354 Shorter Contributions 

t ! to General Geology 

= 0 

.... 

J 1959 

en 

d 

en 

• GEOLOGICAL SURVEY PROFESSIONAL PAPER 354 l 

U') 

Cf) 

..... 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 
CONTENTS 

[The letters in parentheses preceding the titles designate separately published chapters] 

(A) Intrenched meanders of the North Fork of the 

Shenandoah River, Virginia, by John T. Hack 

and RobertS. Young _________________________ _ 

(B) Interpretation of the composition of trioctahedral 

micas, by Margaret D. Foster _________________ _ 

(C) Giant waves in Lituya Bay, Ala,ska, by Don J. 

Miller ____________________ - - - - - - - - - - - - - - - - - - -

(D) Early Cretaceous (Albian) Ammonites from the 

Chitina Valley and Talkeetna Mountains, Alaska, 

by Ralph W. Imlay __________________________ _ 

(E) Interpretation of the composition of lithium micas, 

by Margaret D. Foster _______________________ _ 

Page 

1 

11 

51 

87 

115 

(F) Zones and zonal variations in welded ash flows, by Robert L. Smith _____________________________ _ 

(G) Deposition of uranium in salt-pan basins, by Kenneth 

G. Bell _____________________________________ _ 

(H) Foraminifera from Onotoa Atoll, Gilbert Islands, by 

Ruth Todd __________________ ---------------

(I) Occurrence and significance of marine animal remains 

in American coal balls, by Sergius H. Mamay and 

Ellis L. Y ochelson ___________________________ _ 

(J) Lituyapecten (new subgenus of Patinopecten) from 

Alaska and California, by F. Stearns MacNeiL __ _ 

(K) Stratigraphic Occurrence of Lituyapecten in Alaska, 

by Don J. Miller _____________________________ _ 

m 

U.S. GOVERNMENT PRINTING OFFICE: 1961 0 -579101 

Page 

149 

161 

171 

193 

225 

241 
Intrenched Meanders of the 

North Fork of the Shenandoah 

River, Virginia

GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-A
Intrenched Meanders of the 

North Fork of the Shenandoah 

River, Virginia

By JOHN T. HACK and ROBERT S. YOUNG

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-A

A study of the channel of a river 

intrenched in hard rocks 

and an analysis of factors 

that cause it to meander

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. - Price 20 cents (paper cover)
CONTENTS

Abstract, ________________

Introduction_ ___________

Geology of the river basin.. 

River channel.____________

Longitudinal profile.__ 

Channel cross section-- 

Bed material- ______...

Discharge. __________

Page 

1

1

2

4

4

4

4

6

Page

River channel Continued

Summary of channel characteristics. _. _____________

Controls of meander formation_________________________

Intrenchment hypothesis..____-________--_____--__ __

Meandering streams in rocks other than the Martinsburg 

shale _______________-._..__-_____-_--______--__-_-_-

Conclusion._______ __ _____ _________________________

References,______-___-_______-____--_--_.__.---___-

9

9

10

ILLUSTRATIONS

PLATE 1. Outcrops of Martinsburg shale. A, Outcrop near Woodstock showing typical beds of fine sandstone and siltstoue

alternating with shale; B, Outcrop near Edinburg showing typical prismatic cleavage cutting a spheroid_ _ _ _ _ Facing 6 

FIGURE 1. Map of the meandering reach of the North Fork of the Shenandoah River.______________________________ ._ 2

2. Simplified geologic map of the drainage basin of the North Fork, Shenandoah River_______ _______ __________ 3

3. Longitudinal profile of the North Fork of the Shenandoah River.._______ _______ __________________________ 4

4. Graph on logarithmic scales comparing the dimensions of the channel of the North Fork at different places along

the stream, _________________________________________________________________________________________ 5

5. Profiles of valley floor.__._______________ . ______ __________________________________________________ 8

TABLES

TABLE 1. Summary of data on size of fragments on the streambed, North Fork, Shenandoah River..._ 

2. Mean annual discharge at gaging stations on the North Fork of the Shenandoah River_____

iii

522689 59
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

INTRENCHED MEANDERS OF THE NORTH FORK OF THE SHENANDOAH RIVER, VIRGINIA

BY JOHX T. HACK AND ROBERT S. YOUNG

ABSTRACT

The North Fork of the Shenandoah River traverses rocks of 

different lithology in areas of greatly different topography. 

The spectacular meanders in which the river travels 3.2 times 

as far as the direct down valley distance coincide with the outcrop area of a belt of Martinsburg shale. Along the entire 

river the channel is adjusted to carry bed material of approximately uniform size, and passes through the meandering reach 

without significant change in channel cross section. The profile of the stream follows a simple logarithmic curve from 

source to mouth. The upland on either side of the stream, 

however, slopes downstream at a grade much steeper in the 

area of the meanders than in the nonmeaiidering area upstream. 

