The lead-gray clay, as well as the uppermost lighter colored material, 

is lumpy and uncleavable, showing no' traces of lamination and bedding. It has the same peculiar structure as was seen in the torn-up 

and redeposited clays of the lower Potomac, but it has not the irregular 

admixture of sand shown by that. On the contrary, the material is 

quite uniformly a pure and plastic clay.

From an examination of this spot, as well as of numerous localities 

between Baltimore and Washington, it is clear that much of the lignite found in the variegated clays does not come from vegetation 

entombed for the first time in these clays, and there taking the form 

of that material. It almost certainly 'comes from the lower Potomac 

formation, the lignite of that group being eroded, comminuted, and 

redeposited in the upper Potomac or the variegated clay group. Logs 

and limbs of trees now changed to lignite are not uncommon in the 

lower Potomac, lying, more or less entire, as they were originally 

entombed; but such, features are rarely seen in the upper Potomac. 

The clay above described extends down to the water level, and at the 

base of the bluff no lower Potomac was visible.

Fifty yards above the bluff, and between it and the wharf, the top of 

the lower Potomac may be seen. This spot is important, because we 

have here the variegated clay group superposed on the lower Potomac, 

a contact that is very rarely visible. Unfortunately the contact is near 

water level, and is seen only close to the edge of the water, so that it 

is not well displayed. The lower Potomac shows over the space of a
FONTAINE.] EXPOSURE AT FORT WASHINGTON. -137

few yards at low tide only 4 or 5 feet of material. The greater portion 

of the lower Potomac visible at this spot is composed of the typical 

light-gray sand with clay balls, which is much stained with irregular 

markings, due to oxide of iron. This sand contains dark-gray clay, 

, almost exactly like the overlying variegated clay when this latter is 

unweathered. It occurs in lenticular pockets, which are composed 

partly of the relics of large fragments of the original clay pockets, 

and partly .of the smaller and confusedly redepositecl fragments of 

these layers. The smaller recemented fragments cover and bury the 

larger fragments.. The larger fragments of the original clay pockets 

may be distinguished by the possession of cleavage and by their content of plant impressions. The redeposited and cemented clay has 

the usual lumpy nature and confused structure, and in these points 

resembles strongly the variegated clay which overlies the lower 

Potomac.

There are strong indications here that the variegated clays are 

formed of material eroded from the upper pa.rt of the Lower Potomac. 

If this be true, then the upper part of this latter formation was originally composed mainly of argillaceous matter. This COD elusion would 

be drawn from the exposures on Covington street, also near Federal 

Hill, Baltimore, where a remnant of this upper argillaceous portion is 

still visible, apparently furnishing from its ruins the clays of the variegated clay group which rests upon it. Unfortunately, at Fort Washington the variegated clay is not visibly superposed on the lower 

Potomac, for at the spot where the latter is exposed the river has cut 

away the variegated clay so as to leave a low bench of the older formation. There is certainly some unconformity between the two members 

of the Potomac formation here, established by an interval of time in 

which a considerable amount of erosion took place in the lower Potomac.

Lignite occurs in both the sand and the clay. It is sometimes in 

small fragments, but not rarely considerable portions of trunks and 

limbs of trees may be found, a feature rarely occurring in the variegated 

clay. Some comminuted lignite is found in the sand and clay, but the 

greater portion in the latter is evidently, so far as size is concerned, in 

the state in which it was first embedded.

The redeposited and cemented clay takes the form of lenticular 

pockets, and is sometimes mixed with some sand. In the larger fragments of clay, which retain the cleavage of the original layers, we find 

fragments of the leaves of. Dioonites buchianus, portions of ste*ms of 

Frenelopsis ramosissima, fragments of both an undetermined angiosperm leaf, and of other undetermined leaves.

EXPOSURES FROM ACCOTINK TO NEAR ALEXANDRIA.

