DRAINAGE SYSTEMS AND THE GLACIAL PERIOD.
We will begin the consideration of this part of our subject, also, with
the presentation of the salient facts in North America, since that field
is simpler than any field in the Old World.
The natural drainage basins of North America east of the Rocky Mountains
are readily described. The Mississippi River and its branches drain
nearly all the region lying between the Appalachian chain and the Rocky
Mountains and south of the Dominion of Canada and of the Great Lakes.
All the southern tributaries to the Great Lakes are insignificant, the
river partings on the south being reached in a very short distance. The
drainage of the rather limited basin of the Great Lakes is northeastward
through the St. Lawrence River, leaving nearly all of the Dominion of
Canada east of the Rocky Mountains to pour its surplus waters northward
into Hudson Bay and the Arctic Ocean. With the exception of the St.
Lawrence River, these are essentially permanent systems of drainage. To
understand the extent to which the ice of the Glacial period modified
these systems, we must first get before our minds a picture of the
country before the accumulation of ice began.
_Preglacial Erosion._
Reference has already been made to the elevated condition of the northern
and central parts of North America at the beginning of the Glacial
period. The direct proof of this preglacial elevation is largely derived
from the fiords and great lake basins of the continent. The word "fiord"
is descriptive of the deep and narrow inlets of the sea specially
characteristic of the coasts of Norway, Denmark. Iceland, and British
Columbia. Usually also fiords are connected with valleys extending still
farther inland, and occupied by streams.
Fiords are probably due in great part to river erosion when the shores
stood at considerably higher level than now. Slowly, during the course
of ages, the streams wore out for themselves immense gorges, and were
assisted, perhaps, to some extent by the glaciers which naturally came
into existence during the higher continental elevation. The present
condition of fiords, occupied as they usually are by great depths of
sea-water, would be accounted for by recent subsidence of the land. In
short, fiords seem essentially to be submerged river gorges, partially
silted up near their mouths, or perhaps partially closed by terminal
moraines.
It is not alone in northwestern Europe and British Columbia that fiords
are found, but they characterize as well the eastern coast of America
north of Maine, while even farther south, both on the Atlantic and on
the Pacific coast, some extensive examples exist, whose course has been
revealed only to the sounding-line of the Government survey.
The most remarkable of the submerged fiords in the middle Atlantic region
of the United States is the continuation of the trough of Hudson River
beyond New York Bay. As long ago as 1844 the work of the United States
Coast Survey showed that there was a submarine continuation of this
valley, extending through the comparatively shallow waters eighty miles
or more seaward from Sandy Hook.
[Illustration: Fig. 50.--Map showing old channel and mouth of the Hudson
(dewberry).]
The more accurate surveys conducted from 1880 to 1884 have brought to
our knowledge the facts about this submarine valley almost as clearly
as those relating to the inland portion of it above New York city.
According to Mr. A. Lindenkohl,[CA] this submarine valley began to be
noticeable in the soundings ten miles southeast of Sandy Hook. The depth
of the water where the channel begins is nineteen fathoms (114 feet).
Ten miles out the channel has sunk ninety feet below the general depth
of the water on the bank, and continues at this depth for twenty miles
farther. This narrow channel continues with more or less variation for
a distance of seventy-five miles, where it suddenly enlarges to a width
of three miles and to a depth of 200 fathoms, or 1,200 feet, and extends
for a distance of twenty-five miles, reaching near that point a depth
of 474 fathoms, or 2,844 feet. According to Mr. Lindenkohl, this ravine
maintains for half its length "a vertical depth of more than 2,000 feet,
measuring from the top of its banks, and the banks have a nearly uniform
slope of about 14°." The mouth of the ravine opens out into the deep
basin of the central Atlantic.
[Footnote CA: Bulletin of the Geological Society of America, vol. i, p.
564; American Journal of Science, June, 1891.]
With little question there is brought to light in these remarkable
investigations a channel eroded by the extension of the Hudson River,
into the bordering shelf of the Atlantic basin at a time when the
elevation of the continent was much greater than now. This is shown to
have occurred in late Tertiary or post-Tertiary times by the fact that
the strata through which it is worn are the continuation of the Tertiary
deposits of New Jersey. The subsidence to its present level has probably
been gradual, and, according to Professor Cook, is still continuing at
the rate of two feet a century.