It is concluded that the meanders are caused by strong planar 

and prismatic structures in the Martinsburg shale that favor 

northwest-southeast differential erosion. The topography of 

the adjacent upland is graded to the present river. There is 110 

reason to believe that the river has been intrenched from an 

erosion surface on which the relief was less than that of the 

present upland or that changes in base level have influenced the 

meander development.

INTRODUCTION

The North Fork of the Shenandoah River, Va., is 

a classic example of a meandering- stream intrenched 

in hard rocks. Photographs of these meanders have 

appeared in several textbooks of geology and geomorphology (Thompson, 1917, p. 175 ; Monnett and Brown, 

1950, p. 110; Thornbury, 1954, p. 146). The meanders 

are prominently displayed in an illustration in a paper 

by Rich (1939) describing a section across the Appalachians. Butts (1940, p. 508) offers an interpretation 

of the origin of the meander belt, and Fisher (1955) 

describes the physical character of the meanders.

A similar meander belt on Conodoguinet Creek, near 

Harrisburg, Pa., has received at least an equal amount 

of attention as an example of an intrenched stream and 

has been studied by Strahler (1946).

Most authors who mention either of these meandering streams imply that they are inherited from a surface of low relief and were intrenched into the valley 

floor during rejuvenation, as a consequence of uplift 

or change in base level. As pointed out by Thornbury, 

however (1954, p. 145), geomorphologists are not

1 University of Virginia, formerly of the Virginia Division of Geology 

(now the Virginia Division of Mineral Resources).

agreed that all intrenched meanders have been formed 

in this way. For example, Cole (1930) has shown that 

meanders may form in hard rocks by erosion along 

joint planes. Gregory and Moore (1931, p. 136) believe that many of the meanders that are spectacular 

features of the Colorado Plateau region were formed 

by lateral corrasion of the hard rock strata.

The writers believe that the meandering reach of the 

North Fork of the Shenandoah River is simply a river 

segment with a smooth and regular prohle, adjusted for 

the transportation of a certain load. The enclosing 

rocks have special characteristics favoring erosion in 

a preferred direction, and the adjusted channel slope 

is maintained in spite of the increase in sinuosity. No 

change in base level is necessarily involved in the formation of the meanders.

The meandering reach of the North Fork is shown 

in figure 1, a map prepared from the Strasburg and 

Edinburg quadrangles of the U.S. Geological Survey. 

The reach begins at Edinburg and ends at Strasburg, 

14 miles downvalley. The sinuosity (the distance 

measured along the stream, divided by the distance 

measured along the valley) attains the remarkably high 

value of 3.2. The narrow valley bottom is 100 to 150 

feet below the general level of the adjacent shale hills. 

Measured along the channel, the river has an average 

gradient of 5.5 feet per mile, whereas the valley floor, 

measured across the hilltops between the bends, has a 

gradient of 17 feet per mile. As shown on the geologic 

map, figure 2, the meander belt is localized within the 

outcrop area of the Martinsburg shale (Orclovician). 

This formation, as described on page 6, is lithologically unique in the region and its close relation to the 

meanders led the writers to undertake this study. The 

relation is not confined to the North Fork; similar 

meanders occur in other areas of Martinsburg shale, as 

on the Middle River, Augusta County, Va., and on 

Conococheague Creek, Washington County, Md. In 

these areas the meandering reaches correspond closely 

to the limits of the same formation.

The writers' work on the Shenandoah River grew out 

of bedrock geologic mapping by R. S. Young in the

l
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

CONTOUR INTERVAL 200 FEET

DATUM IS MEAN SEA EEVEL

Base from U S Geological Survey topographic 

map of the Strasburg quadrangle

FIGURE 1. Map of the meandering reach of the North Fork of the Shenandoah River. Redrawn from the Strasburg, Va., quadrangle of the

U.S. Geological Survey (1947).

Ediiiburg-Woodstock area in 1951-52, and studies of 

streams by J. T. Hack in the Shenandoah Valley in 

1952-57. Measurements of the stream channel were 

made in 1957. The writers are grateful to colleagues 

in the Virginia Division of Geology and the U.S. Geological Survey for their suggestions and critical review.