On the turnpike leading from Accotink to Alexandria the exposures 

are very few and poor. Little is to be seen besides the debris arising 

from the disintegration of the Potomac and Pleistocene deposits. As
138 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

we approach Alexandria, in the top of the hill overlooking the mouth of 

Hunting Creek we may see a brownish-red, sandy clay, which is without 

stratification, and which seems to be Pleistocene. This apparently rests 

on a cobblestone bed which contains some large masses of Cambrian 

quartzite. The cobblestone bed rests oil a layer of current-bedded,. 

brownish-gray sand, in which many small pebbles are dispersed. The 

exposures are then interrupted. At a lower level in the hill there is 

an isolated outcrop of a white, highly argillaceous sand, which appears 

to be the representative of the light-colored sand member of the Potomac. This differs somewhat from the material on the same horizon in 

White House Bluff. It is of a finer grain and more uniform composition and texture arid is more argillaceous.

Near the bridge at the mouth of Hunting Creek a light-gray to 

white argillaceous sand has been excavated and utilized to some extent 

for building purposes. It is mottled with yellowish-brown. This 

seems to be Pleistocene. On the opposite side of the creek, between ifc 

and the Potomac Eiver, the Pleistocene sands and loams are well displayed. The loams have long been used for the manufacture of bricks.

The low-lying plain on which the city of Alexandria is built is composed of Pleistocene deposits. The Potomac formation is barely visible 

at a few points near water level. It is the usual light-colored sand. 

As the high hills west of the town that overlook this plain are composed of Potomac material, it follows that in the vicinity of the river 

there has been extensive erosion of the Potomac, by which this formation has been cut down much below its ordinary'level, the Pleistocene 

lying upon. it. This is not an unusual feature along the Potomac Eiver, 

for, as we have seen, it is quite common to find the high land near 

the river formed of Potomac material, while the banks are Pleistocene.

The Pleistocene deposits near the mouth of Hunting Creek are, in 

their lower portion, composed of well stratified sands and clays, gray, 

yellow, red, and brown. The uppermost portion is a fine, massive, brick 

loam of gray color. This latter is about 15 feet thick at its maximum, 

but, being at the surface, it has been much eroded.

EXPOSURES NEAR SEMINARY STATION.

Seminary Station is on the Alexandria and Fredericksburg Eailroad, 

2 miles west of Alexandria. In the vicinity of this place there are 

several partial exposures.

On the road leading from a mill near the station to the theological 

seminary there is a small exposure showing at base about 12 feet of a 

brownish-gray sand and over that 5 to 6 feet of mottled claystone, 

marbled with reddish and brownish colors. The basal sand is, in the 

main, dark-gray, but it has broad, undulating bands, lenticular in form, 

of a brownish-red color. The sand contains embedded clay balls. 

There are thenqe no exposures on this road till a point is reached near 

the top of the hill not far from the seminary. Here we may see 10 feet
FONTAINB.] EXPOSURES NEAR ALEXANDRIA. 139

of yellowish, sandy clay, mottled with light-gray, and occasionally with 

peach-blossom and pink colors. It has some layers of gravel in it.

On the road leading from Alexandria to the Episcopal high school 

there are some slight exposures. A gray clay, of sandy nature, and 

mottled with red and reddish-brown, occurs near the high school. It 

may be seen on the road leading from the high-school gate up the hill 

to the old redoubts on the top of the hill. Immense numbers of large 

Cambrian quartzite cobbles occur on the western slope of this hill and 

in the valley west of the line of redoubts. Similar cobbles with occasional large masses of pre-Cambrian rocks may be seen on the surface 

between the high school and Alexandria. It is noteworthy that this 

surface in its highest parts is not less than 200 feet above the plain 

on which Alexandria is built. It is on a level with the tops of the 

hills, composed of Potomac material, that overlook Alexandria. These 

materials are wanting on the surface of the Pleistocene which underlies Alexandria.

In order that some idea may be formed of the materials composing 

these higher lands the following section is given. It represents only 

matter near the surface. It was obtained at a spring on the road leading from the high school to Alexandria, at a level from 30 to 50 feet 

below the top of the hill before mentioned, on which the old redoubts 

are situated.

SECTION ON ROAD BETWEEN ALEXANDRIA AND THE EPISCOPAL

HIGH SCHOOL. .

1. The highest material seen is a yellowish-brown sand. This contains undulating thin layers and streaks of gray sand, and shows in its 

base small balls of clay. The thickness is 2 feet. The clay is found 

in the form of numerous small balls, cakes, or short films, and is clearly 

material that has been eroded and redeposited in the sand.

