THE PLIOCENE CITRONELLE FORMATION OF THE GULF COASTAL 

PLAIN. 

By GEORGE CHARLTON MATSON. 

PRESENT INVESTIGATION. 

In the spring of 1910 the writer, working 

under the direction of T. Wayland Vaughan, 

geologist in charge of Coastal Plain investigations, undertook a study of the later Tertiary 

formations of the Gulf Coastal Plain. According to the plans outlined before the work was 

begun, the beds that had formerly been 

grouped under the names Lafayette formation 

and Grand Gulf formation were to be studied 

with a view to their possible separation into 

more satisfactory stratigraphic unit~ that 

might be correlated with other formations 

which, on the basis of their fossils, had been 

assigned to their proper positions in the geologic 

time scale. The original plan included a study 

of the post-Vicksburgian Tertiary deposits 

from western Florida to Mississippi River and 

correlations with formations previously recognized in Florida, southern Alabama, and Louisiana. This plan was subsequently modified 

to extend the investigation as far west as Sabine 

River. The field work was interrupted and 

the office work was delayed by calls for geologic 

work in other areas, so that the preparation 

of the reports could not be begun until the 

spring of 1914. 

SCOPE OF THIS REPORT. 

In this paper it is proposed to discuss the 

general character and relations of beds of 

Pliocene age and to leave for subsequent 

publications the details of lithologio character 

and distribution of the Pliocene beds recognized. Although not abundantly fossiliferous, 

these Pliocene deposits contain some fossil 

plants, and at two localities-near Lamberts, 

north of Mobile, and at Red Bluff, on Perdido 

Bay-it was possible to obtain collections 

adequate for correlation. The fossil plants 

from these places are described elsewhere in 

this report by Mr. Berry, who also discusses 

their geologic age. 

EARLY INVESTIGATIONS. 

The area considered in this report has been 

studied by a number of geologists, though few 

of them had adequate opportunity to examine 

the formations in detail. The major portion 

of the time at their command was naturally 

devoted to the fossiliferous formations, and 

the nonfossiliferous beds were assigned to 

groups that by some investigators were probably regarded as separable into formations 

provided enough time could be given to their 

study. The result of this method of classification was to group together beds differing in age 

and having very extensive geographic distribution. The beds of each of these groups 

were composed of materials of similar lithologic character that appeared to be nonfossiliferous. The lithologic character of the materials naturally depended on two factorsthe source of the material and the processes to 

which the sediments had been subjected si,p.ce 

their removal from their original position. 

The sediments of the Coastal Plain were 

transported to approximately their present 

position by streams that existed when deposition began, and in any broad drainage basin 

the material for the different formations from 

the older to the younger was derived either 

wholly or in part from the older lands to the 

north, even though shore currents shifted 

soine sediments from one place to another and 

eroded and redeposited materials along the 

strand line. In some places, however, no 

terrigenous deposits were laid down and the 

formations are made up either wholly or in 

part of organio or chemically precipitated 

material. 

As the detrital deposits came from a common 

source, they vary only in accordance with the 

character of the changes to which they have 

been subjected since they were eroded from the 

older land surface. The processes of weathering that have affected the formations of the 

167 
168 SHORT'ER CONT'RIBUT10NS TO GENERAL GEOLOGY, 1916. 

Coastal Plain since their deposition have been 

comparatively uniform, and for this reason the 

sediments of different ages in the Coastal Plain 

that are now available for examination are 

very similar. In the later part of Tertiary 

time, except in Florida, there was a general 

absence of strictly marine conditions, and 

organic deposits of that age are either thin or 

0 F 

both because of lack of time and because of 

lack of funds for their prosecution. 

FORMATION NAME. 

The name Citronelle formation is applied to 

sediments of Pliocene age, chiefly nonmarine, 

that occur near the seaward margin of the Gulf 

Coastal Plain, extending from a short distance 

~ Fayette 

sandstone 

S.EO ="""""'=='=====SEQ """""'""""""""""'IOcO====:::JISO Miles 

c 0 

FIGURE 15.-Map of Louisiana, Mississippi, and Alabama showing approximately areas covered by Citronelle formation and Fayette 

sandstone and boundary between Catahoula sandstone and underlying form.ations. Sections along lines A-A', B-B', G-G', D-D', E-E' 

are given in Plate XXXII; sections along lines F-G, G-H, and H-Iin figure 20 (p. 219). 

entirely lacking. The Coastal Plain forma- east of the western boundary of Florida westtions in general are composed of clays and ward to Texas. (See fig. 15.) Citronelle, a 

sands derived from the older land, transported town on the Mobile & Ohio Railroad, in the 

to their present position by the streams and in northern part of Mobile County, Ala., was 

places by marine currents and waves and sub- chosen as the type locality because of the exjected since their deposition to similar pro- cellent exposures of the formation in its vicincesses of weathering. As a result the deposits ity, especially to the north along the railroad 

are so similar that they can be divided into dis- for a distance of 3 or 4 miles. The best collectinct formations only by laborious investigation tion of fossas was obtained from a clay bed a 

with careful attention to detail. Such studies few miles south of the type locality, near a stawere beyond the reach of early investigators, tion called Lamberts, where a flora sufficiently 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. . 169 

well preserved to permit correlation of the beds 

with the Pliocene was found. 

SYNONYMY. 

The Citronelle formation is the equivalent of 

a portion of the deposits formerly classified as 

"Drift," "Orange sand," and "Lafayette." 

The name can not be regarded as a synonym for 

any of the older terms because in all the earlier 

descriptions the old names were made to include not only the Citronelle formation, but 

overlying alluvial and colluvial sands and 

gravels and extensive areas of sand and O'ravel 0 

lying farther inland and belonging to a number 

of different terranes. In addition, the earlier 

applications of the old names were such as to 

include beds of Pleistocene age, forming a fringe 

between the Pliocene beds and the coast and 

extending into the river valleys. The development and use of the different names will be 

mentioned briefly in discussing the earlier publications. 

The Lafayette of some writers may have 

included not only the Citronelle but portions 

of the older formations, together with the 

alluvial material forming the flood plains and 

terraces along the streams and the colluvial 

sands, gravels, and clays distributed on the 

slopes. In some places portions of the unweathered Miocene and · Oligocene beds were 

called Lafayette, though in most localities 

only the weathered portions were included. 

The use of the term "Lafayette" was so elastic that exposures on the valley slopes might 

all have been included in the formation, even 

though the outcrops of the older beds were 

unweathered, or the unaltered beds might 

have been called "Grand Gulf," the weathered 

deposits "Lafayette," arid the relations of the 

~eposits explained by a8suming that the contact · between the ''Lafayette'' and ''Grand 

Gulf'' was irregular. 

PREVIOUS DESCRIPTIONS. 

One of the earliest reports on the geology 

of the Gulf Coastal Plain contains a description of what is called "Diluvium or northern 

drift." 1 The areal distribution of the materials described indicates that the gravels of 

glacial origin borderin~ the Mississippi are 

included under that term, together with the 

alluvial deposits on some of the smaller streams 

and also, it is thought, some of the beds of 

1 Wailes, B. L. C., Report on the agriculture and geology of Missis-

\ippi, pp. 245-253, Mississippi Geol. Survey, 1854. 

Pliocene age. The siliceous pebbles mentioned 

in this report; especially the fossiliferous cherts, 

are similar to those found in some of the grav~ 

els of the Citronelle formation, though this can 

hardly be regarded as conclusive evidence for 

correlating these deposits with the Citronelle1 

because such materials may be expected in: 

other formations derived from the same sources; 

Th~ occurrence of the porphyry mentioned by 

Wailes would probably be limited to gravels of 

glacial origin, because it is unlikely that such 

materials would be derived from the sedimentary formations occurring in the drainage basin 

of the Mississippi and its tributaries. 

In a report by Harper, 1 published three years 

after that of Wailes, the name "Orange sand" 

is used to designate materials that had been 

called "Diluviuru." in the earlier publication. 

This name, which was taken from a report by 

Safford, 2 continued in use for many years, 

though it was necessary to redefine it in order 

to harmonize the application by different writers. It is clear from the following quotation 

that Harper 3 intended to apply this term to a 

part of the deposits herein named Citronelle 

formation: 

This cover is thickest where the Eocene meets the Mi~ 

cene. It decreases southward towards the seacoast and 

the State of Louisiana; but nevertheless hills of the Orange 

sand, with the characteristic pebble stratum, are found 

here and there even very near the seacoast. 

The application of the terms Eocene and Miocene by this writer is so unusual and so different from the present usage that for the benefit 

of those who may not have access to Harper's 

geologic map, it seems advisable to say that he 

included in the Eocene the entire area south of 

the outcrop of the Jackson formation. It is in 

this area that his Orange sand includes the Citronelle formation. A discussion of the other 

beds classed as Orange sand by Harper may be 

omitted because they lie outside of the area 

included in this report. 

The reports cited above were based on brief 

reconnaissances in which observations were 

made at widely separated localities. Although 

some of the later investigations were of similar 

character, a tendency toward more detailed 

study is shown in the character of the reports. 

The deposits included in the Citronelle formaI Harper, L., Preliminary report on the geology and agriculture of the 

State of Mississippi, p. 162, Mississippi Geol. Survey, 1857. 

2 Safford, J. M., Geologica~ reconnaissance of Tennessee, pp. 148, 162, 

1856. 

a Harper, L., op. cit., p. 162. 
170 SHORTER CONTRIBUTIONS T'O GENERAL GEOLOGY, 1916. 

tion were first systematically discussed on the from which they are derived. The report con~ 

basis of extensive field investigations by Hil- tains some speculation as to the means of transgard,t who included beds belonging to the portation of these pebbles to their present 

Citronelle formation in the "Orange sand." positions. 

He said: After the publication of the papers cited 

But the formation which gives character to the surface there was a revision of the nomenclature beconformation of the State-whose presence is the rule, cause of the confusion resulting from the 

and whose absence the exception requiring special men- differences of usage in different areas. The 

tion; which forms the main body of most ridges and, to a very great extent, their surface also--is that which has necessity for this revision was emphasized when 

been very appropriately designated by Prof Safford, the McGee 1 correlated his Appomattox formation 

State geologist of Tennessee, as the Orange sand formation. with "at least a part of the Orange sand of 

. The term "Orange sand," as used by Hilgard, Hilgard and other southern geologists." 

mcludes some· of the deposits herein named In 1891 the name "Orange sand" was reCitronelle formation and also portions of older placed by "Lafayette forn1ation," the type 

and younger deposits correlated with them on locality for the new name being in Lafayette 

the basis of lithologic character. County, Miss.,2 where, according to Berry,3 the 

. In a discussion of the "Orange sand" Hop- " Orange · sand" is of Eocene (Wilcox) age. 

kins 2 correlated some of the materials in This change was made at a conference in which 

Louisiana with the Orange sand of Hilgard. Hilgard, McGee, LeConte, and Loughridge parIn his section across the ·State the name ticipated, and the decision reached by them 

"Prairie diluvium" includes the beds now was subsequently indorsed by Safford:1 Mecalled the Citronelle formation, as well as both Gee was the first geologist to apply the term 

older and younger beds. A subsequent report Lafayette to deposits distributed over the entire 

by I-Iopkins 3 contains a more comprehensive area of the Coastal Plain and extending across 

discussion of the" drift," in which he included a the edges of the older formations between 

large amount of material varying in age from Washington and the Rio Grande. 5 Although 

th.e oldest beds exposed in Louisiana to the he regarded his Lafayette as somewhat more 

Pleistocene. Among these beds are the de- restricted than Hilgard's Orange sand, it is 

posits here named Citronelle formation, which dear that he included in the Lafayette the beds 

Hopkins mentioned in connection with the now known as the Citronelle formation. This 

distribution of the drift in some of the parishes is shown by numerous references to the localisouth of the outcrop of the beds later desig- ties where this formation is at the surface,' by 

nated Catahoula sandstone. The geologic map the maps accompanying the report, and by 

accompanying Hopkins's report shows the nu1nerous diagrams showing the relations of the 

area occupied by the Citronelle formation as formations exposed on the Gulf Coastui Plain. 

covered by what he calls "drift." A third The accompanying sections (figs. 16 and 17) 

publication by this author discussed the illustrate the relations of the "Lafayette" to 

"drift" in more detail, giving special attention older and to younger formations, and if the 

to its constitution and origin.4 The identifi- thickened portion of the "Lafayette" as 

able fossils· occurring in pebbles are listed, but represented in these sections is separated from 

unfortunately some of the localities where these a portion of the thin layer on the upland it will 

pebbl~s were collected were not given, though represent approximately the Citronelle formaHopkms stated the age of the geologic forma- tion. As the present report does not deal with 

tions that supplied these pebbles and the pos- the major portion of the deposits coating the 

sible geographic distribution of the localities surface of the upland, it has seemed best to 

1 Hilgard, E . W., Report on the geology and agriculture of the State 

of Mississippi, pp. 4-29, Mississippi Geol. Survey, 1860. 

• Hop~s, F. V., Louisiana Geol. Survey First Ann. Rept., for 1869 

(mcluded m the annual report of the Louisiana State Seminary of Learning and Military Academy), p. 78, 1870. 3 Hopkins, F. V., Louisiana Geol. Survey Second Ann. Rept., pp. 

21-26, 1871. 

. • Hopkins, F. V., Louisiana Geol. Survey Third Ann. Rept. (included 

m annual report of D. F. Boyd, superintendent Louisiana State u niversity, for 1871), pp. 190-203, 1872. 

omit them from the discussion. 

1 McGee, W J, Three formations of the middle Atlantic slope: Am . 

Jour. Sci., 3d ser., vol. 35, p . 330, 1888. 

2 Hilgard, E. W., Orange sand, Lagrange, and Appomattox: Am. 

Geologist, vol. 8, pp. 129-131, 1891. 

a Berry, E. W., The Lafayette formation: Jour. Geology, vol. 19, pp. 

249-256, 1911 . 