Similar submarine channels are found extending out from the present
shore-line to the margin of the narrow shelf bordering the deep water of
the central Atlantic running from the mouth of the St. Lawrence River,
through St. Lawrence Bay, and through Delaware and Chesapeake Bays.[CB]
All these submerged fiords on the Atlantic coast were probably formed
during a continental elevation which commenced late in the Tertiary
period, and reached the amount of from 2,000 to 3,000 feet in the
northern part of the continent.
[Footnote CB: See Lindenkohl in American Journal of Science, for June,
1891.]
[Illustration: Fig. 51.--New York harbor in preglacial times looking
south, from south end of New York Island (Newberry).]
To this period must probably be referred also the formation of the gorge,
or more properly fiord, of the Saguenay, which joins the St. Lawrence
below Quebec. The great depth of this fiord is certainly surprising,
since, according to Sir William Dawson, its bottom, for fifty miles above
the St. Lawrence, is 840 feet below the sea-level, while the bordering
cliffs are in some places 1,500 feet above the water. The average width
is something over a mile.
It seems impossible to account for such a deep gorge extending so far
below the sea-level, except upon the supposition of a long-continued
continental elevation, which should allow the stream to form a cañon to
an extent somewhat comparable with that of the cañons of the Colorado and
other rivers in the far West. Then, upon the subsidence of the continent
to the present level, it would remain partially or wholly submerged,
as we find it at the present time. During the Glacial period it was so
filled with ice as to prevent silting up. The rivers entering the Pacific
Ocean, both in the United States and in British Columbia, are also lost
in submerged channels extending out to the deeper waters of the Pacific
basin in a manner closely similar to the Atlantic streams which have been
mentioned.
During this continental elevation which preceded, accompanied, and
perhaps brought on the Glacial period, erosion must have proceeded with
great intensity along all the lines of drainage, and throughout the whole
region which is now covered, and to a considerable extent smoothed over,
by glacial deposits, and the whole country must have presented a very
different appearance from what it does now.
A pretty definite idea of its preglacial condition can probably be formed
by studying the appearance of the regions outside of and adjoining that
which was covered by the continental glacier. The contrast between the
glaciated and the unglaciated region is striking in several respects
aside from the presence and absence of transported rocks and other
_débris_, but in nothing is it greater than in the extent of river
erosion which is apparent upon the surface. For example, upon the
western flanks of the Alleghanies the regions south of the glacial limit
is everywhere deeply channeled by streams. Indeed, so long have they
evidently been permitted to work in their present channels that, wherever
there have been waterfalls, they have receded to the very head-waters,
and no cataracts exist in them at the present time. Nor are there in the
unglaciated region any lakes of importance, such as characterize the
glaciated region. If there have been lakes, the lapse of time has been
sufficient for their outlets to lower their beds sufficiently to drain
the basins dry.
On entering the glaciated area all this is changed. The ice-movement
has everywhere done much to wear down the hills and fill the valleys,
and, where there was _débris_ enough at command, it has obliterated the
narrow gorges originally occupied by the preglacial streams. Thus it has
completely changed the minor lines of superficial drainage, and in many
instances has produced most extensive and radical changes in the whole
drainage system of the region. In the glaciated area, channels buried
beneath glaciated _débris_ are of frequent occurrence, while many of the
streams which occupy their preglacial channels are flowing at a very much
higher level than formerly, the lower part of the channel having been
silted up by the superabundant _débris_ accessible since the glacial
movement began.
_Buried Outlets and Channels._
It is easy to see how the great number of shallow lakes which frequent
the glaciated region were formed by the irregular deposition of glacial
_débris_, but it is somewhat more difficult to trace out the connection
between the Glacial period and the Great Lakes of North America, several
of which are of such depth that their bottoms are some hundreds of feet
below the sea-level, Lake Erie furnishing the only exception. This
lake is so shallow that it is easy to see how its basin may have been
principally formed by river erosion, while it is evident that such
must have been the mode of its formation, since it is surrounded by
sedimentary strata lying nearly in a horizontal position.
[Illustration: Fig. 52.--Section across the valley of the Cuyahoga River,
twenty miles above its mouth (Claypole).]
That Lake Erie is really nothing but a "glacial mill-pond" is proved
also by much direct evidence, especially that derived from the depth of
the buried channels of the streams flowing into it from the south. Of
these, the Cuyahoga River, which enters the lake at Cleveland, has been
most fully investigated. In searching for oil, some years ago, borings
were made at many places for twenty-five miles above the mouth of the
river. As a result, it appeared that for the whole distance the rocky
bottom of the gorge was about two hundred feet below the present bottom
of the river, while the river itself is two or three hundred feet below
the general level of the country, occupying a trough about half a mile in
width, with steep, rocky sides. These facts indicate that at one time the
river must have found opportunity to discharge its contents at a level
two hundred feet below that of the present lake, while an examination
of the material filling up the bottom of the gorge to its present level
shows it to be glacial _débris_, thus proving that the silting up was
accomplished during the Glacial period.