GEOLOGY OF THE RIVER BASIN

The valley of the North Fork may be divided into 

three parts, each having distinctive geology, as shown

on the map of the river basin, figure 2. The headwater area above Cootes Store is in folded sandstone 

and shale of Silurian and Devonian age. The interstream divides are steep sandstone ridges rising 1,500 

to 2,000 feet above the shale valleys. Along the main 

streams the valleys are broad and flat floored. The 

channels are flanked by broad flood plains and gravel 

terraces. Eock is not exposed in the channels, and the 

beds and banks are composed of rounded cobbles in 

imbricate arrangement, derived from the flanking
INTRENCHED MEANDERS OF THE NORTH FORK OF THE SHENANDOAH RIVER, VIRGINIA

78 50' JH\ 78 C T

3

u

J S Q

"135

J en a

^ Igl

Martinsburg shale | Q: y 

Calcareous shale and sandstone _J O >

15 <

Limestone and dolomite

Contact

Thrust fault 

T. upper plate

10

_JL_

15 Miles

FIGURE 2. Simplified geologic map of the drainage basin of the North Fork, Shenandoah River, showing relation of the meanders to the

Martinsburg shale, and location of profiles (fig. 5).

sandstone ridges. In the segment of the river 

from Cootes Store to Edinburg the river crosses the 

main Shenandoah Valley, a rolling country with an 

average relief of about 200 feet, that is underlain by 

Cambrian and Ordovician carbonate rocks. The river 

is bordered by terraces, narrower than those upstream 

but nevertheless considerably wider than the channel. 

In a few places terraces are a mile wide. The width of 

the flood plain and terraces diminishes, however, in a 

downstream direction. Banks and bed are composed 

primarily of cobbles of sandstone transported from

upstream. Above Broadway the river crosses the strike 

of the rocks at right angles. Below Broadway it 

parallels the strike following closely a belt of limestone 

at the top of the Cambrian. As far as Mount Jackson 

it remains on the northwest side of prominent and 

resistant cherty beds in the Beekmantowii dolomite 

(Ordovician). Just above Edinburg the river crosses 

the cherty beds and enters a belt of Martinsburg shale. 

Below Broadway, outcrops of limestone occur in the 

bed in many places. From Mount Jackson to Edinburg limestone outcrops are more numerous, and the
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

streambed contains angular cobbles and boulders of 

limestone of local origin equal in number to the sandstone cobbles transported from upstream.

The third distinctive segment of the river is the 

meandering reach extending from Edinburg to beyond 

Strasburg. In this segment the river flows on the 

Martinsburg shale, a tightly folded belt of weak siltstone, fine sandstone, shale, and subordinate calcareous 

shale, that has a prominent fracture cleavage and joint 

system oriented northwestward. The river is flanked 

on the southeast by the steep shale slopes of Massanutten MoiuiTain, a complex syncline. The side of this 

mountain that borders the North Fork has a core of 

hard quartzite, of Silurian age, exposed at the crest. 

The quartzite breaks up in boulders forming talus 

slides and block fields that extend down the hollows in 

the mountainside to the meander bends of the river. 

In contrast to the upstream reaches, the riverbed in the 

meandering segment is mostly exposed bedrock. 

Gravel and cobbles occuply less than one third the area 

of the bed. The channel is bordered by a narrow flood 

plain and in some places by gravel terraces. In three 

places the terraces occupy abandoned meander loops 20 

to 40 feet above river level.

RIVER CHANNEL 

LONGITUDINAL PROFILE

It is of first importance to consider whether the 

profile is smoothly graded or whether there is a break 

in grade, an irregularity, or other change that marks

the beginning of the meandering segment. Examination of the profile, measured along its channel, shows 

that it is uniform. The profile follows fairly closely 

a simple logarithmic curve, as shown in figure 3, 

though there are departures from. it. It is 'a property 

of such a profile that the channel slope (S) at any point 

on the stream is inversely proportional to the stream 

length (L) (the distance from the source to the same 

point.) The ratio of the slopes at any two points (Si 

and $2) is equal to the reciprocal of the ratio of the 

lengths of these points (Zx and L2 ) :

CHANNEL CROSS SECTION

Channel cross section is a significant factor affecting 

the adjustment of the stream to its load. If there were 

changes in the nature of the equilibrium conditions in 

the channel the cross section might be expected to 

change. The results of measurements made by traverses 

with tape and hand level across the channel from flood 

plain to flood plain are shown in figure 4. The variations within a reach are much greater than the variations between the meandering and nonmeandering 

reaches. Therefore 110 significant difference can be 

shown.

BED MATERIAL

Rock fragments resting on the bed of the North 

Fork are coarse throughout and consist of pebbles, 

cobbles, and boulders. The bed material was sampled 

at several localities, including two in the lower or 

meandering segment, two in the middle or limestone 

segment, and three in the upper segment above Cootes 

Store. Size was estimated by measuring the intermediate diameters of more than 50 fragments on the 

streambed selected at regular intervals along a tape 

stretched across the streambed. The size statistics at 

these localities are given in table 1.