4 Hilgard, E. W., op. cit., p . 131. 

6 McGee, W J, The Lafayette formation: U.S. Gl)ol. Survey Twelfth 

Ann. Rept., pt. 1, pp. 430-501, 1891. 
l'. 8. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLA 1 £ XXXII. 

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PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 171 

·After the appearance of the report by McGee 

the name ''Lafayette'' was adopted by writers 

on Coastal Plain geology and soon came into 

general use, in an area extending from Maryland to Rio Grande. The present report, 

however, is restricted to the area from western 

Florida to eastern Texas and does not deal 

with the Atlantic border of the Coastal Plain 

though some of the superficial materials on the 

older Tertiary and the Cretaceous formations 

are not included in the Citronelle. In dealing 

with the thin coatings of weathered sand or 

clay resting on the older beds it is in many 

places difficult to obtain enough physiographic 

evidenee to warrant correlations, because of 

the lack of topographic maps. 

FIGURE 16.-Section through the Coastal Plain in western Alabama; generalized in part from sections constructed and described by E. A. 

Smith and L. C. Johnson. (After McGee.) 

or with the major portion of the region west 

of Sabine River. 

Clendennin 1 discussed the "Lafayette formation" as developed in Louisiana, and the 

geographic distribution as he described it is 

much the same as that of the Citronelle, but 

this author followed McGee by including older 

materials also. 

A paper by Dall and Stanley-Brown 1 gives 

details of a number of sections along Apalachicola River in Florida and notes on exposures 

at Bainbridge and Macon in Georgia. In this 

paper the upper member of each of the sections 

on Apalachicola River is called Lafayette, and 

these beds are correlated with the exposures 

farther north. If any of these exposures are to 

FIGURE 17.-Section through the Coastal Plain in the Mississippi embayment; generalized in part from sections constructed and described 

by E. W. Hilgard and L. C. Johnson. (After McGee.) 

The "Lafayette" in Alabama was deseribed 

by Smith,2 who summarized its distribution as 

follows: 

·while the materials of the Lafayette formation are to be 

found as a superficial covering over the entire Coastal 

Plain of the State; that is, over all the Tertiary and Cretaceous strata, lapping in places even far over upon the 

edges of the Carboniferoui3 and other Paleozoic terranes-:-

it is only in the two Gulf-bordering counties, Mobile and 

Baldwin, that we find it forming one of the structural units, 

intercalated between the Biloxi and Second Bottom 

deposits above and the fossiliferous clayey sands of the 

Pascagoula horizon below. 

The portion of the " Lafayette " which this 

author called a structural unit belongs to the 

Citronelle formation, and some portions of 

what lies above the older formations to the 

north may be outliers of this formation, 

1 Clendennin, W. W., The Florida Parishes of east Louisiana and the 

bluff, prairie, and hill lands of southwest Louisiana: Louisiana Geol. 

Survey, pt. 3, pp. 187-192, 1892. 

2 Smith, E. A., Johnson, L. C., and Langdon, D. W., jr., Report on 

the geology of the Coastal Plain of Alabama, p. 66, Alabama Geol. Survey, 1894. 

be classed with the Citronelle formation it is the 

upper part of the section at Alum Bluff, though 

from their conformity with the clays resting on 

the Miocene the fine sands and clays forming 

the upper portion of the bluff are believed to be 

older than Pliocene. 

The Citronelle formation in Louisiana is 

included in the beds called "Lafayette formation" by Harris, 2 who made special reference to 

the occurrence of this formation on the east side 

of Mississippi River and in the ''Florida parishes." A large part of the information in this 

report seems to have been gleaned from the 

earlier publications by former geologists of 

Louisiana and from the comprehensive paper by 

McGee. The information concerning the territory covered by this formation west of the 

1 Dall, W. H., and Stanley-Brown, Joseph, Cenozoic geology along 

the Apalachicola River: Geol. Soc. America Bull., vol. 5, pp. 169-170, 

1894. 

2 Harris, G. D., Geology and agriculture of Louisiana : Louisiana 

Geol. Survey, pt. 5, pp. 99-107, 1899. 
172 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

Mississippi is somewhat less complete than that 

about the area east of the river, but special 

mention is made of the gravel pit in the southern part of Rapides Parish. Another paper by 

the same author was published in one of the 

subsequent reports of the Louisiana Geological 

Survey.1 This paper is devoted very largely 

to a discussion of the theories of the origin 

of the "Lafayette formation," with some mention of the distribution of the beds referred to 

it. The localities noted in the area covered by 

the Citronelle formation are Neame, where the 

beds are over 275feet above sea level; Sulphur, 

where the same deposits are reported to occur 

in a well extending to a depth of more than 400 

feet below sea level; and Morgan City, where 

they extend to a depth of more than 500 feet. 

The relations of these deposits are shown graphically in a diagram of the country from a point 

north of Jackson, Miss., southward through 

Tangipahoa, Amite, and Hammond to the 

Gulf. 

The reports by Crider 2 afford litt]e additional information concerning the Citronelle 

deposits .. This author included in the "Lafayette formation" the beds in the southern 

part of Mississippi belonging to the Citronelle 

formation and stated that they were reported 

to attain a thickness of over 200 feet. 

Veatch 3 dealt in considerable detail with 

the "Lafayette formation" in Louisiana, including in this formation beds belonging to the 

Citronelle, as shown by his diagram and by his 

statement of the detailed distribution of the 

formation. He said: 

South of the Catahoula and Fleming formations these 

sands and gravels form the surface for miles and then pass 

southward beneath the more recent clays of the Quaternary, forming there the water-bearing beds which furnish 

a portion of the waters used in the irrigation of that region. 

It is possible that in addition to the beds 

described in this quotation some of the deposits classed as Lafayette lying north of the 

area described are outliers of the Citronelle 

formation. 

1 Harris·, G. D., Geology and agriculture of Louisiana: Louisiana 

Geol. Survey, J?t. 6, pp. 32-36, 1902. 

2 Crider, A. F., and Johnson, L. C., Summary of the undergroundwater resources of Mississippi: U. S. Geol. Survey Water-Supply Paper 

159, pp. 12-13, 1906. Crider, A. F., Geology and mineral resources of 

Mississippi: U. S. Geol. Survey Bull. 283, pp. 44-46, 1906. 3 Veatch, A. C., Geology and underground water resources of northern Louisiana: Louisiana Geol. Survey Bull. 4, pp. 43-45, 1906. 

In 1907 Smith 1 discussed the "Lafayette formation," which he made to include the area 

underlain by the Citronelle formation. The . 

term "Lafayette formation" as used in this 

report is not synonymous with Citronelle formation, but some of the beds covering a portion of the "Grand Gulf" and lying south of 

the northern margin of that formation :may 

belong to the Ci trone Ue. 

MatRon and Clapp 2 discuss the "Lafayette 

formation" in Florida in a general way. The 

Citronelle formation comprises only a minor 

part of the area assigned to the "Lafayette" 

in Florida and is not included in any of the 

detailed sections given. In this. report the 

name "Lafayette" is used for the weathered 

portions of deposits of Oligocene, Miocene, and 

Pliocene age and for surface deposits of various 

ages. 

STRATIGRAPIDC RELATIONS. 

The Citronelle formation rests upon the 

Pascagoula clay (Miocene) and overlaps the 

Oligocene formations. Wherever the contact 

with the underlying formations has been observed it is marked by an unconformity, the 

older formations having been eroded before the 

deposition of the Citronelle formation. (See 

Pl. XXXII.) The seaward :margin of the 

Citronelle formation is overlain by beds of 

Pleistocene age, and contacts between the 

Pliocene and younger beds can be seen at many 

places. · They show a marked unconformity, 

the Citronelle formation having been eroded 

and the materials derived from it having been 

incorporated with materials from other sources 

to form the deposits of Pleistocene age. (See 

Pl. XXXIII.) The unconformity ·may also 

be inferred from the. fact that the Pleistocene 

deposits form terraces along the seaward margin of the Citronelle formation and extend into 

the valleys of the major streams, reaching entirely across the Citronelle and resting upon 

the still older formations. The relation of the 

Citronelle to the other formations of the Gulf 

Coastal Plain is shown in the accompanying 

table. 

The Citronelle formation differs from both 

the older and the younger formations in being 

1 Smith, E. A., The underground water resources of Alabama, pp. 

24-25, 302-316, Alabama Geol. Survey, 1907. 

2 Matson, G. C., and Clapp, F. G., A preliminary report on the geology 

of Florida, with special reference to the stratigraphy: Florida Geol. 

Survey Second Ann. Rept., pp. 141-145, 1909. 
Age. 

Pleistocene. 

Eastern Texas. 

Pensacola terrace. ·Hammond terrace. 

Port Hickey terrace. 

St. Elmo terrace. 

Louisiana. 

Pensacola. terrace. 

Hammond terrace. 

Port Hickey terrace. 

St. Elmo terrace. 

Plei&tocene and Tertiary formation& of the Gulf C008tal Plain. 

Misslil!ippi. 

Pensacola terrace. 

Hammond terrace. 

Port Hickey terrace. 

St. Elmo terrace. 

Alabama. 

Pensacola terrace. 

Hammond terrace. 

Port Hickey terrace. 

St. Elmo terrace. 

Western. 

Pensacola terrace. 

Tsala Apopka terrace. 

Newberry terrace. 

Florida. 

Peninsular. 

Un~~--------------------------~·-----------------------j----------------------- 1-----------------------·~----------------~----------

Pliocene. 

Citronelle formation. N o n m a r i n e chiefly, including high-level terraces. 

Yellow and red sands and clay, locally 

gray where unweathered. Gravel near the landward margin. Thickness, 50-

250 feet. 

Citronelle formation. Nonmarine 

chiefly, including high-level terraces. 

Yellow and red sands and clays, locally 

gray where unweathered. Much 

gravel near the landward margin, 

especially in the valley of Red River. 

Thickness, 50-400 feet. 

Citronelle formation. N on m a r in 0 

c~liefly, including high-level terrae(:;. 

l' ellow and red sands and clays, locallv 

gray where unweathered. Mu('h ~avel near the landward n1argin and 10 ~he valleys of the principal streams. 

Tluckneas, 5().....400 feet. 

Citronelle formation. N on mar i n e chiefly, including high-level terraces, 

marine fossiliferous equivalents in 

wells. Yell ow and red sands and 

clays; loc·ally gray where unweathered. 

Grovel near the landward margins con- tains fosgil plants. Tllickues8, 50-340 

feet. 

Citronelle formation. Nonmarine chiefly, including 

high-level terraces; marino 

fossiliferous equivalents in 

wells. Yellow and red 

sands and clays, some gravel. 

Thickness, 50-150 feet. 

Caloosahatchie and 

Nashua marls. 

Marine. 

Uncoruonnity-----1·-------------------------------l--------------------------------l------------------------------ -------------------------------l------------------------1----------------

Miocene. 

Pascagoulaclay. Marineinpart. Blue. 

green, and gray clay, locally calcareous 

and fossiliferous. Some large calcare- ous concretions and many small 

nodules. Some layers of sand and 

sandstone. Thickness, 250-300 feet. 

Pascagoula clay. Marine in part. Blue, 

green, and gray clays, locally calcareous 

and fOBBiliferous. Some layers of sand; 

locally a sandstone near the top. Many 

nodules of calcium carbonate in some layers of the clay. Thickneas, 250-

450 feet. 

Pascagoula clay. Marine in part. Blco 

and gray clays and sands; some thin 

beds of sandstone; locally gravels and 

conglomerates at the base. Fossilifer- ous marls in places and some clays 

with nodules of calcium carbona~. 

Thickness, 50-400 feet. 

Pascagoula day. Marine in p~ut. Blue 

and gray clays and sands. Thickne&'i, 

2CO feet. 

Choctawhatchee 

rine. 

marl. Ma- Jacksonville formation. Marine. 

Uncoruormity-----1------------------------------- l-------------------------------·l------------------------------l------------------.------------l---.------------------~~--------------

Oligocene. 

Hattiesburg clay. Nonmarine. Blue 

an4 gray clay; some layers calcareous. 

Tllin layers of sand and sandstone. 

Thickness, 300-350 feet. 

Catahoula. sandstone. Nonmarine. Gray 

sands, sandstones, fine conglomerates, 

quartzite8, and clays. Thickness, 

about 475 feet. 

H~ttiesburg clay. Nonmarine. Blue 

and gray clay; some beds calcareous. 

Thin layers of sand and sandstone. 

Thlckness, 300-350 feet. 

Catahoulasandstone. Nonmarine. Gray 

sands, eandstones, fine conglomerates, 

quartzites, and clays. Thickness, 600-

800 feet. 

Hattiesburg clay. Nonmarine. B]Je 

and gray clay with thin beds of sa1d 

and sandstone. ThicknCBB, 35Q....tpO 

feet. 

I 

Catahoula sandstone. Gray sands, saUlstones, fine conglomerates, quart~.i , and green clays. Thickness, 2 0 

feet. 

I 

L / Vicksburg VickBburg limestone. Marine. limestone. 

-----------------1---------------------------------~--------------~ 

Fayette sandstone. Marine. Gray sands, sand- --------

Eocene. 

I 

~ 

~~ri~le~:t.~ziTthies~~ and d~k calca ~ Jackson formation. Marine. 

Jackson formation. Marine. FOBBilif- Jackson formation. Marine. Gray erous marls, clays, and some thin beds sands, sandstones, and dark calcareous 

of sandstone. Traceable into typical clays. Thickness, 100-160 feet. 

Jackson of Mississippi. Thickness, 50 

feet. 

30830~-17. (To face page Ii2.) a Redefined. 

IIattiesburgclay. Non- AlumBluffformarinC'." Blue and 

gray clay with thin mation. Ma- ci. Alum Bluff formation. Marine. 

beds of sand and sancl- rinc. ::s 

stone. Thickness,300 ~ •---------------------------- feet. l--------1 ~ • 

Catahoula sandstone. 