2. Next under this comes 6 feet of brownish-yellow sand. This has 

in places small clay pellets, arranged in undulating layers, and some 

lighter colored sand disposed in thin, wavy layers.

3. Under No. 2 comes 5 feet of brownish-red sand. This is full of 

small balls of gray clay that are arranged rudely in layers or are scattered through the mass.

4. Under No. 3 comes 6 feet of brownish-yellow sand with some thin 

layers having a peach-blossom and pink color.

5. The lowest material exposed is a mass of coarse, brownish-yellow 

sand, about 12 feet thick, much affected by current-bedding. A considerable portion of this material is replaced, in a very irregular manner, by a gray argillaceous sand which, in some places, graduates into 

a tenacious and true clay. This forms irregular patches in the sand. 

The color and texture of this material contrasts strongly with the 

enveloping brownish-yellow sand. The gray material looks like matter 

eroded from a source different from that furnishing the brownish sand
140 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

and seeins to have been deposited in masses in that sand. This illustrates a feature rather common -in the Potomac, for one kind of material 

is not uncommonly found embedded in, but distinct from, another kind. 

Toward the top of No. 5 occur layers of gray clay, formed of recemented 

material. Many distinct and large balls of this clay are scattered 

through the brownish-yellow sand.

Over the material shown in this section heavy beds of large cobbles 

occur. These may be well seen on the road ascending from the level of 

the, spring and near by it. Here is an exposure of several feet of gray, 

sandy clay, mottled with yellowish brown. A heavy bed of cobbles 

rests on this.

EXPOSURES ON THE RAILROAD NEAR SEMINARY STATION.

Several exposures may be seen in the cuts on the Alexandria and 

Fredericksburg Railroad going south from Seminary Station. They 

occur beyond the crossing of this road and the Virginia Midland Railroad. The first cut that makes any disclosure of the Potomac is found 

about 1£ miles from the station. This cut shows in its lowest part from 

2 to 3 feet of a light-gray sand,'much stained with brown and yellow. 

It seems to be the usual light-colored Potomac sand. At the northern 

end of the cut this sand is abruptly replaced by a mottled, sandy clay. 

This latter is dark gray mottled with brick red. On one side of the cut 

only 3 or 4 feet of this material is shown, but on the opposite side ifc lias 

thickened to 8 feet. On this side 6 feet of the sand appears beneath it. 

A layer of pebbles intervenes between the sand and the clay.

A few yards farther south the mottled clay swells up to 15 feet and 

overlies 4 to 5 feet of dark-gray sand that is mottled and stained with 

yellowish-brown. Tbe basal sand seems to have been eroded before 

the clay was laid down upon it. At this point the sandy clay is dark 

ash-gray, varying to violet-gray, and is mottled with various reddish

tints,

A. few yards still farther south the basal sand rises in the form of an 

arch to the height of 15 feet, and above it appears 10 feet of the clay. 

The lower portion of the mottled clay here, for the thickness of 3 or 4 

feet, shows parallel seams of the material, cemented and indurated by 

the deposition of lirnonite. At this spot the basal sand is in some 

places almost white in color and it is all strongly marked with currentbedding.

At the south end of the cut the mottled clay is abruptly replaced by 

a coarse, brownish-yellow grit, which is full of layers of small gravel 

and has large cobblestones in its base. All of these .materials are 

covered by heavy deposits of coarse gravel and cobblestones which 

form the surface.

Between the point last described and the localities in the vicinity of 

Telegraph Station there are several cuts of small depth that expose 

materials which, in the main, are similar to those just described. In
EXPOSURES NEAR ALEXANDRIA. 141

several of the cuts the proportion of sandy clay having- various colors 

is quite large, in many cases from 10 to 15 feet being shown with little 

other materials. The colors in these sandy clays are generally distributed in irregular masses and they are often bright. Among them 

brick-red, purplish-red, and reddish-brown are most common. Ashgray and violet-gray colors are also found. Occasional exposures of 

the white sand occur, but this is not so common as the highly-colored 

sandy clay.

EXPOSURES ON LITTLE RIVER TURNPIKE.