TABLE 1. Summary of data on size of fragments on the streambed, North Fork, Shenandoah River

[Results of analyses made in the field by the Wolman method (Wolman. 1954; 

Hack, 1957)]

10 20 30 40 60 80 100 

DISTANCE FROM SOURCE, IN MILES

FIGURE 3. Longitudinal profile of the North Fork of the Shenandoah 

River. The distance from the source is measured along the longest 

or principal stream of the drainage basin which heads on the Virginia-West Virginia boundary at lat 38 "40' N. The horizontal scale 

is logarithmic.

Locality

Segment

Upper segment. _ _

Middle segment 

(limestone). ___

Lower segment 

(Martinsburg

Miles 

from 

source 

of river

7.8 

15.4 

18.8

46.2

48.2

79.5

92.7

Number

of fragments in 

sample

50 

50 

52

76

103

119

66

Phi 

mean 

size

-6.26 

-6.64 

-6.02

-5.90

-6.00

-5.45

-6.59

Phi

standard 

deviation

1.45 

1.35 

.93

1.40

1.25

1.37

Geometric mean 

(mm)

76 

100

64

59

64

95

Percent

sandstone in 

sample

100 

100 

100

68

86

37

25
INTRENCHED MEANDERS OF THE NORTH FORK OF THE SHENANDOAH RIVER, VIRGINIA

Cootes Store

\

Edinburg

^ 0.06

Q

O 0.04 hH0.03

LlJ

Q

O 0.02

O

h-

£T 

400

h 30°

LJ

LJ 

LL.

- 200

I 

Q

$

100

h- 10 

LJ 

LJLiDEPTH, IN

Ul

-

-

~

-

1

-

_

_

*

1

I

-

: '

1

 V- w 

1 1 1

9

 

*

. 1

 '<* 

1 1 1

 

*

1 1 1 1 1

 

 *

1 1 1 1 1

9

.

1 1 1 1 1

10 50 100 200 

MILES FROM HEAD OF NORTH FORK SHENANDOAH RIVER

FIGURE 4. Graph on logarithmic scales comparing the dimensions of 

the channel of the North Pork at different places along the stream.

These statistics do not apply strictly to the traction 

load that is moved day by day or month by month. 

They describe material that is now on the bed, some 

of which has been moved from upstream, and some of 

which is residual and represents a lag concentrate of 

fragments torn off the bedrock locally but that is not 

yet broken into sizes small enough to have been moved 

in the last flood. It was shown by studies in other 

streams in the Shenandoah Valley (Hack, 1957, p. 76- 

87), however, that siliceous rocks like sandstone and 

chert are moved great distances downstream from their 

source, whereas rocks like limestone and shale are soft 

and are not carried far downstream before they are 

broken into sizes too small to remain as a lag concentrate on the bed. Inasmuch as the bed samples of the 

North Fork all contain sandstone from upstream, in 

addition to locally derived rocks, and the sandstone is 

on the average coarser than the local rocks, it is inferred that the river is competent to move material 

of the size that is on the bed.

The sampling indicated no significant change in size 

of bed material along the stream, as shown by an analysis of variance of the phi mean sizes. The bed material is equally as coarse or coarser near the mouth, in 

the meandering reach at mile 92.7, as it is near the 

source, at mile 7.8. The lack of change of particle size 

along the stream is related to the regular form of the 

longitudinal profile. Studies in a wide selection of 

streams in the Shenandoah Valley and in nearby Maryland and Virginia (Hack, 1957) show that the range in 

the size of fragments on the streambeds is from mean 

sizes more than 600 millimeters to mean sizes less than 

10 millimeters. These size variations are related to 

differences in discharge, channel slope, and to other 

hydrologic variables as well as to the material itself, 

and it was shown that streams with the competence to 

move the material of the same size along their entire 

course, have longitudinal profiles like that shown in 

figure 3, of the North Fork. In such profiles the fall 

is proportional to the logarithm of the length, and the 

channel slope, is inversely proportional to the length.

The problem is illuminated by consideration of the 

relative proportions of the materials on the bed. It is 

noteworthy that in the upper segment the bed material 

is all sandstone. This is an area of high relief in which 

many tributaries enter the stream from ridges composed 

predominantly of sandstone of Silurian and Devonian 

age. Although the river itself is flowing in a shalevalley, the bedrock is so soft, relative to the sandstone 

introduced by the tributaries, that the bedrock is broken 

up and carried away in small fragments, and the proportion that remains on the bed is too small relative 

to the sandstone to appear in the sample. In the middle 

segment (Cootes Store to Edinburg), where limestone 

is the dominant bedrock, sandstone on the bed diminishes in relation to limestone. This is because the part 

of the drainage basin underlain by sandstone diminishes 

as the basin enlarges in the limestone region. In the 

lower segment, underlain by Martinsburg shale, the 

proportion of sandstone diminishes still further, and 

fragments of Martinsburg shale, although much softer, 

make up three-fourths of the total material in transit 

on the bed.