Gray sands, sandstones and green clay. Thickn~llS (m a x 1 -

mum), about 200 feet. 

Chattahoochee 

formation. 

Marine. 

St. Stephens limestone. Marine. 

b According to C. W. Cooke. 

0 

(,) 

:a 

(,) 

C1 "' 

A 

~ 

Cnattahoochee formation. Marine. 

Marianna limestone. Marine. 

Ocala limestone. b Marine. 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 173 

predominantly sandy, with many lenses and 

scattered pebbles of chert gravel. To understand fully the application of the name it is 

necessary to know the mode of deposition of 

the formation. After the Miocene strata had 

been laid down the Coastal Plain was eroded 

into broad, shallow valleys having approximately the same positions as those of the present streams. These valleys were filled by the 

deposition of Pliocene alluvial sands and gravels, and near the coast the deposits were extended across the interstream areas. This 

filling formed a broad plain, which was later 

partly eroded, while at the same time the 

margin of the formation was pushed farther 

seaward and three successively lower plains 

were built, their sediments resting on the older 

deposits. The original deposit formed a seaward-sloping plain, and each successive addition formed a new plain, which was represented 

also in the stream valleys. (See Pl. XXXII.) 

Some portions of the deposits were doubtless 

reworked by the waves, especially in the interstream areas, and this accounts for the more 

complete rounding of the pebbles in portions 

of the seaward areas of the formation than in 

the stream valleys and the development of 

flat plains with shallow ponds in the interstream areas. The closing stage of the deposition of the successive plains is marked by the 

fine sandy silts laid down at flood stages while 

the streams were eroding their beds to lower 

levels. The deposition was pushed far seaward in some places, as shown by sands with 

Pliocene fossils at Pensacola and Mobile and 

by a leaf-bearing clay at Red Bluff, on Perdido 

Bay, Ala. 

The sediments included in the Citronelle 

formation have sometimes been referred to the 

Pleistocene because of their physiographic resemblance to the terraced deposits of Pleistocene age. Thej differ from the Pleistocene 

deposits of the Gulf coast, however, by being 

more sandy and containing more gravel. The 

evidence of their greater age is shown by their 

mature dissection, which in general exceeds that 

of the coast Pleistocene and glacial deposits, and 

by the weathered condition of the pebbles, many 

of which are composed of chert so completely 

decomposed . as to break or even crumble 

easilyin the hand. The Pliocene age is further 

shown by the presence of the Pliocene fossil 

plants, those at Red Bluff showing that the deposi tion was originally extended some distance 

beyond the southern margin of the general 

area shown in figure 15 (p. 168), and that the 

Pleistocene material was later deposited upon 

the eroded surface of the Citronelle. The line 

between the Pliocene and Pleistocene plains 

is drawn between the 150-foot and 200-foot 

terraces. The Pleistocene age of the plains at 

lower levels is shown by their slight dissection 

and by the presence of crystalline pebbles in 

the fluviatile portion of the 150-foot terrace 

at Natchez, Miss. 

The 150-foot terrace rises gradually landward 

~o 170 feet or more, and its representatives in 

the river valleys are still higher, attaining elevations of somewhat more than 200 feet a short 

distance from the coast. Deposits of this terrace contain the pebbles of crystalline rocks at 

Natchez-the Natchez formation. 

The Pliocene plains are ·more eroded than the 

Pleistocene plains and when compared in the 

same area they may be distinguished by this 

difference. The Pliocene material was considerably eroded before the formation of the 

uppermost Pleistocene terrace, and relatively 

wide valleys were developed even on some of 

the small streams. The formation . of the 

Pleistocene terraces was begun by a submergence that affected the drainage some distance 

from the coast and resulted in the formation of 

flat, swampy areas along most of the streams. 

These readjustments in drainage conditions are 

still marked by swamps in many of the small 

stream valleys in southern Alabama, Mississippi, Louisiana, and southeastern Texas, the 

flat-bottomed valleys being overgrown by · 

swamp vegetation through which the streams 

meander in very poorly defined channels. 

LITHOLOGY. 

The Citronelle formation is predominantly 

sandy but contains varying amounts of clay in 

the form of thin layers or lenses. (See Pl. 

XXXIV.) The relative proportion of materials of different kinds varies from place to 

place, and in any one section it varies from top 

to bottom, though in general sand is everywhere abundant (see Pls. XXXV and XXXVI), 

and at many places there is some gravel. The 

proportion of sand is greatest near the base, 

and the formation contains more sandy material in the vicinity of the principal drainage 

lines than in the interstream spaces. There is 
174 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

also a larger percentage of sand and gravel near 

the landward than the seaward margin of the 

formation (see Pis. XXXVII and' XXXVIII), 

though in making comparisons it is necessary to 

consider large masses and to choose localities 

bearing about the same relations to the principal lines of drainage, because more sand and 

gravel will be found in the vicinit:y- of large 

streams near the coast than on some of the interstream spaces farther inland.. 

The sands of the Citronelle formation are sufficiently rounded to indicate that they have 

been subjected to extensive attrition. The 

pebbles vary greatly in degree of rounding; 

many of them are much waterworn and resemble pebbles found on the beach, others are only 

slightly rounded, and still others are subangular. 

The sand is predominantly quartz, and the pebbles in the Mississippi embayment are mostly 

chert, with a somewhat smaller percentage of 

crystalline quartz and quartzite. Many of the 

chert pebbles contain cavities lined with chalcedony in the form of agates or geodes, and a 

large number of them are fossiliferous, containing fragments of corals, orinoid stem, and other 

organic remains such as are common in the 

Paleozoic limestones and chert. In Alabama 

there are many pebbles of quartz, with a subordinate percentage of quartzite, but in some 

places chert pebbles are numerous. In western 

Louisiana the pebbles are mostly quartz with 

some admixture of chert. 

Certain peculiarities in the distribution of 

materials in the Citronelle formation are worthy 

of mention. In Mississippi the outliers are in 

· general composed of coarse subangular chert 

gravel, with a varying percentage of small, 

well-rounded quartz pebbles. In Louisiana 

and Alabama quartz gravel is common. The 

coarse pebbles have in a measure the same 

composition and evidently came from a common source, and the same is true of the fine 

pebbles. The coarse subangular fragments 

are mostly chert and have been subjected to 

fluviatile wear with very little subsequent 

rounding by waves, but many of the finer, wellrounded pebbles are quartz and have doubtless 

been shaped by prolonged' subaqueous erosion. 

Probably the subangular material was brought 

directly from its original source, and the fine, 

well-rounded material was subjected to wave 

erosion by being deposited in some one or 

more of the older formations of the Coastal 

Plain, from which it was obtained by Pliocene 

streams. This conclusion seems warranted 

because the gravel is a poorly assorted aggregate or' material of all sizes from sand to pebbles an inch or two in diameter. (See Pis. 

XXXIX, XL, and XLI.) If the gravel had 

been subjected to wave action during Pliocene 

time it would have been assorted, and it is 

therefore inferred that the perfect rounding of 

the small pebbles was accomplished by wave 

action during earlier geologic periods. 

The clays vary greatly in character, some of 

the beds being relatively pure and others distinctly sandy. On the whole the sandy clays 

predominate, and in ma;ny places thin layers 

of sand are found in the clay beds. 

The Citronelle formation has a very wide 

range in color. Doubtless when fresh some 

of the materials included in the formation were 

either blue or gray with small percentages of 

red and yellow, but nearly all the exposures 

available for observation at the present time 

have been subjected to so much weathering 

that the original colors have been partly or 

wholly obliterated. The sands in most of the 

sections are either orange or red, though near 

the surface some of them are yellow. This 

predominance of the orange and red colors on 

weathered surfaces led to the designation of 

the materials in this formation, either wholly 

or in part, as "Orange sand." The clays vary 

in color according to the degree of weathering 

to which they have been subjected, beds that 

have been buried to sufficient depth to protect 

them from the oxidizing effect of the atmosphere or surface water being either pale blue 

or gray. Where the oxidation has been slight 

they are mottled, the ·first change producing 

spots or blotches of a light-red or peculiar purplisheolor. As the process of weathering continues the purple color disappears and a deepred color predominates, and still further weathering changes the red color to pale yellow. 

The iron in the clay, which presumably causes 

the coloring, changes during the process of 

oxidation to hydrous oxide, and on complete 

weathering of the clay to a yellow color the 

iron becomes more or less aggregated into 

nodules of varying sizes. Somewhat similar 

aggregations of iron hydroxide occur in the 

sands in the form of concretions and geodes, 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXIII 

.A. CITRONELLE FORMATION (?) OVERLAIN BY PLEISTOCENE SAND AND LOAM ON THE 

WEST BANK OF ESCAMBIA RIVER SOUTH OF MOLINO, FLA. 

B. ANOTHER VIEW NEAR THE SAME LOCALITY, SHOWING UNCONFORMABLE CONTACT. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXIV 

A.. CLAY PIT OF THE SOUTHERN BRICK CO., 1 MILE NORTH OF 

GOULDING, FLA. 

B. SANDSTONE .AND CLAY, PIT OF THE SOUTHERN BRICK CO., 1 MILE 

NORTH OF GOULDING, FLA. 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 175 

or as shells inclosing cores of clay. In other 

places the iron accumulates in the form of 

plates cementing thin layers of sand or as a 

filling for more or less definite cracks that 

cross the strata at high angles. Where the 

iron hydroxide is disseminated among the sand 

grains or pebbles it transforms the bed into a 

sandstone or conglomerate. This cementatio"n 

apparently takes place near the surface and as 

a rule does not extend for any great distance 

along a bed, so that in most places the sandstone and conglomerate are in the form of 

blocks of varying sizes. (See Pl. XXXVIII, 

A.) The irregularity in size and shape of these 

blocks usually prevents them from being utilized in construction, even where they are ftrmly 

cemented, but south of Citronelle, at the 28-

mile siding on the Mobile & Ohio Railroad, a 

hard sandstone belonging to this formation 

has been quarried to a depth of over 5 feet, and 

the rock obtained at this locality has been used 

in the construction of a church. 

The texture of the formation in any particular locality depends entirely on the arrangement and relative percentages of the different 

kinds of materials. In general, the sands form 

more or less continuous beds containing lenses 

of pebbles or clay that in some places have 

considerable horizontal extent and in others 

are limited to only a few feet. Many clay lenses 

are only a few feet to a few rods in extent, 

though in places they interlock with other lenses 

lying above and below them, and in that way 

present the appearance of a continuous bed, 

unless it is possible to examine the section in 

detail. In the sands cross-bedding and cross 

lamination are the rule rather than the exception, the layers of clay and pebbles in many 

places being inclined at high angles. Smooth, 

even bedding is comparatively rare except in 

the clay lenses. 

In this formation, as in many others, pebbles 

of clay are common, occurring at many stratigraphic horizons and having a wide geographic 

distribution. Most of them are more or less 

perfectly rounded balls of clay embedded in 

sand in such a way as to form a lenticular clay 

conglomerate. In places there are more or less 

rounded blocks of clay in which the original 

lamination is still preserved. Here and the.re 

this lamination coincides in direction with that 

of the sands, but as a rule it differs. Doubtless 

many of the pebbles were eroded from the 

subjacent clay by the currents that transported 

the sand. Some of the pebbles and probably 

most of the angular fragments were derived 

from the undercutting of small cliffs developed 

by waves or currents. (See Pl. XLII.) This 

explanation for blocks of clay embedded in 

sand was first suggested by T. Wayland 

Vaughan 1 to account for similar clay fragments found in some of the sands of the Wilcox 

formation, but it is also applicable to such clay 

fragments in the Citronelle formation. 

STRUCTURE. 

In the Citronelle formation the arrangement 

of the sediments in lenticular beds makes it 

impossible to determine the structure by observations on the elevation of beds (see Pis. 

XXXIX, A, and XL, B) except at a few localities where the conditions are unusually favorable. The prevalence of cross lamination and 

cross-bedding renders determinations of dip 

with a clinometer valueless because the inclinations observed may not agree in direction 

with the dip of the beds, and they may vary 

in direction or be in opposite directions in a 

single section. Under such unf~vorable conditions the recognition of local structural features is everywhere difficult and in most places 

impossible. 

The inclination of the base of the formation 

can be measured in many places, especially 

near the landward margin. The results of such 

measurements show that the dip is not uniform, 

and in addition to irregularities resulting from 

deposition on an uneven surface there is a general increase in the rate of inclination of the 

base of the formation toward the coast. These 

facts are well shown by the section across southwestern Mississippi and eastern Louisiana, 

where the contact of the Citronelle with the 

older formations descends at an average rate 

of a little less than 6 feet to the mile from a 

point 6 miles north of Wesson to Osyka, Miss., 

and at a much higher rate from Osyka, Miss., 

to Hammond, La. (See Pl. XXXII.) Other 

sections show steeper inclinations, but they 

are based on less accurate and less detailed 

information. The base of the Citronelle formation near the coast is drawn at the transition 

1 Unpublished notes. 

• 
176 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

from sands, sandy clays, and gravel to finergrained sediments, chiefly clays, containing 

characteristic fossils of the Pascagoula clay, 

and the correlations are probably approximately 

correct, though there may be errors of a few 

feet in the position of the contact because of 

slight inaccuracies in the well logs used for the 

purpose of obtaining measurements of depths 

to the base of the Citronelle formation. Where 

the computatiOns of the rate of dip of the base 

of the formation are made on a line several 

miles in length, the amount of error in the rate 

per mile is minimized. 

The actual dip of any portion of the formation may be less than the inclination of the 

base, because deposition probably began near 

the coast and was gradually extended inlan<;I .. 

This is suggested by the fact that the coarse 

gravels that form outliers along the landward 

margin of the formation. rest on finer deposits 

belonging to the Citronelle farther south. For 

example, the coarse gravel rests on the Pascagoula clay near Wesson, but at Brookhaven 

it overlies about 130 feet of red sand with interbedded clay that is included in the Citronelle. 