As the water of Lake Erie is for the most part less than one hundred
feet in depth, and is nowhere much more than two hundred feet deep, it
is clear that the preglacial outlet which drained it down to the level
of the rocky bottom of the Cuyahoga River must have destroyed the lake
altogether. Hence Ave may be certain that, before the Glacial period, the
area now covered by the lake was simply a broad, shallow valley through
which there coursed a single river of great magnitude, with tributary
branches occupying deep gorges. Professor J. W. Spencer has shown with
great probability that the old line of drainage from Lake Erie passed
through the lower part of the valley of Grand River, in Canada, and
entered Lake Ontario at its western extremity, and that during the great
Ice age this became so completely obstructed with glacial _débris_ as to
form an impenetrable dam, and to cause the pent-up water to flow through
the Niagara Valley, which chanced to furnish the lowest opening.
In speaking of the present area of Lake Erie, however, as being then
occupied by a river valley, we do not mean to imply that it was not
afterwards greatly modified by glacial erosion; for undoubtedly this was
the case, whatever views we may have as to the relative efficiency of ice
and water in scooping out lake basins.
In the case of Lake Erie, we need suppose no change of level to account
for the erosion of its basin, but only that, since the strata in which it
is situated were deposited, time enough had elapsed for a great river to
cut a gorge extending from the western end of Lake Ontario through to the
present bed of Lake Erie, and that here a great enlargement of the valley
was occasioned by the existence of deep beds of soft shale which could
easily be worn away by a ramifying system of tributary streams. Rivers
acting at present relative levels would be amply sufficient to produce
the results which are here manifest.
But in the case of Lakes Ontario, Huron, Michigan, and Superior, whose
depths descend considerably below the sea-level, we must suppose that
they were, in the main, eroded when the continent was so much elevated
that their bottoms were brought above tide-level. The depth of Lake
Ontario implies the existence of an outlet more than four hundred feet
lower than at present, which, of course, could exist only when the
general elevation was more than four hundred feet greater than now.
The existence of an outlet at that depth seems to be proved also by the
fact that at Syracuse, where numerous wells have been sunk to obtain
brine for the manufacture of salt, deposits of sand, gravel, and rolled
stones, four hundred and fifty feet thick, are penetrated without
reaching rock. Since this lies in the basin of Lake Ontario, it follows
that if the basin itself has been produced by river erosion, the land
must have been of sufficient height to permit an outlet through a valley,
or cañon, of the required depth, and this outlet must now be buried
beneath the abundant glacial _débris_ that covers the region.
Professor Newberry, who has studied the vicinity carefully, is of the
opinion that there is ample opportunity for such a line of drainage
to have extended through the Mohawk Valley to the Hudson River. But,
at Little Falls, a spur of the Adirondack Mountains projects into the
valley, and the Archæan rocks over which the river runs are so prominent
and continuous that some have thought it impossible for the requisite
channel to have ever existed there. Extensive deposits of glacial
_débris_, however, are found in the vicinity, especially in places some
distance to the north, and in Professor Newberry's opinion the existence
of a buried channel around the obstruction upon the north side is by no
means improbable.
The preglacial drainage of Lake Huron has not been determined with any
great degree of probability. Professor Spencer formerly supposed that it
passed from the southern end of the lake through London, in the western
part of Ontario, and reached the Erie basin near Port Stanley, and so
augmented the volume of the ancient river which eroded the buried cañon
from Lake Erie to Lake Ontario. But he now supposes, though the evidence
is by no means demonstrative, that the waters of Lake Huron passed into
Lake Ontario by means of a channel extending from Georgian Bay to the
vicinity of Toronto.
With a fair degree of probability, the basin of Lake Superior is
supposed by Professor Newberry to have been joined to that of Lake
Michigan by some passage, now buried, considerably to the west of the
Strait of Mackinac, and thence to have had an outlet southward from the
vicinity of Chicago directly into the Mississippi River. Of this there
is considerable evidence furnished by deeply buried channels which have
been penetrated by borings in various places in Kankakee, Livingston,
and McLean Counties, Illinois; but the whole area extending from Lake
Michigan to the Mississippi is so deeply covered with glacial _débris_
that the surface of the country gives no satisfactory indication of the
exact lines of preglacial drainage.