Although not shown in table 1, wherever sandstone 

bed material is mixed with shale and limestone fragments, the sandstone component is the coarsest. The 

mean size of all the bed material sampled averages 68
6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

millimeters. The mean of all sandstone fragments is 

78 millimeters, and the mean of all limestone and shale 

fragments is 59 millimeters. Since sandstone is the 

most resistant material it determines the size of the 

other components of the bed material. Fragments of 

locally derived rock that are much smaller are quickly 

transported downstream, whereas larger fragments 

accumulate at their source until they are broken into 

sizes small enough to be moved by traction. The size 

of the sandstone fragments must be a function of several factors including inherent properties of the rock, 

such as its toughness, and the spacing of joints, and of 

external factors such as the distance from the source 

and rate of transport. If the sandstone were all derived from the headwater area above Cootes Store the 

size of fragments on the bed probably would diminish 

downstream as it does in the Middle River (Hack, 1957, 

fig. 38). In the North Fork, however, the load of large 

sandstone fragments transported only short distances is 

renewed downstream by tributaries like Smith Creek 

and by rock discharged into the stream from the slopes 

of Massanutten Mountain.

DISCHARGE

The North Fork shows a regular increase in discharge as the drainage area becomes larger. Data on 

the average discharge are available at three gaging 

stations along the river as shown in table 2.

TABLE 2. Mean annual discharge at gaging stations on the North

Fork of the Shenandoah River 

[Commonwealth of Virginia, Division of Water Resources, 1952]

Location of gage

Cootes Store

Mount Jackson .. _

Strasburg __ _

Drainage 

area 

(square 

miles)

215

509

772

Length of 

record

(years)

25

7

25

Average 

annual 

discharge 

(cfs)

184

351

568

SUMMARY OF CHANNEL CHARACTERISTICS

The North Fork of the Shenandoah River has a 

smooth longitudinal profile so regular that it follows 

closely a simple logarithmic curve throughout its 

length. Measurement of the bed material indicates 

that the channel slope and cross section are adjusted 

with the available discharge so as to transport bed 

material of the same size. This size is determined in 

part by the physical properties of sandstone beds of 

Silurian and Devonian age, the most resistant rocks 

of the drainage basin. The adjustment of various factors seems to be precise with regard to the bed material and longitudinal profile. This adjustment is 

maintained throughout the entire river course in spite

of great changes ill bedrock conditions and in the 

sinuosity of the river.

CONTROLS OF MEANDER FORMATION

The close correlation of the meanders with the outcrop area of the Martinsburg shale (fig. 2) shows that 

the meanders are related in origin to this formation. 

Furthermore, other rivers in the Shenandoah Valley 

have elongate meanders in the same rock formation. 

The straight reaches of the North Fork within the 

meander belt all trend uniformly northwestward, although as reported by Fisher (1955) the river has some 

short rectilinear reaches less than 1,000 feet long at the 

bends that trend N. 40° E.

The Martinsburg shale consists of thin laminae alternately of fine-grained sandstone, siltstone, and shale. 

Calcareous beds occur at some horizons, particularly 

near the base of the formation. A typical lithologic 

unit is exposed in a roadcut shown in plate 1. A section 25 feet thick at this locality contains 8 fine-grained 

sandstone beds averaging 2.3 feet in thickness and 7 

siltstone and shale beds averaging 1.0 feet in thickness. 

The sandstone beds are notably more resistant to erosion than the shale. In the river channel the fine 

sandstone beds stand up as slablike corrugations, separated by grooves of shale, in places 2 feet deep. The 

corrugations are prominent and striking features of 

the riverbed for long distances, and at low-water 

periods form straight lines crossing the water surface 

northeastward at right angles to the channel banks.

The prominent slabs or laminations are cut by a 

strong joint system in which the most sharply defined 

planes are alined north west-southeast. There is also 

a prominent fracture cleavage. The siltstone beds, in 

particular, are broken into crude match-shaped 

prisms whose long axes are oriented northwestward 

parallel to the principal joint planes. The prismatic 

cleavage is independent of the bedding. This structure 

is illustrated in plate 1. Spheroids are common in the 

Martinsburg, especially in the more massive siltstone 

beds. These bear no apparent relation to the fracture 

cleavage, which cuts through them.

Another characteristic of the Martinsburg shale that 

may favor meander development is its relatively low 

resistance to erosion. The shale is commonly found 

in the lowest parts of the valleys and is nowhere a 

ridge maker.