The gravels are covered near Summit by still 

younger sediments that are more argillaceous 

than those below. 

Near the coast the upper portion of the 

formation was laid down on the seaward slope 

of the earlier deposits, and in this portion 

the actual dip is probably greater than the dip 

of the base of the formation. In the absence 

of reliable data to use in computing the true 

dip, however, it seems best to give the inclination of the base. The exact dip of the base 

of the formation is not easy to det·ermine, 

because near the coast there are no exposures 

that reach the contact with the Pascagoula 

formation and it is difficult to interpret the 

information supplied by some of the logs of 

wells. Such computations as have been made 

give discordant results that are probably not 

entirely due to the incomplete information 

but are partly explained by local variations 

in the rate of dip. 

In the northern part of Mobile County, Ala., 

the base of the Citronelle formation is about 

240 feet above sea level, and at Citronelle, 4 

miles south of the county line, the contact 

with the underlying Pascagoula clay is at 

220 feet above sea level, giving an average inclination of about 5 feet to the mile. From 

Citronelle southward to Semmes the contact 

descends from 220 feet above sea level to 92 

feet below sea level, or at the rate of about 

16 feet to the mile. 

At Laine, Miss., where the surface is· about 

10 feet above sea level, characteristic fossils 

of the Pascagoula formation were obtained 

from the De Lamorton well at a depth of 615 

feet (?), and the base of the gravelbeds was 

reached at 605 feet. This well is 53 miles 

S. 28° W. from the place in Mobile County, 

Ala., where the elevation of the base of the 

formation was determined, indicating a dip 

of nearly 16 feet to the mile, provided the gravel 

reached in the well is at the base of the Citron- · 

elle formation; but as the gravel may be somewhat above the contact the dip thus determined 

should be regarded as the minimum inclination of the base of the formation rather than 

the exact amount. 

Near Maxie the base of the Citronelle formation is about 215 feet above sea level, · and at 

Bond, 6 miles S. 20° E., a well reached the 

contact of this formation with the clays of the 

Pascagoula at an elevation of 115 feet. The 

steep dip between these two points, nearly 17 

feet to the mile, is explained by the fact that the 

formation was deposited on an uneven surface. 

Computations of the rate of dip between Maxie · 

and Laine give about 14 feet to the mile, and 

the direction is about S. 45° E. At Fontainebleau, 1 about 20 feet above sea level, fossils belonging to the Pascagoula clay were obtained in 

a well at a depth of 500 feet. These fossils 

probably came from beds near the top of the 

Pascagoula, and they show that the dip of the 

Citronelle formation between Maxie and Fontainebleau is between 14 and 15 feet to the mile 

in a direction S. 35 ° E. 

In western Mississippi measurements of the 

dip of the Citronelle formation have been made 

farther inland than in eastern Mississippi and in 

Alabama. (See Pl. XXXII.) Six miles north 

of Wesson the base of the formation is 435 feet 

above sea level, and just east of Brookhaven it 

is 330 feet above sea level. Here the contact of 

the Citronelle with the older formations descends about 115 feet in 15 miles, or at the rate 

of nearly 8 feet to the mile. From Brookhaven 

to Osyka the contact descends from 330 to 100 

1 Crider, A. F., and Johnson, L. C., Summary of the underground 

water resources of Mississippi: U.S. Geol. Survey Water-Supply Paper 

159, p. 32, 1906. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXV 

.A. SANDS AND SANDY CLAYS WITH PLATES OF LIMONITE-CEMENTED 

SANDSTONE, NORTHERN ESCAMBIA COUNTY, FLA. 

B. SAND CONTAINING A LENS OF CLAY, WITH A PLATE OF SANDSTONE 

CEMENTED WITH LIMONITE AT THE CONTACT OF THE SAND AND 

CLAY, NORTHERN ESCAMBIA COUNTY, FLA. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXVI 

A. SAND AND CLAY, FLOMATON, ALA. 

B. SAND AND GRAVEL, FLOMATON, ALA. 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 177 

feet above sea level, or 230 feet in about 39 

miles, which is at the rate of 6 feet to the mile. 

From Osyka southward the dip is difficult to 

determine because of meager information, but 

it apparently becomes steeper, amounting to 

approximately 810 feet between Osyka and 

Hammond, a distance of 35 miles, which would 

be at an average rate of about 23 feet to the 

mile. These determinations are all made along 

a line extending nearly north and south. 

In Louisiana many wells have been drilled 

south of the margin of the Citronelle formation, 

but the materials penetrated in most of them 

are not described in the records with sufficient 

exactness to permit correlations. Between 

Centerville, Miss., and Bass, La., the rate of dip 

of the base of the formation is about 19 feet to 

the mile, and it must increase rapidly toward 

the south, as is shown by the great thickness of 

the overlying Pleistocene deposits at New Orleans. Between Osyka, Miss., and Bass, La., 

the rate of descent is about 10 feet to the mile 

and the direction about southwest. This is less 

than the slope from C~nterville to Bass and 

probably indicates that there was a delta in the 

Mississippi Valley during this epoch rather than 

an embayment. West of the Mississippi few 

data that can be used in computing the dips of 

the formation have been obtained. About a 

mile north of the Jennings oil field, where the 

strata are undisturbed and the surface is only 

about 25 feet above sea level, Miocene fossils 

were found at a depth of 1,960 feet. 1 The base 

of the Citronelle formation near Woodworth, 

about 70 miles north of the Jennings oil P'lld, is 

approximately 140 feet above sea level. This 

would give a dip of nearly 30 feet to the mile, 

provided the fauna found near Jennings came 

from the top of the Pascagoula clay .. This dip 

represents a maximum, and the actual rate of 

descent between the two points is probably not 

so great. 

The figures given in the foregoing discussion 

should be regarded as only approximate measures of the rate of inclination of the base of the 

formation. The general dip of the deposits 

beneath the Pleistocene along the southern 

margin of the formation is shown in Plate 

XXXII and amounts to less than 20 feet to the 

mile. The general dip is toward the south, 

though there are doubtless many local variations in direction. Dips determined from the 

1 Harris, G. D., Oil and gas in Louisiana: U.S. Geol. Survey Bull. 429, 

p. 58, 1910. 

elevations of the bases of outliers and from logs 

of wells drilled where the Citronelle formation 

is at the surface are much lower than 20 feet to 

the mile, and the rate is variable because the 

formation was deposited on a surface that was 

uneven and had a steep slope near the Pliocene 

strand line. This steep slope accounts for the 

fact that near the coast the dip becomes steeper, 

probably exceeding 20 feet to the mile. In the 

vicinity of New Orleans the original slope near 

the Pliocene strand line may have been increased by subsidence during Pleistocene time, 

as shown by the great thickness of Pleistocene 

beds (2,443 feet) overlying the Tertiary formations.1 

The thickness of the Pleistocene beds at 

Fort Morgan, Ala., suggests a steep descent of 

the Pliocene Citronelle formation. The log of 

the well at that place, shown in Plate XLIII, 

in pocket, was made from a series of samples 

of drillings, and the subjoined list of fossils was 

furnished by Mr. Dall. 

Fragments of echinoderms and two Recent 

shells, Donax variabilis Say and Paramya Depth (feet). 

subovata Conrad.......................... 30-32 

Dentalium acus Dall, Nassa acuta Say, Ervilia sp. junior, Ostrea, and echinoid fragments, all Recent ....................... . 

Recent shells, Mulinia lateralis Say, Donax 

variabilis Say, Area transversa Say, etc ..... 

Recent species, Abra requalis Say, Area 

transversa Say, Strigilla earnaria Linne, 

Mulinia lateralis Say, Leda, Pecten, and 

Dolium fragments ....................... . 

Strigilla earnaria Linne, Recent ........... . 

Ostrea £ragmen ts ......................... . 

Recent species, Ervilia cf. E. nitens Montgomery, Phaeoides crenella Dall, Mulinia 

lateralis Say, Diplodonta sp., Modiolus 

demissus fragments, Cadulus and · Cymatium fragments ........................ . 

Fragments of Phacoides, Mulinia, Ostrea, 

and Margin ella sp.; probably Recent ..... . 

Divaricella cf. D. dentata Wood, fragments; 

Mulinia cf. M. lateralis Say; Cadulus frag, 

ments; and Phacoides sp., young; prob· 

30-40 

32-87 

100-112 

169-175 

175-256 

217-321 

322-337 

ably all Recent................ . . . . . . . . . . 523-570 

Nassa acuta Say, upper Miocene to Recent .. 1, 076-1,089 

Ostrea fragments ...........•.........•.... 1, 089-1, 121 

Conus pealei Green, Recent .••...•.•.•...... 1, 244-1,290 

Mulinia lateralis Say, Recent to Miocene .•.. 1, 290-1,330 

Chione sp., fragments ..................... 1, 350-1,370 

Ostrea fragments, Anachis cf. A. avara Say, 

and a fragment of Turritella sp .....•.... 1, 378-1,573 

The material above 1,290 feet is nearly all Quaternary; 

that below may J>QSsibly be Pliocene. 

1 Harris, G. D., op. cit., p. 170. 
178 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916~ 

This well is 35 miles S. 35° E. from St. Elmo, 

where the Pliocene reaches an elevation of 130 

feet above sea level. The surface of the Citronelle formation descends about 1,220 feet 

between these two points, or at the rate of 

more than 35 feet to the mile, but the original 

inclination may have been much less, because 

although the Pliocene has been eroded at both 

localities the amount of post-Pliocene erosion 

was greater at Fort Morgan, which is near the 

axis of Mobile Bay. 

Near Pensacola the available information 

suggests a somewhat lower dip for the top of 

the Citronelle formation. At Pine Orchard, 

nearly 7! miles north of Pensacola, the top of 

the formation is about 160 feet above sea level, 

and about 4! miles south of Pensacola the shells 

obtained at a depth of 256 to 276 feet below 

sea level were tentatively referred to the Pliocene by Mr. Dall. This indicates a maximum 

dip of 416 feet in a distance of about 12 miles, 

or at the rate of about 35 feet to the mile, 

approximately toward the south. 

From the foregoing statements it seems evident that there is a rapid increase in the rate 

of inclination of the top of the Pliocene near 

the coast at Pensacola and Mobile and north of 

New Orleans, and similar conditions doubtless 

prevail along the entire shore line. This apparent steeper dip at Pensacola and Mobile is, 

however, partly explained by the fact that in 

the wells cited the seaward equivalent of the 

Citronelle formation may have been reached 

some distance above the · level of the fossilbearing beds. Still, even after liberal allowances have been made for inaccuracies due to 

this cause, it is. obvious that the top of the 

Citronelle formation must originally have had 

a steep seaward slope near the coast. 

THICKNESS. 

The thickness of the Citronelle formation is 

variable, ranging from a few feet near the landward margin of the formation to several hundred feet near the coast. Exact determination 

of the thickness near the coast is difficult 

because it is necessary to rely in part on logs 

of wells for data, and the information obtained 

from this source is not entirely satisfactory. 

The uncertainty is increased by the fact that 

the overlying Pleistocene beds consist of sands 

and clays resembling those belonging to the 

Citronelle formation. 

In southern Alabama the Citronelle formation forms a high upland reaching nearly 350 

feet above sea level, and in the adjacent valleys 

the older formations are not exposed in the 

valleys where the elevation is less than 100 

feet. Thus the formation may have a thickness of more than 250 feet. The log of a well 

drilled about 12 miles west of Mobile shows 

sand and gravel to a depth of 245 feet and an 

additional thickness of pink sand and clay 

amounting to 95 feet. This gives a minimum 

thickness of 245 feet and a maximum of 340 

feet. 

In Mississippi the thickness of the outliers of 

the Citronelle formation as a rule does not 

exceed 20 to 30 feet. At Bond a well penetrated 90 feet of sands and gravel belonging to 

this formation before encountering the underlying Pascagoula; The ·De Lamorton well at 

Laine passed through sand containing some 

tree trunks at a depth of 141 to 153 feet. The 

materials beneath this sand are described as 

sand, shale, and gumbo to a depth of 615 feet, 

where Pascagoula fossils were obtained. These 

materials included a gravel bed with its base 

at 605 feet, and if this is at the base 9f the 

Citronelle the thickness of the formation at 

that place may be as great as 452 feet. At 

Fontainebleau the Pascagoula clay was reached 

at a depth of 500 feet,t and as the Pleistocene 

beds are probably as thick there as at Laine, 

the thickness of the Citronelle formation at 

Fontainebleau is estimated to be about 350 

feet. 

A well at Osyka passed through 150 feet of 

sand and gravel belonging to the Citronelle 

formation, and the formation has an additional thickness of about 50 feet beneath the 

upland. Farther south, at Hammond, the 

thickness may amount to several hundred 

feet, though the formation is not readily separable from the Pleistocene, and accurate determination of thickness is impossible. Near 

Brookhaven the Citronelle formation has a 

thickness of about 130 feet, but it thins to a 

few feet within a distance of 20 to 30 miles 

farther north. 

Information concerning the thickness of the 

Citronelle formation in Louisiana is meager. 

The formation has been observed through a 

vertical range of over 150 feet, and may safely 

t Crider, A. F., and Johnson, L. C., Underground water resources 

of Mississippi: U. S. Geol. Survey Water-Supply Paper 159, p. 32, 1906. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXVII 

A. 

B. 

SILT UNDERLAIN BY SAND AND GRAVEL OF THE CITRONELLE FORMATION, ROCKY 

SPRINGS, MISS. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXVIII 

A. 

B. 

GRAVEL PIT 1 MILE SOUTH OF WEATHERSBY, MISS. 