Little Eiver turnpike runs nearly due west from Alexandria, leading 

to and beyond Fairfax. Exposures of such material as was seen 

between the high school and Alexandria occur occasionally on this 

road, but as there are few excavations, and as the surface is covered 

by a great sheet of debris arising from the waste of the Potoinac, the 

exposures are very imperfect and disconnected. There are many 

cobblestones on the surface, often of large size, composed mainly of 

Cambrian quartzite.

The superficial capping extends for a long distance west of Alexandria along this road. Seven miles west of that town an outcrop of 

Potomac may be seen in a gully on the side of the road. This rests on 

eroded and much-decayed crystalline rocks. These are fine grained 

mica-schists and chlorite-schists. The lowest portion of the Potomac 

is a brownish-gray sand, 3 feet thick, which contains in its base some 

yellowish-gray sandy clay. On this rests a gray sandy clay, mottled 

with .yellowish-brown, 1 foot thick. Above this comes 3 feet of brownish-gray sand, mottled and banded with yellowish-brown. This latter 

contains some small clay balls and pebbles. Above the last-named 

material we have 2£ feet of yellowish sandy clay, containing sonic 

particles of gray clay that look like eroded and redeposited fragments. 

On the top of the whole is the usual surface cobble deposit. In a thin 

seam of clay occurring in the 1-foot layer some fragments of coniferous 

plants were found.

About half a mile west of this point the Potomac has nearly disappeared, and is represented by a few feet of reddish-brown sandy clay. 

Half a mile west of the last-named point the crystalline rocks form 

the surface, and all traces of the Potomac have disappeared.

It is noteworthy that the abundant surface cobbles that have accompanied us up to this point disappear when we pass into the terrane of 

the crystalline rocks and give place to the angular quartz fragments 

and other substances set free by the decay of these rocks. This is 

strong evidence tBat these cobbles came originally from the Potomac 

formation. We may place the western limit of the Potomac on the 

Little Eiver turnpike at about 7 miles from Alexandria. It then sweeps 

around to the northeast, toward the Potomac Eiver, so as to enter the 

District of Columbia a little west of Washington.
1 42 THE POTOMAC' FOEMATION IN VIRGINIA. [BULL. us. 

THE POTOMAC AT AND BETWEEN WASHINGTON AND BALTIMORE.

On entering the District of Columbia two members of the Potomac 

formation may be recognized. The lower is that traced through Virginia, and this is the only member recognized in that State. From 

the predominance of sand and sandstone in this it may be called the 

sandy member. The other, or upper member, is composed of sands 

and clays, mostly the latter, both being usually highly colored with 

tints due to oxide of iron. The clays greatly predominate. They have 

the colors arranged in irregular spots, patches, and seams, and on 

account of this they have been called by Mr. Philip Tyson and Professor Eogers the variegated clay group. This is the upper member 

of the Potomac described at Fort Washington.

The sandy lower member of the Potomac is visible at Washington 

and at several points between Washington and Baltimore, in the vicinity of the Baltimore and Ohio Eailroad. The farthest point north at 

which it has as yet been seen is Baltimore. This member is deeply 

buried under the variegated clays, and it is reached only in the deepest 

excavations. At Baltimore it contains all the plants there found whose 

horizon has been determined, with one exception. These occur in flaggy 

sandstone and shale at the top of the sandy member. The fossil which 

forms a possible exception is the cycad, Tysonia marylandica, found by 

Mr. Tyson. The exact horizon of this is not fully established, but 

from Mr. Tyson's account its place is either the base of the variegated 

clays or the top of the sandy member of the Potomac. The upper 

member of the Potomac, or the variegated clay member, is younger 

than the basal sandy member, as is shown by its superposition and its 

want of conformability, the former resting on the eroded surface of the 

latter. This upper member extends far beyond Baltimore to the north. 

It has been traced by Mr. McGee to and beyond the head of Chesapeake 

Bay.

THE GEOLOGICAL POSITION OF THE POTOMAC FORMATION.

In dealing with the question of age only the lower or sandy member 

will be taken into consideration, for this is the only portion of the formation known to be present in Yirgiuia. In fixing the age of the 

Potomac formation we are aided somewhat by its stratigraphical relations to known formations and to a greater extent by the plant remains 

that it contains.

The stratigraphical relations of the formation unfortunately do not 

fix within narrow limits its place in the geologic column. In Virginia 

the youngest formation upon which the lower, or sandy member of the 

Potomac is seen to rest, is the older Mesozoic or Ehetic formation. 