Examination of the meanders (fig. 1) shows that, unlike alluvial meanders, they consist of long straight 

reaches connected by 180° bends. The amplitude of 

the meanders is irregular and in places is more than 

twice the value of the wavelength. Alluvial meanders, 

on the other hand, are more symmetrical. The curves, 

or arcs, that form the bends generally exceed 180° of
GEOLOGICAL SUKVEY PROFESSIONAL PAPEK 354 PLATE 1

A. OUTCKOP 1N7EAR W'lQDSTOCK 

Showing typical beds of fine sandstone and siltstone alternating with shale

B. OUTCROP NEAR EDINBL'RG

Showing typical prismatir cleavage cutting a spheroid. The small talus apron 

at right is composed of siltstone prisms

OUTCROPS OF MARTINSBURG SHALE
INTRENCHED MEANDERS OF THE NORTH FORK OF THE SHENANDOAH RIVER. VIRGINIA

arc, and the amplitude is regular and may be nearly 

equal to but rarely larger than the wavelength. In 

alluvial meanders, crossovers (where the current 

crosses from one side of the stream to the other) generally occur at the same point on the bends. In. the 

North Fork, crossovers occur not only 011 the, bends 

but also in the straight reaches. This phenomenon is 

a consequence of the extraordinary amplitude of the 

meanders.

The North Fork meanders bear at least one similarity to alluvial meanders, however. Their wavelength 

is regular and is a function of the discharge or width 

of the stream. Opequoii Creek, for example, and the 

Middle River, which are smaller streams in the Shenandoah Yailey, have meanders of the same type but the 

wavelengths are smaller. Leopold and AVolmaii (1957, 

p. 58) have shown that there is a regular relation between the wavelength of the bends in a stream and the 

channel width and discharge. Streams with a channel width of 250 feet, like the North Fork, have an 

average meander wavelength of about 3,000 feet. The 

wavelength of the North Fork meanders is 4,000 feet 

which is in agreement with Leopold and \Vol man's 

data, considering the range in variation.

In summary it has been shown that the North Fork 

has throughout its length just the slope and cross section required with the gradually increasing discharge 

to transport the more resistant rock fragment eroded 

in the drainage basin. The extraordinary elongate 

meanders are formed in the least resistant rock of the 

drainage basin and follow closely planes of si ructural 

weakness in that rock. In the upstream reaches the 

riverbed is mostly covered by cobbles and boulders. 

\vhereas in the meandering reaches the bed is mostly on 

exposed bedrock. The river travels a much greater 

distance within the meandering reaches for a given 

down valley distance than it does in the nonmeai icier ing 

reaches, and a larger proportion of the valley floor is 

occupied by stream channel.

The explanation for the meanders, then, is that in 

the Martinsburg shale the river channel was cut along 

lines of structural weakness in the rock while the slope 

and cross section necessary to transport cobbles and 

boulders derived from upstream or from t ributaries was 

maintained. The lesser amount of sandbtone and other 

transported cobbles on the bed suggests that in the 

meandering reaches, greater energy has been expended 

in the excavation of rock by elongation of the channel 

and less by the transportation of rock fragments. Instead of a bed composed largely of cobbles the river 

has a bed mostly of shale arranged in project ing 

laminae that resemble a washboard. These projections may supply a frictional resistance to streamflowT 

similar to the resistance supplied upstream by gravel

and cobbles and thus maintain the equilibrium of grade 

and cross section. Spacing of the meanders, as shown 

by a comparison with typical alluvial meanders, is a 

function of hydrologic factors rather than factors related to the bedrock.

INTRENCHMENT HYPOTHESIS

The argument in the preceding pages disposes of the 

hypothesis of meander origin suggested by Charles 

Butts (1940, p. 508), which supposes that the meanders 

are inherited from alluvial meanders formed on a 

peneplain and that they have been preserved but modified in shape as the river was rejuvenated during a 

postulated Pleistocene regional uplift. The meanders 

are coincident wit h the outcrop area of the Mart insburg 

shale, and are related in origin to structural properties 

of that formation rather than to an alluvial deposit 

that has been removed.

ft has been noted by many geologists, however, that 

the North Fork is bordered by steep valley walls, and 

the stream has often been referred to as intrenched, or 

entrenched (for example Thornbury, 1954, p. 146. fig. 

6.2). The word, "intrenched." itself implies that the 

river has cut down through bedrock in a narrow valley 

but I hat at one time it flowed across a more open plain 

such as a peneplain or at least an open valley less narrow than the present one. If this were the case presumably the meanders developed during intrenchment.

The topography adjacent to the river is now considered to determine whether the intrenchment hypot hesis is necessary in explaining the valley. Figure 5 

shows a series of transverse profiles across the Shenandoah Valley from Massanutten Mountain to the sandstone ridges on the northwest. Profiles A-A' to 

D-D' cross the valley in the limestone area above the 

meandering reaches of the river, whereas profiles 

E-E' to H-llr cross the meanders. The topography 

as illustrated by these profiles is hilly and irregular. 