Photographs by E. W. Shaw. 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 179 

be credited with a minimum thickness of that 

amount, and this agrees with the thickness assigned to the formation in the well at Ludington. (See Pl. XLIII, in pocket.) East of 

Mississippi River the well at Bass penetrated 

90 feet of yellow clay and 10 feet of sand, like 

the materials exposed at Port Hickey. This 

portion of the section is referred to the Pleistocene. It rests on 220 feet of clay that is regarded as the southward extension of the clays 

found in the upper part of the Citronelle formation in southern Mississippi, and the cherty 

sands and chert gravels immediately below this 

clay may safely be correlated with the sand and 

gravel that form the basal part of the Citronelle formation. This would give a thickness 

of 400 feet for the formation at Bass, and if the 

subjacent clay is included the thickness is still 

greater. Near the coast the deposits of Pleistocene sand, gravel, and clay can not be separated from the Citronelle formation, and its 

thickness has not been determined, though it 

probably thins rapidly in that direction. 

TOPOGRAPHY. 

GENERAL FEATURES. 

The Citronelle formation occupies an area 

where the characteristic topography of the seaward margin of the Coastal Plain changes to 

the more diversified topography of the Tertiary 

beds. The contrast between the varied physiographic expression of this formation and the 

relatively uneroded terraces of Pleistocene age 

farther south is striking and would serve as a 

basis for mapping the seaward margin of the 

formation except where remnants of the Pliocene plains have been preserved. The landward. margin of the formation could not be so 

readily distinguished by this method, because 

the topography of the older formations is s~ milar to that of the Citronelle. 

The original surface of the formation was a 

series of plains with gentle slopes toward the 

Gulf (see Pl. XXXII), but its present topography is the result of dissection so thorough 

that only a small percentage of the area remains 

level, the surlace being mainly reduced to 

slopes interrupted by terraces, though near the 

coast a coating of rna terials of more recent age 

obscures the original surface. The general 

homogeneity of the deposits that constitute the 

formation permits the drainage channels to form 

under the influence of the original slope and 

independent of the stratigraphy. Each of the 

major streams has numerous tributaries, providing a dendritic drainage that is adequate for 

nearly all the area covered by the formation. 

In a few places may be seen remnants of the 

original surface of the formation, with drainage 

so deficient that they are partly submerged 

during periods of prolonged rainfall. On these 

remnants the variations in level are so slight 

that they are scarcely noticeable except where 

there are ponds either in the form of small 

rounded or elongated depressions o-vergrown 

with cypress and other swamp vegetation. 

Some of the higher hills, especially those with 

flat tops, are remnants of the surface of the 

oldest plain, and by noting their elevations it 

is in places possible to reconstruct an upland 

having a :maximum elevation of a little more 

than 500 feet above sea level. However, some 

of the hills are mere erosion remnants, the 

original surface having been wholly removed, 

though the rate of degradation has been sufficiently uniform locally to preserve a certain 

amount of uniformity in elevation over small 

areas. This condition is found where a bed of 

resistant material, such a.s gravel, is encountered, and a general seaward inclination results 

because such beds have a slight dip in that 

direction. The slope of the original surface of 

the formation remained nearly undisturbed 

after the time of deposition, because the formation was raised bodily to its present height 

with only slight change in attitude resulting 

from a gentle warping of its surface. The 

presence of remnants of terraces in the valleys 

that cross the Pliocene upland shows that the 

Citronelle formation has been terraced since 

the formation of the highest plain. The lower 

plains resemble the upper one in topography, 

but they are much narrower and the continuity 

of their deposits with those beneath the upper 

plain suggests that the materials of the lower 

plains were laid down in the closing stages of 

the Pliocene. 

The range in elevation of the exposures belonging to the Citronelle formation amounts to 

over 450 feet, because the formation crops out 

in a few places near the level of the Gulf, and 

its outliers cap some of the high hills in the 

areas underlain by Oligocene formations. The 

average difference is about 250 feet, and if the 

upper surface of the formation is considered, 

the range is from about 170 feet on the sea-
180 SHORTER CONT'RIBUTIONS TO GENERAL GEOLOGY, 1916. 

ward margin to more than 500 feet on some of 

the higher outliers farther inland. In general, 

the increase in altitude is comparatively rapid, 

the surface rising from about 170 feet to approximately 300 feet within a distance of a few 

miles, and upon this portion of the formation 

narrow seaward-facing plains are developed. · 

PLAINS. 

SUBDIVISIONS AND GENERAL CHARACTER. 

The surface of the Citronelle formation is 

divided into four plains, designated, in descending order, the Brookhaven · plain, the 

Sardis plain, the Canton plain, and the Loxley 

plain. (See Pl. XXXII.) This arrangement is 

also in the order of age, the Brookhaven plain 

being the oldest and the others being successively younger. Each plain may be divided 

into two portions that merge with each other 

at their points of contact and may be called 

stream terraces and interstream plains. It has 

been customary to speak of these features as 

fluviatile and marine, though the propriety of 

such a designation may well be questioned, 

because some portions of the interstream plains 

are of as distinctly fluviatile origin as any 

portion of the stream terraces. The exact 

position of the contact between the deposits of 

the two types no doubt varied during ·the 

progress of deposition, the fluviatile sediments 

being pushed beyond the limits of the adjacent 

valley walls where the material was being 

carried seaward in the form of deltas and 

retreating into the valleys where estuaries 

were developed. For these reasons the terms 

stream and interstream, although arbitrary, 

are used ·instead of the more expressive but 

less accurate terms· fluviatile and marine. 

BROOKHAVEN PLAIN. 

DISTRIBUTION. 

The Brookhaven plain, named from Brookhaven, Miss., where it is well known, is the 

oldest and highest of the Pliocene plains and 

formerly occupied an area larger than the 

combined area of all the other Pliocene plains. 

Its interstream portion forms a belt from 10 to 

more than 60 miles in width, extending from 

western Florida across Alabama, Mississippi, · 

and Louisiana into southeastern Texas. The 

original width of this plain can not now be 

determined accurately because some portions 

of the deposit have been removed by erosion, 

but it must have been more than 30 miles 

wide north of the Alabama-Florida boundary 

and 60 miles wide in western Mississippi, from 

which it narrowed rapidly to somewhat more 

than 10 miles in southeastern Texas. 

The stream terraces of the Brookhaven plain 

are not well known outside of the Mississippi 

Valley, where they have been studied by 

E. W. Shaw.1 

ALTITUDE. 

Because of subsequent erosion a!ld deformation the altitude of the Brookhaven plain as it 

now appears is variable, being from about 350 

to 420 feet above sea level in western Alabama, 

from 420 to more than 520 feet in western 

Mississippi, about 320 to 380 feet in southern 

Louisiana, and about 340 feet in southeastern 

Texas. 

The landward boundary of the plain is not 

marked by any distinct scarp but by outliers 

of sand and gravel, locally overlain by sandy 

clay. These outliers are widely distributed, but 

their detailed distribution has not been mapped. 

SLOPE. 

The interstream portion of the Brookhaven 

plain slopes seaward at the rate of about 6 

inches to the mile, though local variations 

are to be found in places where the surface has 

been eroded. From the highest point on this 

plain in western Mississippi, where its surface is 

as much as 520 feet above sea level, the plain · 

slopes eastward to about 420 feet above sea 

level in western Alabama, or at an average of 

about 6 inches to the mile. The slope from 

western Mississippi to southeastern Texas is 

from 520 to 340 feet, or at an average rate of 

about 5 inches to the mile, in a direction south 

of west. As the greater portion of this descent 

appears to be in the eastern portion of Louisiana, the actual rate of slope near Mississippi 

River may have been nearly a foot to the mile, 

though the subsequent removal by erosion of 

this portion of the plain has made exact determinations impossible. The width of this plain 

in Alabama is variable but is much less than in 

Mississippi, and the slope toward the south is 

apparently about 2 feet to the mile. 

DISSECTION. 

The interstream portion of the Brookhaven 

plain has been deeply dissected by streams, 

and the original surface is preserved only on the 

1 Shaw, E. W., unpublished notes. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XXXIX 

A. IRREGULAR STRATIFICATION OF SAND AND GRAVEL, GRAVEL 

PIT 272 MILES SOUTH OF BROOKHAVEN, MISS. 

B. COARSE FRIABLE CONGLOMERATE 172 MILES SOUTH OF BROOKHAVEN, MISS. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XL 

A. GRAVEL 2 MILES SOUTH OF ROCKY SPRINGS, MISS. 

B. GRAVEL PIT 272 MILES SOUTH OF BROOKHAVEN, MISS. 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 181 

divides and in isolated hills. These divides are 

generally in the form of even-topped ridges or 

in some localities fiat areas several miles in 

width. A large proportion of this dissection 

was accomplished during the closing stages of 

the Pliocene, and the valleys developed at that 

time have not been broadened except at a 

few localities. This is shown by the fact that 

the Pliocene plains younger than the Brookhaven are represented by streain terraces 

in the valleys that cross the Brookhaven plain. 

LITHOLOGY AND SOILS. 

The material underlying the Brookhaven 

plain is in general coarser than that underlying 

the younger plains. Fine sediments, however, are the rule on the surface of the plain, 

and the soil derived from them is either loam 

or sandy loam, with subordinate amounts of 

clay. Where the plain has been sufficiently 

eroded to uncover the coarse gravels, as, for 

example, near Crystal Springs and Hazlehurst, 

:Miss., stony or gravelly loams are common. 

STRATIGRAPHIC RELATIONS OF THE DEPOSITS. 

The strata forming the interstream areas of 

the Brookhaven plain rest unconformably on 

the underlying formations, of upper Tertiary 

age, and merge with the materials beneath the 

next younger (Sardis) plain. The history of 

the formation and dissection of the Brookhaven plain forms a part of the history of the 

Citronelle formation, as a whole, which is discussed elsewhere. 

SARDIS PLAIN. 

Below the level of the Brookhaven is another plain that is well developed in the 

vicinity of Sardis, Miss., and is called the 

Sardis plain. The stream terrace of this plain 

in the Mississippi Valley north of the latitude 

of Vicksburg has been studied by E. W. Shaw.1 

DISTRmUTION. 

Stream terraces.-A stream terrace of the 

Sardis plain probably originally occupied an 

area in the Mississippi Valley extending from 

eastern Adams County, Miss., nearly to central 

Louisiana. This is the area where it should 

have been widest, because here it was formed by 

the combined action of Mississippi River and 

1 Shaw, E. W., unpublished notes. 

its largest tributaries in the region. Subsequent erosion has reduced this terrace to a 

band less than 15 miles wide bordering the 

east side of Mississippi River. The width of 

this terrace on Pearl River, farther east in the 

same latitude, was from 15 to 20 miles, and the 

width on both Leaf and Chickasawhay rivers, 

still fp,rther east, was nearly as great. On 

Alabama and Tombigbee rivers in Alabama 

this terrace was apparently twice as wide as 

on the rivers in eastern Mississippi. In Louisiana the stream terraces belonging to the 

Sardis plain have been so much dissected by 

erosion that it is difficult to estimate their 

width. 

lnterstream areas.-The interstream portion 

of the Sardis plain borders the southern margin 

of the Brookhaven plain and extends from 

central Alabama westward to southeastern 

Texas. In western Mississippi it has a breadth 

of about 30 to 35 miles, and it narrows gradually eastward to the Alabama-Florida 

boundary, where it is only about 15 miles wide. 

In western Louisiana and eastern Texas the 

interstream areas are still narrower, having an 

average width of about 10 miles. 

ALTITUDE. 

The altitude of the stream terraces of the 

Sardis plain is in general greatest near Mississippi River and declines gradually eastward 

to Florida and westward to Texas. They have 

a general slope in the direction of stream flow 

except in western Mississippi, where deformation has altered the original slope. On the 

east side of Mississippi River the stream terrace of the Sardis plain has an altitude of about 

380 feet above sea level east of Vicksburg and 

near Woodville and Centerville. Near Natchez 

it is about 75 feet higher. In the Pearl River 

valley the plain slopes from about 410 feet a 

few n1iles north of Jackson to about 400 feet 

east of Brookhaven and to 360 feet near the 

Mississippi-Louisiana boundary. On Chickasawhay River and along the streams in Alabama 

and west of Mississippi River in Louisiana the 

slope appears to be steeper, though exact 

determinations of altitude have not been procured. 

The altitude of the inner margin of the interstream plain in western Mississippi is about, 

420 feet, in western Alabama about 350 feet, 
182 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

and in western Louisiana about 320 feet. 

The outer margin of the plain has an altitude 

of about 350 feet in southwestern Mississippi, 

310 feet in western Alabama, and 270 feet in 

western Florida. In western Louisiana and 

eastern Texas the outer margin has an altitude of about 270 feet. 

The width of the plain is variable, being 

from 30 to 35 miles in western Mississippi, 10 

to 15 miles near the Alabama-Florida boundary, and 8 to 10 miles in eastern Texas. The 

narrowing of the plain both to the east and . 

west of its place of maximum width is gradual, though marked local variations are found 

where the terrace has ·been partly removed by 

erosion. 

SLOPE. 

The slope of the stream terraces of the 

Sardis plain was probably originally steep, but 

the exact rate can be determined on very few 

streams because there are few reliable determinations of altitude. In western Mississippi 

deformation has given the stream terrace a 

slope away from the coast from Natchez to 

Vicksburg at the rateof about 6 inches to the 

mile. From Natchez southward it slopes seaward at the rate of about 9 inches to the mile. 

The slope of the interstream plains has been 

less affected by changes resulting from deformation and, if the steep descent from the 

Brookhaven plain is omitted, amounts to about 

1 foot to the mile in western Mississippi, 

between 2 and 3 feet to the mile near the 

Alabama-Florida boundary, and about 3 feet 

to the mile in western Louisiana and eastern 

Texas. 

DISSECTION. 

The Sardis plain has undergone somewhat 

less erosion than the Brookhaven plain, 

though locally the stream terraces have been 

entirely removed. In general the terraces 

have been reduced in area by the meandering 

of the major streams and have been separated 

into small isolated plains by the erosion of 

tributary streams. 