The interval of time, however, between the deposition of the Ehetic 

and the deposition of the Potomac beds must have been a considerable one. There are several reasons for coming to this conclusion: 

(1) Where the superposition of the Potomac on the Ehetic is visible
FONTAINE.] GEOLOGIC POSITION OF THE POTOMAC. 143

the latter is seen to have been greatly worn before the deposition of 

the former. (2) The lithologic and structural character' of the two 

formations is very different, implying a total change in the conditions 

of deposition. (3) The Ehetic is made up of sandstones and shales 

which are distinctly bedded, so that the dip and strike can be easily 

made out. The materials composing these beds were well sorted by 

water action. Before the deposition of the Potoinac the lihetic strata 

had been consolidated and, in the main, indurated, so as to form firm 

sandstones and shales, .or even slates. The Ehelic beds are in many 

places crushed, contorted, and faulted, all of which changes took 

place before the Potomac age. No traces of them are found in the 

Potomac. The Ehetic is also penetrated by numerous dikes of igneous 

rock, none of which pass into the Potomac beds. The interval of time 

separating the two formations must, then, have been long enough to 

permit the occurrence of important geologic changes. These resulted 

in the draining off of the Ehetic waters and in the lateral compression 

of the Ehe'tic areas, which caused crumpling and faulting of the strata 

and outpours of igneous rock. The Ehetic basins were elevated and 

formed into dry land, subjected to great erosion, and the most eastern 

of them then depressed and brought under water again. Certainly no 

Ehetic species of plant survives into the Potomac.

In Virginia the oldest formations coming next above the Potomac 

are the Eocene. In Maryland, and probably northward, the group of 

variegated or iron-ore clays comes next above the Virginia or sandy 

member of the Potomac, but with an unconformity produced by 

erosion. The age of this group is unknown, and its relation to the 

New Jersey Amboy clay has not as yet been determined. This Maryland group, which at present must be regarded as an upper member of 

the Potomac formation, is probably somewhat older than the New 

Jersey beds. If it should prove to be not older, it is most probably of 

the same age as the last-named formation.

The New Jersey beds, as is shown by their fossil plants, are certainly considerably younger than the Virginia member of the Po.tomac. 

So far as is yet known, the Amboy clay is not younger than the Cenomanian of Europe.

So far, then, as can be determined by the stratigraphy, the Virginia Potomac is considerably older than the Cenomanian, and much 

younger than the Ehetic. The evidence from the stratigraphy, so far 

as it goes, agrees well with that of the fossils found in the Potomac.

The Wealden formation is most probably not uppermost Jurassic, 

but the estuary and marsh equivalent of the oldest marine Neocomian. 

What will be said therefore concerning the Neocomian will include 

the Wealden.

The animal remains found in the Potomac are very rare. At Dutch 

Gap a single impression of the posterior part of a fish was found. 

This has a homocercal tail, and is about as large as a shad. Its affinities have not been determined. At the bank near Brooke. and at
144 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

White House Bluff, three or four species of Estheria have been found. 

They were identified as Estheria by Dr. C. A. White, but as yet their . 

geologic significance has not been made out. These are all the animal 

fossils that have been found.

When we consider the great numbers of fossil plants yielded by the 

Potomac beds, it is surprising that no traces of insects have been met 

with. The negative evidence in this case is entitled to more than common weight. A very large amount of material was collected at widely 

separated localities and carefully worked oven. A close search was 

made always for animal fossils. We are then fairly entitled to conclude that in the areas of Potomac material now exposed animal life 

was not abundant. The rarity of animal fossils can not be attributed 

to any character of the rocks unfavorable to their preservation, for 

many of the clays are well adapted to this.

The flora of the Potomac seems to have been an abundant one. It 

was rich in species of certain groups, but, as compared with modern 

floras, it was poor in types. A large amount of fossiliferous material 

was obtained from points located at intervals between James Kiver and 

Baltimore. The fossils found will give a fair idea of the general character of the flora. This flora has been studied by me, and is described 

in Monograph XV of the United States Geological Survey. The comparison of these plants with those of known fossil floras shows somewhat complex relations.

There is present in the Potomac flora a Jurassic element which is 

large in the very considerable number of genera that characterize that 

system. Some few of the genera begin as far back as the Rhetic. 