Some of the most remarkable instances of ancient river channels buried
by the glacial deposits have been brought to light in southwestern Ohio,
where there has been great activity in boring for gas and oil. At St.
Paris, Champaign County, for example, in a locality where the surface
of the rock near by was known to be not far below the general level, a
boring was begun and continued to a depth of more than five hundred feet
without reaching rock, or passing out of glacial _débris_.
Many years ago Professor Newberry collected sufficient facts to show that
pretty generally the ancient bed of the Ohio River was as much as 150
feet below that over which it now flows. During a continental elevation
the erosion had proceeded to that extent, and then the channel had been
silted up during the Glacial period with the abundant material carried
down by the streams from the glaciated area. One of the evidences of
the preglacial depth of the channel of the Ohio was brought to light at
Cincinnati, where "gravel and sand have been found to extend to a depth
of over one hundred feet below low-water mark, and the bottom of the
trough has not been reached." In the valley of Mill Creek, also, "in the
suburbs of Cincinnati, gravel and sand were penetrated to the depth of
120 feet below the stream before reaching rock." But from the general
appearance of the channel, Professor J. F. James was led to surmise
that a rock bottom extended all the way across the present channel of
the Ohio, between Price Hill and Ludlow, Ky., a short distance below
Cincinnati, which would preclude the possibility of a preglacial outlet
at the depth disclosed in that direction. Mr. Charles J. Bates (who was
inspector of the masonry for the Cincinnati Southern Railroad while
building the bridge across the Ohio at this point) informs me that Mr.
James's surmise is certainly correct, and that his "in all probability"
may be displaced by "certainly," since the bedded rocks supposed by
Professor James to extend across the river a few feet below its present
bottom were found by the engineers to be in actual existence.
In looking for an outlet for the waters of the upper Ohio which should
permit them to flow off at the low level reached in the channel at
Cincinnati, Professor James was led to inspect the valley extending
up Mill Creek to the north towards Hamilton, where it joins the Great
Miami. The importance of Mill Creek Valley is readily seen in the fact
that the canal and the railroads have been able to avoid heavy grades
by following it from Cincinnati to Hamilton. As a glance at a map will
show, it is also practically but a continuation of the northerly course
pursued by the Ohio for twenty miles before reaching Cincinnati. This,
therefore, was a natural place in which to look beneath the extensive
glacial _débris_ for the buried channel of the ancient Ohio, and here
in all probability it has been found. The borings which have been made
in Milk Creek Valley north of Cincinnati, show that the bedded rock
lies certainly thirty-four feet below the low-water mark of the Ohio
just below Cincinnati, while at Hamilton, twenty-five miles north of
Cincinnati, where the valley of the Great Miami is reached, the bedded
rock of the valley lies as much as ninety feet below present low-water
mark in the Ohio.
Other indications of the greater depth of the preglacial gorge of the
Ohio are abundant. "At the junction of the Anderson with the Ohio, in
Indiana, a well was sunk ninety-four feet below the level of the Ohio
before rock was found." At Louisville, Ky., the occurrence of falls
in the Ohio seemed at first to discredit the theory in question, but
Professor Newberry was able to show that the falls at Louisville are
produced by the water's being now compelled to flow over a rocky point
projecting from the north side into the old valley, while to the south
there is ample opportunity for an old channel to have passed around this
point underneath the city on the south side. The lowlands upon which the
city stands are made lands, where glacial _débris_ has filled up the old
channel of the Ohio.
Above Cincinnati the tributaries of the Ohio exhibit the same phenomena.
At New Philadelphia, Tuscarawas County, the borings for salt-wells show
that the Tuscarawas is running 175 feet above its ancient bed. The
Beaver, at the junction of the Mahoning and Shenango, is flowing 150 feet
above the bottom of its old trough, as is demonstrated by a large number
of oil-wells bored in the vicinity. Oil Creek is shown by the same proofs
to run from 75 to 100 feet above its old channel, and that channel had
sometimes vertical and even overhanging walls.[CC]
[Footnote CC: Geological Survey of Ohio, vol. ii, pp. 13, 14.]