The interstream plains are extensively 

eroded near the principal streams, but on 

some of the divides there are still remnants of 

the original surface from 5 to more than 10 

miles wide. Most of these remnants are 

crossed by shallow channels of wet-weather 

streams that provide adequate drainage, but 

in a few places, as, for example, near Atmore 

and Georgetown, Ala., the surface of the plain 

is so flat that shallow ponds form during wet 

weather. Even near the Mississippi Valley at 

Centerville and Woodville some portions of this 

plain are wen preserved, being trenched by very 

shallow valleys, but the conditions here are 

favorable . for the preservation of the original 

surface, because during the development of the 

drainage of southwestern Mississippi the stream 

encountered ledges of hard rock :similar to that 

at Fort Adams at depths of 40 to 120 feet below 

the surface of the Sardis plain, and this hard 

rock retarded the erosion and resulted in the 

partial preservation of the original Pliocene 

surface where it would otherwise have been 

destroyed. 

LITHOLOGY AND SOILS. 

The materials beneath the Sardis plain are 

essentially the same as those beneath the 

Brookhaven plain, consisting of coarse crossbedded sands and gravels near the base of the 

deposits overlain by finer sands and sandy 

clays. In general the gravels are finer than in 

the deposits of the Brookhaven plain, though 

near Mississippi River coarse gravels are found 

beneath the landward margin of the Sardis 

plain. Gravels are not abundant except in the 

area where sediments were supplied by Mississippi River. The upper member of the 

plain is a sandy clay of red or yellow color, 

and where the surface of the underlying 

Tertiary formations was high, as at Clinton 

and Woodville, Miss., this material rests 

directly upon them, the basal sands and 

gravels being absent. 

The soils derived from the weathering of the 

surface materials of the Sardis plain are either 

loam or sandy loam, though in a few places 

where the upper member has been removed by 

erosion gravelly loam is found. 

STRATIGRAPHIC RELATIONS OF THE DEPOSITS. 

Near the north edge of the Sardis plain the 

stream-terrace deposits rest unconformably on 

the deposits of the Brookhaven plain. Farther 

north remnants of the Brookhaven plain are 

above the level of the base of the Sardis plain, 

and the deposits of the latter plain rest unconformably on older Tertiary formations. 

The interstream deposits of the Sardis plain 

are continuous with those of the Brookhaven 

plain and were deposited when the sedimenta-
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XLI 

A CLOSE VIEW OF GRAVEL AND CONGLOMERATE AT PIT SHOWN IN 

PLATE XL, B. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XLII 

A. CLAY FRAGMENTS DERIVED FROM UNDERCUTTING BY A SMALL STREAM, SULPHUR CREEK, 

LIVE OAK COUNTY, TEX. 

B. A CLOSE VIEW OF THE FRAGMENTS SHOWN IN PLATE XLII, A. 

Photographs by T. W. Vaughan. 
PLIOCENE CITRONELLE FORMATION OF GULF COAS'TAL PLAIN. 183 

tion was pushed farther seaward because of a 

slight general change in the altitude of the land. 

The change was apparently gradual, and the 

zone of rapid sedimentation was carried progressively seaward with continuous deposition 

along the margin of the older plain. This may 

have resulted in partial unconformity between 

the deposits of the two plains, though probably 

the unconformity was not greater than at 

many other places within the deposits beneath 

the plains. 

ANTON PLAIN. 

The Canton plain borders the seaward and 

streamward n1argins of the Sardis plain and 

originally formed a continuous belt at a 

slightly lower level than the Sardis plain. 

The name is applied by Shaw 1 to a stream terrace of this plain at Canton, Miss., and is 

extended to other stream terraces that can 

be correlated with that one and to the interstream plains that merge with the stream terraces at the seaward ends of the valley. 

DISTRmUTION. 

Stream terraces.-Although the stream terraces of the Canton plain are better preserved 

than those of the older plains, they are represented only by mere fragments of the original 

deposits. The. terrace in the Mississippi Valley doubtless had a width of many miles, but 

remnants in western Mississippi are 10 to 12 

miles frdm the present position of the river. 

This terrace has probably been entirely removed by erosion in the area between Mississippi and Ouachita rivers and it has not been 

recognized between Ouachita and Red rivers. 

The stream terraces of the Canton plain have 

been noted at a number of places in the valley of 

the rivers of eastern Mississippi and westernAlabama, but the observations have not been sufficiently systematic to warrant discussing the distribution of the terraces in detail. In western 

Louisiana and southeastern Texas still less 

detailed information has been obtained concerning their distribution, though a poorly 

preserved terrace at an elevation of 285 feet on 

the east side of Sabine River between N eame 

and Evans Ferry is tentatively correlated with 

the Canton plain. 

lnterstream areas.-The interstream portions 

of the Canton plain are well developed 1n 

1 Shaw, E. W., unpublished notes. 

Louisiana east of Mississippi River and extend 

across southern Mississippi and Alabama into 

western Florida. In Louisiana the interstream 

areas of this plain extend westward from a 

point a few miles west of Woodworth, passing 

north of De Ridder and across the State line 

into eastern Texas. 

The original width of the interstream areas 

of the Canton plain in southern Mississippi and 

the adjacent portions of Louisiana was about 

12 to 15 miles, and they narrow eastward to 

Baldwin County, near the Alabama-Florida 

boundary line, where the width is approximately 6 to 8 miles. In western Louisiana and 

eastern Texas the interstream areas are about 

as wide as those in Alabama. 

ALTITUDE. 

The stream terraces of the Canton plain are 

lower than those belonging to the older plains, 

though considerable local variation is found, 

the altitude depending on the size of the stream. 

As a rule the stream terraces of all the plains ' naturally rise to higher levels and away from 

the coast have steeper gradients on the Email 

streams than on the large. An exception is 

found in the Mississippi Valley, where the terrace in the vicinity of Newmans Grove, east of 

Vicksburg, rises to only about 300 feet above 

sea level and the altitude increases slightly 

toward the coast, being nearly 380 feet east of 

N atohez. Thence the terrace slopes southward to about 350 feet near the southern boundary of Mississippi. Both inner and outer margins of the stream terraces were originally 

marked by scarps, but in many places these 

features have been destroyed by erosion. 

The interstream areas in southern Mississippi range in altitude from about 350 feet on 

the landward margin to about 280 feet on the 

seaward margin. The altitude decreases gradually from western Mississippi to western Alabama, where the landward margin is· about 310 

feet above sea level and the seaward margin 250 

feet and there is a still further decrease in 

western ' Florida, where the altitude of the landward margin is about 270 feet and that of the 

seaward margin about 220 feet. A similar but 

slightly more rapid descent west of Mississippi 

River brings the surface of the plain to about 

the same altitude in western Louisiana and 

eastern Texas that it has near the western 

boundary of Florida. 
184 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

The margins of the interstream plain are not 

bounded by distinct scarps but they are marked 

by an increase in gradient. At many places 

the margins have been destroyed by erosion, 

because the changes in gradient have furnished 

favorable places for the development of lines of 

drainage. 

SLOPE. 

The slope of the stream terraces of the Canton 

plain is in general seaward, though the rate is 

not easily determinable because of the lack {)f 

detailed information. The terrace on the east 

side of Mississippi River from Newmans Grove 

to the latitude of Natchez forms an exception 

because of deformation subsequent to deposition. The slope of this terrace is reversed, 

being northeastward, and it amounts in the 

aggregate to 30 feet in 60 miles, or about 6 

inches to the mile. From the latitude of 

Natchez to the northern boundary of Louisiana the slope of this terrace is southward at 

the rate of about 9 inches to the mile. 

The in terstream portions ·of the Can ton 

plain slope more steeply than those of the Sardis and Brookhaven plains, the average slope 

being from 2 to 3 feet to the mile, if the steepened landward margin is disregarded. Local 

variations are common, however, and in many 

places the original slope has been destroyed by 

erosion. 

DISSECTION. 

Both stream and interstream portions of the 

Canton plain have been greatly dissected, and 

the remnants now appear as isolated areas 

having nearly level surfaces or as even-topped 

divides between the n1ajor streams. In places 

these divides are several miles in width, but 

the undrained areas are smaller than those on 

the Sardis plain. This is attributed to the fact 

that the Can ton plain was narrow and the 

original surface had a steeper slope than the 

Sardis plain. 

LITHOLOGY AND SOILS. 

The lithology of the Canton plain is, in general, like that of the Citronelle formation as a 

whole. Gravel is less abundant in the deposits 

beneath this plain than in those beneath the 

Sardis and Brookhaven plains, though it is 

reported in some wells and is exposed in · a few 

deep valleys. The surface of the plain is composed of fine sandy clay and silt similar to the 

material forming the surface of the older plains 

and the soils are chiefly loam and sandy loams. 

STRATIGRAPHIC RELATIONS OF THE DEPOSITS. 

The materials underlying the Canton plain 

doubtless have the same stratigraphi.c relations 

as those beneath the Sardis plain, but the 

exposures are not deep enough to permit satisfactory observations. 

LOXLEY PLAIN. 

The lowest of the Pliocene plains that are 

underlain by deposits of the Citronelle formation is well preserved in the vicinity of Loxley, 

Baldwin County, Ala., and is therefore named 

the Loxley plain. 

DISTRmUTION. 

Stream terraces.-The Loxley plain has a 

width of only a few mi1es where it is represented by remnants of stream terraces. In the 

Mississippi Valley the original extent of the 

stream terrace belonging to this plain can not 

be easily determined, though in the vicinity of 

Sicily Island, where the terrace is the result 

of erosion and deposition by Mississippi and 

Ouachita rivers, it may· have been 30 to 40 

miles wide. The average width in the Mississippi Valley between Sicily Island and Vicksburg was probably not much greater than 25 

miles. The original width was apparently less 

than 10 miles in the valleys of the rivers in 

eastern Mississippi and western Alabama and 

Florida, except in the Alabama and Tombigbee 

drainage basins, where this terrace is more 

than 25 miles wide at its southern margin. 

Interstream areas.-The interstream areas of 

the Loxley plain occupy a belt between the 

Canton plain and the oldest Pleistocene terrace. 

They vary much less in width than the older 

terraces of the Citronelle formation, being as 

wide in western Florida and Alabama as in 

southern Mississippi and eastern Louisiana. 

The average width of these areas is about 8 

miles and the maximum is about 15 miles. On 

the eroded outer margin of the plain the underlying deposits have been exposed in places 

by the removal of the younger beds, and if 

the maximum width should be extended to include these exposures it would amount to 

slightly more than 30 miles. However, these 

more or less isolated exposures represent surfaces eroded during the · Pleistocene epoch. 

The width of the interstream portions of the 

Loxley plain in southwestern Louisiana and 

southeastern Texas is about 8 miles. This 

plain is well developed in an area that extends 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 185 

from Forest Hill to De Ridder, La., and thence regarded as approximate, because neither landwestward, passing a few miles south of Newton, ward nor seaward margins of the interstream 

Tex. portions of the Loxley plain are very sharply 

ALTITUDE. defined. 

The stream terraces of the Loxley plain vary 

in altitude, being higher on the small streams 

than on the large, and they increase in altitude 

away from the coast. This generalization, 

however, does not apply to the terraces of the 

rivers in western Mississippi, where the altitude 

has been changed by deformation. East of 

Vicksburg the terrace in the Mississippi Valley 

is about 280 feet above sea level, and east of 

Natchez the altitude is nearly 40 feet higher. 

From Natchez southward to the Louisiana 

boundary it declines to about 270 feet above 

sea level. The terrace on Pearl River has an 

altitude ranging from about 230 feet northwest of McNeil to about 300 feet near Monticello and 320 feet near Jackson. The altitudes 

observed on Leaf River range from about 250 

feet near Hattiesburg to about 200 feet near 

Merrill, and those in western Alabama, on the 

west side of the Tombigbee, from about 250 

feet east of Seaboard to about 200 feet northwest of Mobile. On Conecuh River they range 

from 200 feet near the Alabama-Florida boundary to about 230 feet east of Kirkland. Terraces of this age were recognized as far east as 

Yellow River, in northern Florida, where the 

altitudes obtained by barometric readings were 

somewhat greater than 200 feet, and near Crestview, where the altitude on . the interstream 

plain is about 190 feet. 

On the west side of Mississippi River, in 

Louisiana, the gravel deposits on the top of the 

Sicily Island hill and on the upland west of 

Ouachita River are tentatively referred to a 

stream terrace of the Loxley plain, and other 

stream terraces at altitudes of 200 to 225 feet 

were noted farther west, on the west side of 

Red River near Woodworth and on Sabine 

River west of Neame. 

In Louisiana east of Mississippi River the 

interstream areas stand at 280 to 220 feet 

above sea level, and the altitude decreases 

slightly from the Mississippi Valley eastward, 

being 250 to 210 feet in southern Mississippi, 

250 to 190 feet in western Alabama, 230 to 170 

feet near Loxley, in southern Baldwin County, 

Ala., and 220 to 160 feet near Yellow River, in 

western Florida. AU these figures should be 

West of Mississippi River the decrease in 

altitude is more rapid than east of it, bringing 

the inner margin of the plain down to about 

220 feet above sea level in central Louisiana. 

Farther west, near Forest Hill, in central 

Louisiana, the altitude ranges from 220 to 180 

feet. From central Louisiana to southeastern 

Texas the altitude is nearly uniform and about 

the same as near Forest Hill. Along the Kansas City Southern Railway, which crosses this 

plain, the landward margin just north of De 

Ridder has an altitude of about 220 feet and 

the seaward margin north of Bon Ami is 

approximately 40 feet lower. South of Newton this plain is represented by the tops of the 

hills that rise 180 to 220 feet above sea level. 

SLOPE. 

The stream terraces of the Loxley plain have 

a general slope in the direotion of stream flow 

except in a part of the Mississippi Valley, where 

deformation since the materials were laid down 

has caused a reversal of the normal direction. 

Between southern Warren County and southern Jefferson County, Miss., the Loxley stream 

terrace slopes northeastward at about the same 

rate as the stream terrace of the Canton plain. 