This element shows indications of decadence. The number of species 

of each genus is very small; generally only one or two. Yer7 few 

individuals of the species are met with, and they are usually local in 

occurrence. The species are nearly or quite all peculiar to the Potomac.

There is an important Wealden element in the flora. Many species

of Potomac plants are identical with species found in the Wealden of 

Europe, and this is the oldest known fossil flora that gives any considerable number of plants identical with Potomac species. Some of 

these species of the European Wealden are abundant and widely diffused plants in. the Potomac. But while the species common to the 

European Wealden and the Potomac are noteworthy, there is a still 

larger number of important species found jn the Potomac which are so 

nearly allied to Wealden species that they are with difficulty distinguished from them. These, although regarded as new species peculiar 

to the Potomac, are probably forms representing Wealden species, 

being modified by differences of environment.

The Jurassic and the Wealden elements combine to give a Jurassic 

or Mesozoic facies to the flora, and hence, so far as they go, give it a 

comparatively ancient character. The Jurassic or Mesozoic type of 

flora is, as is known, characterized by the overwhelming predominance
FONTAINE.] GEOLOGIC POSITION OF THE POTOMAC. 145

of four elements, viz: Equiseta, ferns, cycads, and conifers, and by the 

absence of angiosperms.

The formations which possess the largest number of species identical 

with those of the Potomac are those of the Middle Neocoinian or 

Urgonian. The strata of this age which occur in Greenland (in Koine 

and other localities) and the Wernsdorf beds of the northern Carpathians yield an Urgonian flora, which Heer and Schenk have described. 

In the plants coming from these regions we find the largest number of 

forms identical with Potomac species. The number of Potomac species 

nearly allied to TJrgouian forms is still larger. These identical and 

nearly allied species include many of the most characteristic, abundant, 

and widely diffused species of the Potomac. If we are to determine 

'the age by the largest number of important species identical with those 

}f known fossil floras, then we would without hesitation set it down as 

ranging from the Lower through the Middle Neocomian. A. very large

and important element of the Potomac flora is peculiar to this series. 

In this we find without doubt the most abundant, characteristic, and 

widely diffused species. As these are new, they can not give any direct 

evidence concerning the age of the formation, but, indirectly, the existence of such a large proportion of peculiar forms is favorable to the 

assumption that the age is Neocoinian. The flora of this formation is 

one of the least known, and any large collection of richly fossiliferous 

material from beds of Neocoinian age could not fail to furnish a great 

number of new species.

Then again, the relatively great development of the conifers, along 

with the existence of an important cycadaceous element, points strongly 

to the Neocornian as the era of the formation. The survival of a considerable Jurassic element iu the flora also indicates that it can hardly 

be younger than Neocoinian. While much the most important elements 

of the flora indicate an age not more recent than the Urgonian or 

Middle Neocomian, there are some species which point to a more recent 

era of deposition for the formation. There are one or two species which 

are probably identical with forms found by Heer in the Cenomanian 

beds of Greenland. These are local and are represented by very few 

individuals. A few of the species also may be considered as nearly 

allied to some occurring in the Greenland Cenomanian. These Cenomanian types are probably to be regarded as precursors not yet fully 

established, just as the Jurassic types must be considered as survivors 

not yet extinct.

The angiosperm plants present in the flora are much more important 

in giving a more recent facies to the flora. They show quite a large 

number of species, but these are almost always local in occurrence, and 

are represented in most instances by few individuals. In a number of 

cases only one or two specimens were found. It has been generally 

held that any considerable development of angiosperms in a fossil 

flora is strong, indeed conclusive, evidence that its age is not greater 

Bull. 145 10
146 THE POTOMAC FORMATION IN VIRGINIA. [BULL. 145.

than that of the Cenomanian. But apart from the evidence given by 

the older and predominant elements of the flora, there is reason to think 

that the Potomac flora is older than Cenomanian, even if we take into 

consideration the angiosperms alone.