The course of preglacial drainage in the upper basin of the Alleghany
River is worthy of more particular mention. Mr. Carll, of the
Pennsylvania Geological Survey, has adduced plausible reasons for
believing that previous to the Glacial period the drainage of the
valley of the upper Alleghany north of the neighbourhood of Tidioute,
in Warren County, instead of passing southward as now, was collected
into one great stream flowing northward through the region of Cassadaga
Lake to enter the Lake Erie basin at Dunkirk, N. Y. The evidence is
that between Tidioute and Warren the present Alleghany is shallow, and
flows over a rocky basin; but from Warren northward along the valley
of the Conewango, the bottom of the old trough lies at a considerably
lower level, and slopes to the north. Borings show that in thirteen
miles the slope of the preglacial floor of Conewango Creek to the north
is 136 feet. The actual height above tide of the old valley floor at
Fentonville, where the Conewango crosses the New York line, is only 964
feet; while that of the ancient rocky floor of the Alleghany at Great
Bend, a few miles south of Warren, was 1,170 feet. Again, going nearer
the head-waters of the Alleghany, in the neighbourhood of Salamanca,
it is found that the ancient floor of the Alleghany is, at Carrollton,
70 feet lower than the ancient bed of the present stream at Great
Bend, about sixty miles to the south; while at Cole's Spring, in the
neighbourhood of Steamburg, Cattaraugus County, N. Y., there has been
an accumulation of 315 feet of drift in a preglacial valley whose rocky
floor is 155 feet below the ancient rocky floor at Great Bend. Unless
there has been a great change in levels, there must, therefore, have
been some other outlet than the present for the waters collecting in the
drainage basin to the north of Great Bend.[CD]
[Footnote CD: For a criticism of Mr. Carll's views, see an article
on Pleistocene Fluvial Planes of Western Pennsylvania, by Mr. Frank
Leverett, in American Journal of Science, vol. xlii, pp. 200-212.]
While there are numerous superficial indications of buried channels
running towards Lake Erie in this region, direct exploration has not
been made to confirm these theoretical conclusions. In the opinion of Mr.
Carll, Chautauqua Lake did not flow directly to the north, but, passing
through a channel nearly coincident with that now occupied by it, joined
the northerly flowing stream a few miles northeast from Jamestown.[CE] It
is probable, however, that Chautauqua did not then exist as a lake, since
the length of preglacial time would have permitted its outlet to wear a
continuous channel of great depth corresponding to that known to have
existed in the Conewango and upper Alleghany.
[Footnote CE: Second Geological Survey of Pennsylvania, vol. iii.]
The foregoing are but a few of the innumerable instances where the local
lines of drainage have been disturbed, and even permanently changed,
by the glacial deposits. Almost every lake in the glaciated region is
a witness to this disturbance of the established lines of drainage by
glacial action, while in numerous places where lakes do not now exist
they have been so recently drained that their shore-lines are readily
discernible.
An interesting instance of the recent disappearance of one of these
glacial lakes is that of Runaway Pond, in northern Vermont. In the early
part of the century the Lamoille River had its source in a small lake
in Craftsbury, Orleans County. The sources of the Missisquoi River were
upon the same level just to the north, and the owner of a mill privilege
upon this latter stream, desiring to increase his power by obtaining
access to the water of the lake, began digging a ditch to turn it into
the Missisquoi, but no sooner had he loosened the thin rim of compact
material which formed the bottom and the sides of the inclosure, than the
water began to rush out through the underlying and adjacent quicksands.
This almost instantly enlarged the channel, and drained the whole body of
water oft 3 in an incredibly short time. As a consequence, the torrent
went rushing down through the narrow valley, sweeping everything before
it; and nothing but the unsettled condition of the country prevented a
disaster like that which occurred in 1889 at Johnstown, Pa. Doubtless
there are many other lakes held in position by equally slender natural
embankments. Artificial reservoirs are by no means the only sources of
such danger.
The buried channel of the old Mississippi River in the vicinity of
Minneapolis is another instructive example of the instability of many
of the present lines of drainage. The gorge of the Mississippi River
extending from Fort Snelling to the Falls of St. Anthony at Minneapolis
is of post-glacial origin. One evidence of this is its narrowness when
contrasted with the breadth of the valley below Fort Snelling. Below
this point the main trough of the Mississippi has a width of from two to
eight miles, and the faces of the bluffs on either side show the marks
of extreme age. The tributary streams also have had time to wear gorges
proportionate to that of the main stream, and the agencies which oxidise
and discolor the rocks have had time to produce their full effects. But
from Fort Snelling up to Minneapolis, a distance of about seven miles,
the gorge is scarcely a quarter of a mile in width, and the faces of
the high, steep bluffs on either side are remarkably fresh looking by
comparison with those below; while the tributary gorges, of which that of
the Minnehaha River is a fair specimen, are very limited in their extent.