It is not feasible on the basis of this topography to 

reconstruct a hypothetical peneplain or erosion surface 

of low relief. The hilltops are too irregular in altitude 

and in relative height above the streams. Furthermore, study of the geology of the area shows thai the 

height of the hills is closely related to their lithologic 

composition. The high hills in the western part of 

profile A-A', for example, are underlain by limestone 

containing thick beds of chert. If the area ever were 

a peneplain its surface must have been above the present 

topography.

The narrow, bedrock valley of the North Fork typified by the cross section 77-77' (fig. 5) is adequately 

explained without recourse to the idea of peneplanation 

or a surface of low relief. The topography along the 

North Fork is an equilibrium or graded topography
8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

A'

1600'- 

1400' -

1000' -

600' -

- 1400'

r looo'-

- 600' -

1400' -,

600'-

D

1400' -,

1000'-

D'

600'-< 600' -

1000'- 

600'-

1000' -i

600'-

DATUM IS MEAN SEA LEVEL 

Beginning of meanders

10 MILES

_J

LINE CONNECTING POINTS IN CHANNEL OF RIVER AT 

SECTIONS A-A' TO H-H'

LONG PROFILE OF CHANNEL (INCLUDES BENDS)

FIGURE 5. Profiles with greatly exaggerated vertical scale showing the relief of the floor of the Shenandoah Valley in the outcrop area of 

Cambrian and Ordovician limestone and shale. The location of the profiles is shown on figure 2.

like that described by Hack and Goodlett (in press, 

1960) in an area nearby, or by Strahler (1950) in the 

Verdugo Hills, California. As shown by many recent 

geomorphological works, principally that of Horton 

(1945), drainage networks are characteristically regular in pattern and the density of drainage of streams 

of the same order tends to be nearly constant in areas 

of the same climate and geology. As a consequence the 

interstream divides form a system of ridges rising above 

the streams about the same height. A large trunk 

stream in such a network of valleys and ridges appears 

to be intrenched because it is bordered mostly by the

steep slopes of interstream areas. These slopes are 

graded for the transportation of soil and rock fragments by the process of creep, and hence are convex 

upward. The streams that intersect the trunk streams 

are mostly first-order streams and they intersect 

at a high angle. The appearance of intrenchment is 

enhanced if the topography is underlain by a nonresistant rock on which the profiles of the interstream 

areas are convex upward and therefore appear rounded 

or flattish.

Such a topography may be said to be composed of 

slopes that are everywhere in a state of continuous
INTRENCHED MEANDEES OF THE NORTH FORK OF THE SHENANDOAH RIVER, VIRGINIA 9

adjustment so that the materials weathered and eroded 

from the surface are transported through a network 

of channels into larger and larger trunk streams. Depending on the rate of erosion and the total relief, theaverage slopes of such a topography may be very steep 

or very gentle.

The longitudinal profile of the North Fork shown 

at the bottom of figure 5 by the line Y-Y', is drawn 

along the channel and includes the bends of the 

meanders. It is a smooth curve, gently concave upward 

and is of simple form because adjusted to transport 

material of the same size with a gradually increasing 

discharge. The profile above it (that has a larger 

horizontal scale) connects points in the river channel 

along a nearly straight line. It does not include the 

meander bends and therefore represents the profile, as 

the crow flies, drawn along the lowest points of the valley bottom. This curve is steeper in the meandering 

reaches than in the reaches upstream. The adjacent 

network of tributary channels and intervening hills are 

graded to the main trunk channel, and therefore the 

entire topographic surface adjacent to the North Fork 

has a downvalley or northeastward component of slope 

that is steeper in the meandering region than in the 

part upstream.

If the topography adjacent to the North Fork can be 

satisfactorily explained as a graded topography or a 

topography in which the erosional slopes are in equilibrium, then the meanders exist simply because they 

are in the Martinsburg shale, and we can expect that 

any river of comparable size with the gradient necessary 

to transport cobbles and boulders will also meander in 

the Martinsburg. An exception to this generalization 

would be a stream so heavily loaded with waste of 

material derived from upstream that the bed and banks 

would be blanketed by this material thereby insulating tlie river from the shale.

It becomes pointless to speculate on the geologichistory of the North Fork and the manner in which 

it has cut down into the Martinsburg. The river may 

have eroded its way downward in the northwest limb 

of the Massanutten syncline thousands of feet, and may 

have developed and maintained a meandering course 

in the Martinsburg shale in the distant geologic past.

MEANDERING STREAMS IN ROCKS OTHER THAN THE 

MARTINSBURG SHALE

Meandering streams of the Sheiiandoah type are 

common in the central Appalachians, and they are not 

all confined to the Martinsburg shale. Meandering 

reaches are spectacular in the Middle River, Augusta 

County, Va., in Cambrian limestone. The Potomac

River has well-developed meanders that are apparently 

structurally controlled in rocks of Devonian age. 

Meanders almost as striking as those in the Martinsburg shale occur in southwestern Virginia in metamorphic rocks on both sides of the Blue Ridge escarpment. 