Accurate determinations of slope are not possible, because measurements of altitude are difficult to procure on account of the thick mantle 

of loess that covers the Pliocene plains. From 

southern Jefferson County, Miss., to northern 

Louisiana the terrace slopes southward at the 

rate of 1 to 1! feet to the mile. 

On Pearl River the stream terrace of the 

Loxley plain has very little slope between Jackson and Monticello, but south of Monticello it 

slopes toward the coast at the rate of about 

1 foot to the mile. The terrace on Leaf River 

slopes a little more than 1 foot to the mile from 

Hattiesburg to Merrill, and the terrace on the 

west side of Tombigbee River slopes about 

1 foot to the mile. On Conecuh and Yellow 

rivers the slopes may be somewhat higher than 

farther west, amounting to 1! feet to the mile, 

though the altitudes used in making this estimate were determined with a barometer and 

are subject to correction. In western Louisiana and eastern Texas there are few data for 
186 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

estimating the rate of slope of the stream terraces, but such observations as have been made 

indicate slopes of at least 1 foot to the mile. 

The slopes of the interstream areas of the 

Loxley plain amount to 5 or 6 feet to the mile, 

but these :figures include a comparatively steep 

descent along the landward margin of the plain 

from the surface of the Canton plain, and in 

addition some allowance should be made for 

the erosion of the sea ward margin in postPliocene time. Observations on uneroded surfaces indicate a slope of 1 or 2 feet to the mile, 

the rate of slope being apparently greatest near 

Mississippi River and decreasing slightly both 

to the east and to the west. 

DISSECTION. 

The Loxley plain is much less eroded than 

the Sardis and Brookhaven plains, but it shows 

somewhat greater erosion than the next older 

or Canton plain, because it forms a fringe about 

this plain and was attacked :first by the streams 

in post-Pliocene time. In addition, the seaward margin of the.Loxley plain was eroded at 

the time of the formation of the oldest Pleistocene terrace. In general, the surface of the 

Loxley plain has been reduced to slopes except 

· on the divides between the large streams, 

where there are flat areas sever~l miles wide 

covered by small undrained depression. 

LITHOLOGY AND SOILS. 

The lithologic description of the Citronelle 

formation as a whole will apply in a general 

way to the materials beneath the Loxley plain, 

though the exposed materials, except where 

there has been· an unusual amount of erosion, 

are largely sandy clays or silts. Locally fine or 

moderately coarse gravel has been exposed by 

erosion, and well records show that gravel 

exists at many places where it is not exposed. 

In general, the percentage of fine rna terial in 

the deposits beneath the Loxley plain· is 

greater than in those b·eneath the older plains 

of the Citronelle formation. The soils are 

largely loams and sandy loams. 

STRATIGRAPHIC RELATIONS OF THE DEPOSITS. 

The stratigraphic relations of the materials 

underlying the Loxley plain are similar to 

those of the deposits underlying the older 

plains. There is apparently no marked break 

between the sediments beneath the Loxley 

plain and those beneath the Canton plain, 

except possibly in the upper portion of the 

deposit, where a slight subsidence during the 

closing stages of deposition on the Loxley plain 

may have permitted the transgression of the 

sediments upon the margin of the Canton plain. 

The exposures are so poor that opportunities to 

examine the contacts between the deposits of 

these plains are rare. In the stream valleys 

the terrace sediments of the Loxley, plain were 

deposited unconformably on those that were 

laid down during earlier Pliocene time, and 

farther up the streams they are conformable 

on still older formations. 

PALEONTOLOGIC CHARACTER. 

The scarcity of organic remains in the Citronelle formation has long been a bar to its 

satisfactory discrimination and correlation. 

Heretofore no identifiable fossils have been 

discovered except fragments of silicified wood, 

which might have been derived from some older 

formation by erosion. The numerous exposures that were studied during this investigation furnished no remains of either fresh or 

salt water organisms, though possibly this 

might be explained in part by the weathered 

character of the outcrops. It is probable that 

the processes of weathering would remove 

traces of shells from sands, but even where 

unweathered layers of sand or clay have been 

examined they have, except in two places, 

been found to be nonfossiliferous. Failure to 

:find remains of marine organisms warrants the 

assumption that the beds are l~argely nonmarine, and this view is strengthened by the 

character and arrangement of the sediments. · 

The absence of fresh-water fossils is probably 

due to the scarcity of animal remains capable 

of preservation, together with the unfavorable 

conditions for preservation resulting from the 

mode of origin and character of the deposits. 

The only identifiable organic remains obtained from the Citronelle formation were 

fossil plants collected in a cut on the Mobile & 

Ohio Railroad south of Lamberts and at Red 

Bluff, on the shore of Perdido Bay, in Alabama. 

At both places the fossil plants occur in beds 

of dense clay, stained black by organic matter, 

and they owe their preservation to the character of the matrix, which excluded the air, 

thus preventing their destruction by weathering. These fossils are described by Mr. Berry 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 187 

on pages 195-208, and further reference to them 

is. unnecessary. The relation of the leaf-bearing 

clays to the other beds in the exposures where 

they were collected is similar to the relations 

shown by many of the other lenses of clay that 

occur in the formation. 

The leaf-bearing clays are near the top of the 

formation, and those at Red Bluff are somewhat 

younger than those at Lamberts. The plants 

collected at Lamberts were obtained from a 

bed that is younger than the outliers that cap 

the high hills to the north, but the upper 

portion of the formation near the coast is 

younger still. The fossils at Red Bluff were 

found in the upper beds near the coast, though 

no doubt some higher layers have been removed by erosion at this locality. The leafbearing clays are younger than most of the 

coarse gravels that form outliers along the 

landward margin of the formation, because 

the gravels pass under the clay beds on the 

uplands and are found in wells near the coast. 

Beds deposited contemporaneously with those 

near the coast extend into the valleys, where 

they form terraces in the valleys of the principal streams. These terraces contain coarse 

gravels a short distance away from the coast 

and present conditions unfavorable for the 

presence of fossils. The beds penetrated by 

wells at Fort Morgan and Pensacola are apparently marine, and marine Pliocene fossils may 

have been found in wells elsewhere along the 

coast, though samples of drillings are seldom 

preserved and information concerning such 

fossils is fragmentary. 

ORIGIN~ 

The Citronelle formation contains practically 

every variety of clastic sediments, and nearly 

all of them may be represented in a section 

only a few feet thick, many small exposures 

ranging through all the intermediate stages 

from coarse gravel to fine clay. In mineral 

composition the deposits are similar to ordinary clastic sediments, being made up largely 

of quartz or silicates, such as kaolin. The 

many variations in physical character, together 

with rapid alternations of materials of different sizes (see Pis. XXXVI-XXXIX), indicate 

changing conditions of sedimentation such as 

are characteristic of areas where there are 

many variations in direction and velocity of 

the currents transporting the materials. These 

30830°--17----13 

conditions prevail in the valleys of streams 

subject to changes in velocity produced by 

floods and quiet water. Similar conditions are 

found along shore lines where rivers supply 

detritus of varying coarseness to be reworked 

by waves. More detailed observations on sedimentation might reveal characteristic differences between fluviatile deposits and those 

made along the margin of the sea, with intergr.adations where deposits of the two kinds 

merge, but in the absence of such observations it is not always easy to distinguish the 

deposits of the two classes. 

The character and arrangement of the sediments comprising the Citronelle formation 

suggest that they are in part fluviatile, in part 

estuarine, and in part shallow-water deposits 

made at or near the strand line, where there 

was some wave action. The coarse gravels 

are much less rounded than similar materials 

that have been subjected to prolonged subaqueous erosion and are therefore thought to 

be of fluviatile origin. This belief is supported 

by the . confused character of the stratification, which shows lenticular bedding and an 

alternation of imperfectly assorted sediments 

of different grades. It is possible, however, 

that some of the gravels were deposited along 

the shore, where they were partly reworked 

and redeposited by waves. 

The absence of remains of marine organisms 

and the character and arrangement of the 

Citronelle sediments suggest that they are 

mainly fluviatile. Attempts to explain the 

lack of marine fossils by supposing that .they 

have been removed by solution during the 

processes of weathering are not entirely satisfactory, because comparatively fresh exposures 

of the formation have been examined at many 

localities without finding traces of marine 

organisms. 

On pages 194-195 Mr. Berry discusses the 

ecologic conditions of the plants whose fossil 

remains have been found. He suggests barrier 

beaches, possibly with sand dunes, a mile or 

more wide, crossed by inlets and inClosing 

shallow lagoons. These lagoons, he thinks, 

must have been a mile or more in width, and 

still wider where large rivers formed estuaries. 

Dunes were probably built along the landward 

margins of the lagoons. This description agrees 

with the general features of portions of the 

Gulf coast to-day and may represent condi-
188 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

tions that existed at the places where the plants 

·grew. It does not, however, seem possible to 

account in this way for the various kinds of 

deposits found in the formation throughout a 

large portion of the area where it is exposed. 

The conditions described by Mr. Berry might 

account for all the sediments except the fluviatile deposits, though it is more probable that 

at the mouths of some of the large streams like 

the Mississippi the sediments were built into 

broad deltas. Under the influence of shore 

currents some of the materials brought to the 

shore by the streams were spread across the 

interstream spaces until they coalesced to form 

extensive deposits. This condition harmonizes 

with the theory of the formation of lagoons, 

beaches, and dunes alongshore. As the sea 

encroached upon the land the beaches and · 

dunes would be destroyed and the lagoons filled 

by the action of the waves, and thus distinctive 

shore features would not be recognizable in the 

formation. Probably at som~ stages of deposition rapid subsidence would produce estuaries 

where at times of relative quiescence deltas had 

been formed. Sometimes estuaries and deltas 

might both exist along the same shore, just as 

the broad Delta of the Mississippi is flanked 

on one side by the estuaries of southwestern 

Louisiana and on the other side by the estuaries of southeastern Mississippi and southern 

Alabama. 

Certain features indicate that the conditions 

were not uniform and that there may have 

been many changes in the site of most rapid 

sedimentation. For example, at Brookhaven 

there is a thick deposit of sand at the base of 

the formation, overlain by coarse gravel, and 

this in turn by sands and clays. This alteration would result from a shifting of the direCtion of the principal currents that transported 

the sediments, or from reversal of movement 

along the strand line during the progress of 

deposition, or from uplift farther inland, causing enough increase in velocity to permit the 

streams to spread gravel over the basal sands. 

While material was being laid down upon the 

upland the site of rapid sedimentation was 

shifted landward and the beds previously deposited were covered by finer sediments. At 

the same time the older shore features were 

destroyed, and in this way fine sediments were 

deposited some distance landward upon the 

coarse fluviatile sediments. However, that the 

fine materials did not extend entirely across 

the area in which the. Citronelle formation is 

now exposed is indicated by the fact that the 

erosion of the formation has not been sufficient 

to remove a great amount of material and the 

landward edge of the formation is now composed of coarse sands and gravel. If :fine sands 

and clays had been deposited on this portion 

of the area erosion would probably have been 

insufficient to remove them entirely. 

The emergence of the seaward margin of the 

formation permitted the duplication of some of 

the shore phenomena of the earlier depression, 

including the formation of deltas, bars, and 

other shore features, but these features have 

been destroyed by subsequent erosion. Intermittent changes in level during emergence 

account for the terracing seen at high levels in 

the present valleys, and it is probable that the 

fossil plants now found in the upper portion of 

the formation belong to this period. 

The upper sand be~s show a renewal of the 

quiet-water conditions that existed before the 

deposition of the gravel, and in the closing 

stages of Citronelle time the deposits were fine 

sand and silt, such as are laid down by flood 

waters of a muddy stream at times of overflow. 

This overflow began when the streams erod~d 

their channels below the general level of the 

deposits already made and continued as long 

as the waters spread over the flood plains adjacent to the streams. 

Some of the terraces in the stream valleys 

that trench the older' formations represent 

episodes in the post-Pliocene history of the 

region, but those that stand at high levels were 

formed during the closing stages of the deposition of the Citronelle formation. Knowledge 

of the details of the distribution and relations 

of these terraces is still incomplete, but enough 

is known to warrant the conclusion that they 

merge in to the deposits classed with the Citronelle formation. 

The interstream portions of the Sardis, Canton, and Loxley plains were each built against 

the steep front of the next older plain as the 

result of changes in level that shifted sedimentation seaward. The changes may have been 

caused by general uplifting of the land, by 

withdrawal of the sea, or by an upward movement some distance inland that enabled the 

streams to carry their loads of detritus farther 

seaward. Of these possible explanations the 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 189 

last seems the least probable, because such a 

change would have resulted in the deposition 

of a larger percentage of coarse sediment along 

the seaward margin of the formation; moreover, the base-level of the streams was lowered, as is shown by the formation of stream 

terraces that coalesce with the interstream 

plains. This lowering of the base-level might 

hawe been the result either of upward movement of the land or of recession of the water, or 

of both causes operating simultaneously. The 

warping of the plains suggests a land movement 

but it does not prove that such a movement was 

the sole cause or even the most effective cause 

of the changes in base-level, though the warping 

was largely contemporaneous with those 

changes, as is suggested by the fact that the 

post-Pliocene terraces are not deformed. 

The lowering of the base-level resulted in the 

removal of a portion of the deposits already 

formed and their redeposition along the seaward margin of the older beds. The intermittent character of the changes permitted the 

formation of terraces in the stream valleys, and 

at the same time the deposits made along the 

seaward margins of the older beds were built 

into interstream plains either by the coalescing 

of deltas or more probably as the combined 

work of streams and waves. 

The closing stage of the formation of each 

plain seems to have been a partial submergence, 

which permitted the development of small 

lagoons and other depressions on the surface, 

though there is no evidence of marine erosion 

and deposition in the form of beach ridges such 

as would have resulted from vigorous wave 

action. Each successive stage of deposition 

was closed by a renewed lowering of base-level, 

and as the change progressed finer sedim~n t 

was spread over the coarse detritus in the 

stream valleys. Some of the fine silt that 

covers the uneroded portions of the interstream plains may have been deposited as the 

result of a rise in the level of the streams as. 

deltas were built seaward, though these deposits are too widely distributed to be explained as a whole in that way. Moreover, 

evidence of slight submergence is afforded by 

partly filled valleys emerging at the levels of 

the surfaces of some of the younger plains. 