Upon looking for the cause of this condition of things we observe that
the broad trough of the Mississippi River, which had characterised it all
the way below Fort Snelling, continues westward, without interruption, up
the valley of the present Minnesota River, and, what seems at first most
singular, it does not cease at the sources of the Minnesota, but, through
Lake Traverse and Big Stone Lake, is continuous with the trough of the
Red River of the North.
[Illustration: Fig. 53.--Map of Mississippi River from Fort Snelling to
Minneapolis and the vicinity, showing the extent of the recession of the
Falls of St. Anthony since the great Ice age. Notice the greater breadth
of the valley of the Minnesota River as described in the text (Winchell).]
Deferring, however, for a little the explanation of this, we will go
back to finish the history of the preglacial channel around the Falls
of St. Anthony. As early as the year 1876 Professor N. H. Winchell had
collected sufficient evidence from wells, one of which had been sunk to a
depth of one hundred and seventy-five feet, to show that the preglacial
course of the stream corresponding to the present Mississippi River ran
to the west of Minneapolis and of the Falls of Minnehaha, and joined
the main valley some distance above Fort Snelling, as shown in the
accompanying map.
This condition of things was at one time very painfully brought to the
notice of the citizens of Minneapolis. A large part of the wealth of the
city at that time consisted of the commercial value of the water-power
furnished by the Falls of St. Anthony. To facilitate the discharge of
the waste water from their wheels, some mill-owners dug a tunnel through
the soft sandstone underlying the limestone strata over which the river
falls; but it very soon became apparent that the erosion was proceeding
with such rapidity that in a few years the recession of the falls would
be carried back to the preglacial channel, when the river would soon
scour out the channel and destroy their present source of wealth. The
citizens rallied to protect their property, and spent altogether as
much as half a million dollars in filling up the holes that had been
thoughtlessly made; but so serious was the task that they were finally
compelled to appeal for aid to the United States Government. Permanent
protection was provided by running a tunnel, some ways back from the
falls, completely across the channel, through the soft sandstone
underlying the limestone, and filling this up with cement hard enough
and compact enough to prevent the further percolation of the water from
above.
_Ice-Dams._
The foregoing changes in lines of drainage due to the Glacial period were
produced by deposits of earthy material in preglacial channels. Another
class of temporary but equally interesting changes were produced by the
ice itself acting directly as a barrier.
Many such lakes on a small scale are still in existence in various parts
of the world. The Merjelen See in Switzerland is a well-known instance.
This is a small body of water held back by the great Aletsch Glacier,
in a little valley leading to that of the Fiesch Glacier, behind the
Eggischorn. At irregular intervals the ice-barrier gives way, and allows
the water to rush out in a torrent and flood the valley below. Afterwards
the ice closes up again, and the water reaccumulates in preparation for
another flood.
Other instances in the Alps are found in the Mattmark See, which fills
the portion of the Saas Valley between Monte Rosa and the Rhône. This
body of water is held in place by the Allalin Glacier, which here crosses
the main valley. The Lac du Combal is held back by the Glacier de Miage
at the southern base of Mont Blanc. "A more famous case is that of the
Gietroz Glacier in the valley of Bagnes, south of Martigny. In 1818
this lake had grown to be a mile long, and was 700 feet wide and 200
feet deep. An attempt was made to drain it by cutting through the ice,
and about half the water was slowly drawn off in this way; but then the
barrier broke, and the rest of the lake was emptied in half an hour,
causing a dreadful flood in the valley below. In the Tyrol, the Vernagt
Glacier has many times caused disastrous floods by its inability to hold
up the lake formed behind it. In the northwestern Himalaya, the upper
branches of the Indus are sometimes held back in this way. A noted flood
occurred in 1835; it advanced twenty-five miles in an hour, and was felt
three hundred miles down-stream, destroying all the villages on the lower
plain, and strewing the fields with stones, sand, and mud."[CF]
[Footnote CF: Professor William M. Davis in. Proceedings of the Boston
Society of Natural History, vol. xxi, pp. 350, 351.]
In Greenland such temporary obstructions are frequent, forming lakes of
considerable size. Instances occur, in connection with the Jakobshavn and
the Frederickshaab Glaciers, and in the North Isortok and Alangordlia
Fiords.
Frequently, also, bodies of water of considerable size are found in
depressions of the ice itself, even at high levels. I have myself seen
them covering more than an acre, and as much as a thousand feet above
the sea-level, upon the surface of the Muir Glacier, Alaska. They are
reported by Mr. I. C. Russell[CG] of larger size and at still higher
elevations upon the glaciers radiating from Mount St. Elias; while the
explorers of Greenland mention them of impressive size upon the surface
of its continental ice-sheet.