The conclusion above mentioned is based solely upon the fact that in 

no flora older than Cenomanian has any considerable angiosperm element been found up to the present time, but various writers have with 

justice maintained that it is improbable that the apparently sudden 

appearance of augiosperms in great force in the Cenomanian represents 

the true state of the case. It is highly probable that they had numerous precursors and ancestors, which existed intheNeocomian, and perhaps some of them, at least, in the Jurassic. It is probable that some 

of the forms called Protorhipis are ancient angiosperms. The existence 

then of numerous angiosperms in a flora which is predominantly Neocomian, but which contains many surviving Jurassic types, is just what 

we would expect to find. But we have direct evidence of the existence 

of angiosperms in the Necomian. Heer has described from the Kome 

beds of Greenland, which are Urgonian in age, an angiosperm Avhich 

he called Populus primceva. Only a few specimens were found. This 

single occurrence has remained so long unsupported by other discoveries of angiosperms in the Neocomian that doubts have been expressed 

concerning the correct localization of these specimens. It was thoughtpossible that they really came from a younger flora. If the Potomac 

flora is in fact Neocomian, we have in this case a noteworthy illustration 

of the truth that positive evidence, however scanty, should outweigh 

any amount of negative evidence.

The Potomac angiosperms in their general character give evidence of 

an age greater than Cenomanian. It is true that we find in them genera, 

and possibly some species, that survive into the Cenomanian and even 

down to the present time, but taken as a whole they form a peculiar

group, totally unlike the floras of the Dakota and the Amboy beds. 

It is in the flora of the Dakota group, and of the Amboy clays of New 

Jersey, especially the latter, that we would expect to fiud the greatest 

number of plants identical with Potomac forms. Both of these floras 

are Cenomanian probably, and the Amboy flora, so far as yet known, 

is the one that comes next above the Potomac. There are one or two 

species that are probably common to the Potomac and the Dakota 

beds, or that are nearly allied, but.they are long-lived types, that come 

down to the present time with little modification.

By the kindness of Dr. J. S. Newberry, who studied and described 

the New Jersey Amboy flora, I have been enabled to examine a large 

number of drawings of the New Jersey plants. These plants are totally 

different from those of the Potomac. It is not certain that a single 

species survives from the Potomac into the Arnboy beds. What is even 

more significant, even the genera that are most abundant in the Potomac and most characteristic of that formation have no representative
FONTAINE.] GEOLOGIC POSITION OP THE POTOMAC. 147

in the New Jersey flora. It is clear that a very important gap exists 

between these two floras, and that an interval of time separates them, 

in which changes took place that produced an extensive destruction of 

vegetal types and altered the entire character of the flora.

The localization of the species of Potomac angiosperms and their 

slight development, as shown in the very few individuals that in most 

cases represent them, indicate that these forms are, comparatively 

speaking, newcomers and precursors or ancestors of forms destined to 

become the predominant ones. This indication is confirmed by the 

character of a number of the species. They appear to be complex or 

comprehensive types, uniting in one form features that in the process 

of differentiation will later distinguish separate species.

We may then conclude that the Potomac flora is not exactly like any 

known, but on the whole coincides most nearly with that of the Lower 

and Middle Neocoinian. If this be true, then, we find that in this flora 

the development of angiosperms in considerable numbers has been 

pushed back through a long period of time.

The extent of the area over which the Potomac beds are found entitles 

the formation to rank among the more important ones of the United 

States. If we limit its extent to the area occupied by the lower or 

sandy member, we still find it to be very considerable. If we take this 

member as stopping at Baltimore and existing no farther south than 

the Nottoway River, with an eastward extension to a north-and-south 

line passing through Fort Monroe, we would still have it spreading 

over 12,000 square miles of territory. The character of the Potomac 

materials indicates that they were deposited in comparatively shallow 

waters that were fresh or at most brackish. The waters appear to 

have been agitated and subjected to changing, and sometimes powerful, 

currents. As these deposits all show "beach" and "ebb and flow" 

structure, we may conclude that they were laid down in estuaries or 

along shore. From the position and character of the lower Potomac 

deposits between Baltimore and the Kottoway Eiver the idea is suggested that they were mostly formed in the united estuaries of two 

great rivers corresponding to the present Potomac and James. It is 

clear that the waters in which they were deposited were loaded with 

sediment, and this was constantly changing in character. The assumption that the Potomac was formed in or near the estuaries of great 

rivers best explains all the peculiarities of the formation. If this is 

true, we have in the conditions of deposition a resemblance to the Euro 

pean Wealden, the flora of which, in part at least, shows affinities with 

that of the Potomac.