That this · resubmergence came at the (/nd of 

the period of deposition of the materials forming the plains and not after the shaping of the 

existing valleys is proved by the absence of 

deep filling in these valleys. 

SURFACE DEPOSITS. 

Erosion of the surface of the Citronelle formation began at the end of Pliocene deposition 

and has continued down to the present time. 

It was accomplished by the same forces that 

are now active, including stream corrasion, 

surface wash, wind action, and the work of 

organisms; and although the action of these 

agencies may have varied in intensity at different times, evidence of such variation is not 

apparent. There was a constant shifting in 

the sites of most active erosion, ana portions 

of the eroded materials were redeposited many 

times. Much of this redeposited material 

may be seen on the slopes, and where it 

resembles the subjacent deposits of Pliocene 

age it is not readily recognized, though in 

many places it may be distinguished because 

of the concentration of relatively coarse detritus, such as pebbles or fragments of ironcemented sandstone or limonite, which has 

resulted from the partial removal of the finer 

materials. The redeposited materials do not 

form a continuous mantle but are distributed 

on all the slopes and are exposed in many 

shallow gullies, especially along wagon roads. 

They range in age from the earliest period of 

erosion of the Pliocene deposits to the present 

day, but in many places they have been undisturbed for a long time because of protection 

by vegetation. 

Since the beginning of the terracing, in 

Pleistocene time, erosion and redeposition 

have been very largely concentrated on the 

lower slopes and in the bottoms of the valleys, 

and this has resulted in the formation of 

deposits of definite types that will be mentioned 

in discussing the Pleistocene deposits. 

PLEISTOCENE TERRACES. 

During the Pleistocene epoch extensive 

terraces bordered by seaward-facing scarps 

were formed along the seaward front of the 

Pliocene deposits. These terraces extend into 

the river valleys, where ·they merge with ter- · 

races of fluviatile origin. In most places the 

Pleistocene deposits forming these terraces are 

relatively thin, though near Mississippi River 

and other large streams thick constructional 

deposits were made. Names have been applied 
190 SHOR.TER. CONTR.IBUTIONS TO GENER.AL GEOLOGY, 1916. 

to the terraces and are used on tne accompanying diagrams and in some of the sections 

(Pl. XXXII, p. 170), as well as in the text. 

The Pleistocene age of the oldest terrace, 

the St. Elmo, is inferred from the fact that it 

merges, in the ~1ississippi Valley, into the 

Natchez formation, which is regarded as subAftonian.1 The Natchez formation contains 

crystalline pebbles and underlies a terrace that 

was doubtless formed during the Pleistocene 

epoch. The younger terraces, named in the 

order of the age, are the Port Hickey, the 

Hammond, and the Pensacola. The name 

Port Hickey is taken from a locality on Mississippi River where the typical materials of 

the Port Hudson formation are exposed. The 

Hammond terrace is named from Hammond, 

La., and the Pensacola terrace from Pensacola, 

Fla. 

The level, uneroded surfaces of these terraces 

are in sharp contrast with the erosional features 

of the Citronelle formation, but they have 

their counterparts in the uneroded portions of 

the interstream plains of the Citronelle. Shallow depressions extend from the larger river 

valleys diagonally toward the coast, and most 

of them are bordered by low natural levees. 

These channels resemble those now being 

formed across the Mississippi Delta, and they 

show the constructional origin of the subsurface 

deposits. In some of the areas between the 

streams, seaward-facing scarps border the 

inner margins of the Pleistocene plains, and in 

places there are low ridges and shallow ponds 

whose long axes lie parallel to the shore line. 

These features show that wave action has been 

an influential agent in forming the surface, and 

it is probable that both wave and stream action 

prevailed at the same time along different 

portions of the coast and that their relative 

efficiency varied with the changing relations 

of the land and the water. 

The exact relations of the different terraces 

to the glacial deposits is not fully known, though 

the St. Elmo terrace, which appears to be 

older than the great deposits of loess that 

fringe the east side of the Mississippi Valley and 

to be contemporaneous with the crystalline 

gravel at Natchez, is regarded as sub-Aftonian. 

The Port Hickey terrace appears to be younger 

than the principal deposits of loess. If this 

loess is considered to be of Iowan age these 

lChamberlin, T. c., and Salisbury, R. D., Geology, vol. 3, p. 387, 1906. 

relations would indicate that the St. Elmo terrace is pre-Iowan and the Port Hickey terrace 

post-Iowan. The loesslike silt that forms the 

surface of the Port Hickey terrace may be 

of Wisconsin age, and if so, this would place 

the terrace in the interval between the deposition of the Iowan loess and that of the Wisconsin silt. The lower terraces must neces-

. sar1ly be younger than the Wisconsin silt. 

In addition to the terraces of Pleistocene 

age a similar terrace of Recent age now rises a 

few feet above sea level and extends outward 

beneath the shallow waters of t.he Gulf. 

WELL LOGS. 

The accompanying diagrams of · well logs 

(Pl. XLIII, in pocket) show the characteristic'3 

of the Citronelle formation at a number of 

localities and the relation of this and other 

formations penetrated in drilling the wells. 

The correlations with formations exposed at the 

surface are based on such information as could 

be obtained concerning the lithologic character 

of the materials penetrated in the wells, supplemented by the evidence of fossils. In 

general it may be said that the correlations 

represent the most probable subdivisions, 

though the lines between different formations 

are somewhat indefinite. 

Well No. 1, drilled by the Baldwin County 

Oil Development Co. in southeastern Alabama, 

shows at the surface a thin deposit of sand and 

clay that is classed as Pleistocene because it 

underlies a terrace of that age. Beneath this 

is a thick bed of sand containing some grave~ 

and thin beds of rock and clay, correlated with 

the Citronelle formation. The more clayey 

beds immediately underlying the Citronelle 

formation are classed with the Pascagoula clay, 

and the accuracy of this correlation is strengthened by the fact that at depths of 730 feet and 

more in a well drilled near this locality fossils 

that were classed as Miocene 1 were obtained. 

The materials penetrated below 900 feet in 

well No. 1 have been doubtfully correlated 

with several formations, but Claiborne fossils 

were identified from a collection obtained at 

the well. 

The Fort Morgan well (No. 2) was drilled on 

a narrow sandy island opposite the entrance to 

Mobile Bay. This well is unique in showing an 

i Smith, E. A., The underground water resources of Alaba-ma, p. 316, 

Alabama Geol. Survey, 1907. · 
PLIOCENE CITRONELLE FORMATION OF GULF COASTAL PLAIN. 191 

unusually large percentage of sand. Fossils 

were obtained at various depths, and a list is 

given on page 194. According to Mr. Dall's interpretations, the Quaternary material extends 

to a depth of about 1,290 feet. The materials 

below that depth were doubtfully correlated 

with the Pliocene because they contain fossils 

that were supposed to be of Pliocene age. They 

may represent the seaward extension of the 

Citronelle formation. 

Through the courtesy of Mr. A. G. Curtis, 

manager of the Southwestern Gas Co. at 

Shreveport, La., a log was obtained of the Jarvis & Meacham well No. 1, on the level upland 

of Mobile County at Semmes, Ala., about 12 

miles west of :Mobile. This well (No. 3) shows 

a thickness of more than 200 feet of sand and 

clay with a bed of gravel near the base. The 

gravel consisted very largely of coarse chert 

pebbles but included some finer pebbles · of 

quartz. Some of the chert pebbles were more 

than an inch in diameter, and most of them 

were subangular or only slightly rounded, resembling the pebbles found farther north in 

Alabama and in south-central Mississippi. Beneath the sand and gravel were layers of pink 

and white sand and clay extending to a depth 

of 390 feet. All this material is included in the 

Citronelle formation in the diagram. From the 

underlying clays and associated sand beds fossils were obtained at several horizons. They 

were submitted to C. W. Cooke, who reported 

that the characteristic fossils of the Pascagoula 

clay were present. The well penetrated some 

distance into the Oligocene formations, but no 

fossils of that age were collected and the descriptions of the material are not sufficiently 

definite to permit correlations. 

In southern Mississippi, especially near the 

coast, there is a somewhat uniform general 

sequence of beds, although rnany minor variations are recorded in the logs of wells, probably 

because the formations are lenticular. 

The well at Bond, Harrison County (No.4), 

encountered red sandy clay and white sand 

having a total thickness of about 190 feet. 

The upper portion of this material may 

safely be correlated with the Citronelle 

formation, and perhaps all of it should be so 

included. A thick clay bed that begins at 190 

feet and the sand below it are classed as the 

Pascagoula clay, which is in turn underlain by 

a thicker bed of clay, the Hattiesburg clay. At 

780 feet some chert gravel was encountered 

similar to that obtained in the wells at Hattiesburg at the base of the Hattiesburg clay. This 

gravel, associated with sand and clay, was 

reported at intervals to a depth of 1,120 feet, 

and this portion of the well section is correlated with the Catahoula sandstone. 

Farther south, at Wortham, the well of 

the Gulf & Ship Island Railroad (No. 5) 

shows some 20 feet of yellow clay that is 

assigned to the Pleistocene. The underlying 

sandstones, sands, and clays, with a bed of 

gravel at the base, are placed in the Citronelle 

formation. Beneath this formation is a bed of 

massive blue clay, the Pascagoula clay, which is 

in turn underlain by gray sand of uncertain age. 

The De La Morton well at Laine, Miss. 

(No. 6), encountered sand and clay containing 

tree trunks to a depth of nearly 160 feet. Beneath this material clay predominated to about 

565 feet, where a bed of gravel nearly 40 feet 

thick was reached. This clay und the underlying gravel are classed with the Citronelle formation. Characteristic fossils of the Pascagoula clay were found at a slightly greater 

depth and were reported at intervals nearly to 

the bottom of the well. The first casing was set 

at a depth of 881 feet, and apparently son1e of the 

Pascagoula fossils came from a slightly greater 

depth and continued to appear in the cuttings 

long after the drill had penetrated to other formations. Tentative correlations of the deeper 

formations are presented on the diagram. 

At Brookhaven the well belonging to the 

Southern Gravel & Development Co. (No. 7) 

encountered gravel and red sands that are 

classed with the Citronelle formation. The 

material below these beds is all designated 

clay, and subdivision into formations is impossible. 

At McComb (well No. 8) the Citronelle formation has an apparent thickness of about 220 

feet, though some · doubt exists as to the correctness of the correlation of the rna terials 

designated "hard gumbo and dry gravel." 

This gravel did not come from the beds encountered near the surface, because a casing 

had been set in the well at a depth of 117 feet. 

Beneath the Citronelle formation the Pascagoula clay is recognized, and it is possible that 

the underlying Hattiesburg clay was reached. 
192 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

At Osyka (well No. 9) the Citronelle formation is about 150 feet thick and is underlain by 

blue clay and some sand and gravel that are 

correlated with the Pascagoula clay. The 

point of contact between the Pascagoula and 

Hattiesburg clays is not clearly defined, and 

it is not possible to separate the Hattiesburg 

clay from theCatahoula formation. 

The well at Woodville (No. 10) shows an 

unusual condition in southern Mississippi, 

where, because of the presence of relatively 

hard sandstone, a hill of hard Miocene sandstone (Pascagoula clay) rose high enough to be 

above the level of the coarse sediments of the 

Citronelle formation. As a consequence of this 

condition the material deposited on the hill 

represents only a portion of the upper finegrained sediments of the formation. The diagram of this well with its thin deposits of sand 

and clay should be contrasted with the diagram 

of the Bass well~ where the Citronelle formation 

is much thicker. These two diagrams illustrate 

the marked irregularity of the upper surface 

and the steep slope of the Miocene surface toward the coast, though it should be noted that 

the gradient of this surface was probably increased by downward flexing near the Pliocene 

shore. 

At Bass, La., the well belonging to B. A. 

Bass (No. 11) passed through about 100 feet 

of Pleistocene clay, with a little sand at the 

base. Below this is about 360 feet of clay, 

underlain by sand and containing some chert 

. gr:avel, all classed with the Citronelle formation. The sands and clays at greater depths 

are correlated with the Pascagoula clay. 

These cqrrelations are the result of a study of 

a set of samples of the materials penetrated. 

The correlations of the formations penetrated in the Camp-Hinton Lumber Co.'s well 

(No. 12) are probably approximately correct, 

as the materials penetrated are typical of the 

several formations. 

The well at the Natchez ice factory (No. 13) 

encountered about 50 feet of sand and gravel 

that have been assigned to the Citronelle formation and probably underlie the gravels of the 

Natchez formation. The underlying materials are typical of the Hattiesburg clay and 

Catahoula formation. 

Exact correlations of the materials penetrated in the well at Bayou Sara (No. 14) are 

difficult. The beds in the upper 60 feet apparently belong to the alluvium of the Mississippi 

Valley, and the underlying sands are thought 

to belong to the Citronelle formation. Beneath 

these sands are interbedded clays and sands 

of the Pascagoula clay, and still lower is a thick 

mass of clays underlain by interbedded sands 

and clays, not subdivided into formations. 

In western Louisiana, at Ludington, the 

well of the Ludington & Van Schaik Co. (No. 

15) shows 150 feet of sand and gravel that 

belong to the Citronelle formation. Some of 

the older formations have been differentiated, 

but the log of the well below 1,600 feet is difficult to interpret. 

At Fullerton the well of the Gulf Land & 

Lumber Co. (No. 16) encountered about 95 

feet of clays containing some interbedded sand 

and gravel, referred to the Citronelle formation. Beneath this are more massive clay 

beds and some thick sands that are thought 

to belong to the Pascagoula clay. The massive clays below 400 feet are with some uncertainty correlated with the Hattiesburg clay.