[Footnote CG: See National Geographic Magazine, vol. iii, pp. 116-120.]
With these facts in mind we can the more readily enter into the
description which will now be given of some temporary lakes of vast size
which were formed by direct ice-obstructions during portions of the
period.
One of the most interesting of these is illustrated upon the accompanying
map, which will need little description.
[Illustration: Fig. 54.--Map showing the effect of the glacial dam at
Cincinnati (Claypole). (From Transactions of the Edinburgh Geological
Society.)]
While tracing the boundary-line of the glaciated area in the Mississippi
Valley during the summer of 1882, I discovered the existence of
unmistakable glacial deposits in Boone County, Kentucky, across the Ohio
River, from Cincinnati.[CH]; These deposits were upon the height of land
550 feet above the Ohio River, or nearly 1,000 feet above the sea, which
is about the height of the water-shed between the Licking and Kentucky
Rivers. As the Ohio River occupies a trough of erosion some hundreds of
feet in depth, and extending all the way from this point to the mountains
of western Pennsylvania, it would follow that the ice which conveyed
boulders across the Ohio River at Cincinnati, and deposited them upon
the highlands between the Licking and Kentucky Rivers, would so obstruct
the channel of the Ohio as to pond the water back, and hold it up to
the level of the lowest pass into the Ohio River farther down. Direct
evidences of obstruction by glacial ice appear also for a distance of
fifty or sixty miles, extending both ways, from Cincinnati.
[Footnote CH: The existence of portions of this evidence had previously
been pointed out by Mr. Robert B. Warder and Dr. George Sutton (see
Geological Reports of Indiana, 1872 and 1878).]
The consequences connected with this state of things are of the most
interesting character.
The bottom of the Ohio River at Cincinnati is 432 feet above the
sea-level. A dam of 550 feet would raise the water in its rear to a
height of 982 feet above tide. This would produce a long, narrow lake,
of the width of the eroded trough of the Ohio, submerging the site
of Pittsburg to a depth of 281 feet, and creating slack water up the
Monongahela nearly to Grafton, West Virginia, and up the Alleghany as far
as Oil City. All the tributaries of the Ohio would likewise be filled
to this level. The length of this slack-water lake in the main valley,
to its termination up either the Alleghany or the Monongahela, was not
far from one thousand miles. The conditions were also peculiar in this,
that all the northern tributaries rose within the southern margin of the
ice-front, which lay at varying distances to the north. Down these there
must have poured during the summer months immense torrents of water to
strand boulder-laden icebergs on the summits of such high hills as were
lower than the level of the dam.
Naturally enough, this hypothesis of a glacial dam at Cincinnati aroused
considerable discussion, and led to some differences of opinion.
Professors I. C. White and J. P. Lesley, whose field work has made them
perfectly familiar with the upper Ohio and its tributaries, at once
supported the theory, with a great number of facts concerning certain
high-level terraces along the Alleghany and Monongahela Rivers; while
additional facts of the same character have been brought to light by
myself and others. In general, it may be said that in numerous places
terraces occur at a height so closely corresponding to that of the
supposed dam at Cincinnati, that they certainly strongly suggest direct
dependence upon it. The upward limit of these terraces in the Monongahela
River is 1,065 feet, and they are found in various places in situations
which indicate that they were formed in still water of such long standing
as would require an obstruction below of considerable permanence.
One of the most decisive cases adduced by Professor White occurs near
Morgantown, in West Virginia, of which he gives the following description:
"Owing to the considerable elevation--275 feet--of the fifth terrace
above the present river-bed in the vicinity of Morgantown, its deposits
are frequently found far inland from the Monongahela, on tributary
streams. A very extensive deposit of this kind occurs on a tributary one
mile and a half northeast of Morgantown; and the region, which includes
three or four square miles, is significantly known as the 'Flats.' The
elevation of the 'Flats' is 275 feet above the river, or 1,065 feet above
tide. The deposits on this area consist almost entirely of clays and
fine, sandy material, there being very few boulders intermingled. The
depth of the deposit is unknown, since a well sunk on the land of Mr.
Baker passed through alternate beds of clay, fine sand, and muddy trash,
to a depth of sixty-five feet without reaching bed-rock. In some portions
of the clays which make up this deposit, the leaves of our common
forest-trees are found most beautifully preserved.