The New River, Reed Island Creek, Dan River, and 

Blackwater River, for example, display meanders with 

extreme amplitudes. The meanders of the Dan, like 

the meanders of the North Fork, are confined in 

strongly laminated rocks, although the rocks are metamorphic with alternating thin layers of quartz mica 

schist and amphibolite schist.

CONCLUSION

The meanders of the North Fork have been explained 

within a framework of ideas in which rejuvenation and 

a former surface of low relief are not factors. In the 

writers 1 hypothesis the meanders are an essential part 

of a graded, erosional topographic system. The arrangement of the rocks in space and the action of 

hydrologic factors on them is the basis for our explanation of the topography and pattern of drainage, rather 

than a change in regimen or slope resulting from an 

uplift.

The elevation of the mouth of the North Fork is 

determined largely independently of conditions in its 

own drainage basin, for it empties into a much larger 

stream, the South Fork, which determines the local 

base level. The elevations of the headwaters of the 

North Fork are also to some extent determined independently for they are related to the physical and 

chemical resistance of the rocks under the drainage 

divides. Between these two independently fixed points 

the profile of the stream is determined by a combination of factors, some local related to the rocks along 

the channel others related to duties imposed from upstream such as quantity of water and size and amount 

of bed material. The meanders result from the interaction of these factors with those of local origin related to peculiar structural properties of the enclosing 

rock.

Leopold and Wolman (1957), dealing primarily with 

streams flowing in alluvial channels, have demonstrated 

a close relation between hydrologic factors, including 

channel slope, discharge, and load, that apparently determine whether or not a stream will meander. It seems 

from our data and analysis that the meanders of the 

North Fork, even though in bedrock, exist in harmony 

with the same kinds of factors. Probably streams that 

meander in bedrock are actively eroding streams that 

have a relatively small load for the slope and discharge 

and that erode weak rocks with strong linear and planar 

structures.
10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

REFERENCES

Butts, Charles, 1940, Geology of the Appalachian Valley in 

Virginia: Virginia Geol. Survey Bull., v. 52, 568 p.

Cole, W. S., 1930, The interpretation of intrenched meanders: 

Jour. Geology, v. 38, p. 423-436.

Commonwealth of Virginia, Division of Water Resources, 1952, 

Surface water supply of Virginia, Potomac, Rappahannock, 

and York River Basins, 1942-50: Commonwealth of Virginia, Dept. of Conserv. and Devel. Bull. 12, 372 p.

Fisher, C. C., 1955, Elongate meanders of the North Fork of 

the Shenandoah River [abs.] : Geol. Soc. American Bull., 

v. 66, p. 1687.

Gregory, H. E., and Moore, R. C., 1931, The Kaiparowits region; 

a geographic and geologic reconnaissance of parts of Utah, 

and Arizona: U.S. Geol. Survey Prof. Paper 164, 157 p.

Hack, J. T., 1957, Studies of longtitudinal stream profiles in Virginia and Maryland: U.S. Geol. Survey Prof. Paper 294-B, 

p. 45-97.

Hack, J. T., and Goodlett, J. C., (1960), Geomorphology and 

forest ecology of a mountain region in the central Appalachians: U.S. Geol. Survey Prof. Paper 347, in press.

Horton, R. E., 1945, Erosional development of streams and their 

drainage basins; hydrophysical approach to quantitative 

morphology: Geol. Soc. American Bull., v. 56, p. 275-370.

Leopold, L. B., and Wolman, M. G., 1957, River channel patterns: braided meandering and straight: U.S. Geol. Survey 

Prof. Paper 282-B, p. 39-84.

Monnett, V. E., and Brown, H. E., 1950, The principles of physical geology : Boston, Ginn & Co., 450 p.

Rich, John L., 1939, A bird's eye cross section of the Central 

Appalachian Mountains and Plateau, Washington to Cincinnati; Geog. Rev., v. 29, p. 561-586.

Strahler, A. N., 1946, Elongate intrenched meanders of Conodoguinet Creek, Pa.: Am. Jour. Sci., v. 244, p. 31-40.

 , 1950, Equilibrium theory of erosional slopes approached 

by frequency distribution analysis: Am. Jour. Sci., v. 248, 

p. 673-696 and 800-814.

Thompson, H. D., 1947, Fundamentals of earth science: New 

York, John Wiley & Sons, 618 p.

Thornbury, W. D., 1954, Principles of geomorphology: New 

York, John Wiley & Sons, 618 p.

U.S. Geological Survey, 1947, Strasburg quadrangle, Virginia, 

scale 1:62,500, shaded relief.

Wolman, M. G., 1954, A method of sampling coarse riverbed 

material: Am. Geophys. Union Trans., v. 35, p. 951-956.

o