INDEX.

Accotink, beds near...................... 137-138

Accotink road, beds on................... 122-123

Age of the Potomac formation........... 142-147

Alexandria, beds near............. 22,138,139-140

Alum Spring, beds at...................... 68-72

Animal remains of the Potomac formation. 143-144 

Appomattox River, beds on................ 27-30

Aquia Creek, beds on............... 93-94,95-100

Baltimore,Md.,beds near .................. 142

Boilings Bridge, beds at................. 17,25-26

Brooke, beds near....................... 22,85-93

Chafflns Bluff, James River, beds at........ 46

City Point, Va., beds near.................. 18

Clark, W. B., Eocene fossils determined by. 135 

Cockpit Point, beds at and near.......... 105-109

Colchester road, beds on ................. 110-111

Dana, J. D., cited .......................... 11

Deep Bottom, James River, beds at........ 40-44

Drurys Bluff, James River, beds at........ 46-47

Dumfries, beds near...................... 100-101

Dutch Gap, beds at and near............... 31-40

Dutch Gap Canal, beds on.................. 35-36

Estheria, occurrence of..................... 131

Falmouth, beds near ....................... 73-75

Fort Monroe, beds disclosed by well boring

at............................... 19,44-45

Fort Washington, Md., beds at....... 23,135-137

Fossils of the Potomac formation... 12,13,29,34,

35,36,37,42-43, 57, 83, 84,109-110,

114, 117, 132,134,137,142,143-147

Fredericksburg, beds at and near......... 20-24,

59-68,72-73 

Freestone Point, beds near............... 111-113

Freestone Point, Potomac River, beds at and

near.......................... 111-113,123

Geological position of the Potomao formation ..........;................... 142-147

Gunston Cove Bluff, Potomao River, beds

at ................................ 125-126

Hanover Junction, beds at and near...... 19-20,

54-55,56-57

Hazel Run, near Fredericksburg, beds on.. 66 

Indian Head, Potomac River, beds near.. 124-125 

James River, beds on................... 1.. 31-40

Lafayette (= Appomattox) formation, exposuresof...................... 24-25.39-40

Page. 

Little River turnpike, beds near........... 141

Lorton, beds near........................ 115-118

Massaponax River, beds on............. 20,57-58

McGee, W. J., cited........................ 12

Morris Ford, North Anna River, beds

at............................ 19-20,55-56

Mount Vernon, beds at................... 134-135

Neabsco Run, beds near................. 110-111

North Carolina, Potomao strata in...... 16,24-25

Nottoway River, beds on................... 17-18

Neabsco Run, beds on and near.. 101-102,113-115 

lignite deposit near.................. 109-110

Occoquan, beds near.................. 21,113-115

Petersburg, beds at.................. 18-19,26-27

Pohick Church, beds near............... 121-122

Pohick Run, beds on..................... 118-121

Point of Rocks, Va., beds at and near .. 18,27-30 

Potomac formation, age of............... 142-147

Powells Run, section on ................. 102-103

beds near............................ 104-105

Potomac Church, beds at.................. 75-77

Potomac Run, beds on and near......... 75,77-85

Quantico, beds at........................ 100-102

Rappahannock River, beds on ............. 58

Red Bank, Potomac River, beds at......... 133

Richmond, Potomac beds at................. 48-50

the Tertiary at......................... 50-54

Rogers, W. B., cited ...................... 11,12,

13,15,16,17, 20,29,42, 44, 45,123 

Seminary station, beds near ..... 138-139,140-141

Stafford, beds near......................... 94

Stony Point fishing shore, Potomac River,

beds at........................... 123-124

Trents Reach, James River, beds at....... 32-35

Telegraph road, beds on .... 79-85,99-100,113-115

Telegraph station (Lorton), beds near ... 115-118 

Uhler, P. R., Potomac fossil plants collected by........................... 13

"Washington, D. C., beds near.............. 142

Woodbridge, beds near .................... 114

Woodchopping road, beds on .............. 85

Weldon, N. C., beds at..................... 24-25

White House Bluff, Potomac River, beds

at................................ 126-132

Wilton farm, on James River, Potomac

beds at............................. 48

149