"At Clarksburg, where the river unites with Elk Creek, there is a wide
stretch of terrace deposits, and the upper limit is there about 1,050
feet above tide, or only 130 feet above low-water (920 feet); while at
Weston, forty miles above (by the river), these deposits cease at seventy
feet above low water, which is there 985 feet above tide. It will thus
be observed that the upper limit of the deposits retains a practical
horizontality from Morgantown to Weston, a distance of one hundred miles,
since the upper limit has the same elevation above tide (1,045 to 1,065
feet) at every locality.
"These deposits consist of rounded boulders of sandstone, with a large
amount of clay, quicksand, and other detrital matter. The country rock
in this region consists of the soft shales and limestones of the upper
coal-measures, and hence there are many 'low gaps' from the head of one
little stream to that of another, especially along the immediate region
of the river; and in every case the summits of these divides, where they
do not exceed an elevation of 1,050 feet above tide, are covered with
transported or terrace material; but where the summits go more than a
few feet above that level we find no transported material upon them, but
simply the decomposed country rock."
Other noteworthy terraces naturally attributable to the Cincinnati
ice-dam are to be found in the valley of the Kanawha, in West Virginia,
and one of special significance on the pass between the valleys of the
Ohio and Monongahela, west of Clarksburg, West Virginia. According to
Professor White, there is at this latter place "a broad, level summit,
having an elevation of 1,100 feet, in a gap about 300 feet below the
enclosing hills. This gap, or valley, is covered by a deposit of fine
clay. The cut through it is about thirty feet, and one can observe the
succession of clays of all kinds and of different colours, from yellow on
the surface down to the finest white potter's clay at the level of the
railway, where the cut reaches bed-rock, thus proving that the region has
been submerged."[CI]
[Footnote CI: Bulletin of the Geological Society of America, vol. i, p.
478.]
Another crucial case I have myself described at Bellevue, in the angle
of the Ohio and Alleghany Rivers, about five miles below Pittsburg,
where the gravel terrace is nearly 300 feet above the river, making it
about 1,000 feet above the sea. A significant circumstance connected
with this terrace is that not only does its height correspond with that
of the supposed obstruction at Cincinnati, but it contains many pebbles
of Canadian origin, which could not have got into the valley of the
Alleghany before the Glacial period, and could only have reached their
present position by being brought down the Alleghany River upon floating
ice, or by the ordinary movement of gravel along the margin of a river.
Thus this terrace, while corresponding closely with the elevation of
those on the Monongahela River, is directly connected with the Glacial
period, and furnishes a twofold argument for our theory.
A still stronger case occurs at Beech Flats, at the head of Ohio
Brush Creek, in the northwest corner of Pike County, Ohio, where, at
an elevation of about 950 feet above the sea, there is an extensive
flat-topped terrace just in front of the terminal moraine. This terrace
consists of fine loam, such as is derived from the glacial streams, but
which must have been deposited in still water. The occurrence of still
water at that elevation just in front of the continental ice-sheet is
best accounted for by the supposed dam at Cincinnati. Indeed, it is
extremely difficult to account for it in any other way.
There are, however, two other methods of attempting to account for the
class of facts above cited in support of the ice-dam theory, of which the
most plausible is, that in connection with the Glacial period there was
a subsidence of the whole region to an extent of 1,100 feet.
The principal objection heretofore alleged against this supposition is
that there are not corresponding signs of still-water action at the same
level on the other side of the Alleghany Mountains. This will certainly
be fatal to the subsidence theory, if it proves true. But it is possible
that sufficient search for such marks has not yet been made on the
eastern side of the mountains.
The other theory to account for the facts is, that the terraces adduced
in proof of the Cincinnati ice-dam were left by the streams in the slow
process of lowering their beds from their former high levels. This is
the view advocated by President T. C. Chamberlin. But the freshness
of the leaves and fragments of wood, such as were noted by Professor
White at Morgantown, and the great extent of fine silt occasionally
resting upon the summits of the water-sheds, as described above, near
Clarksburg, bear strongly against it. Furthermore, to account for the
terrace described at Bellevue, which contains Canadian pebbles, President
Chamberlin is compelled to connect the deposit with his hypothetical
first Glacial epoch, and to assume that all the erosion of the Alleghany
and Monongahela Rivers, and indeed of the whole trough of the Ohio River,
took place in the interval between the "first" and the "second" Glacial
periods (for he would connect the glacial deposits upon the south side of
the river at Cincinnati with the first Glacial epoch)--all of which, it
would seem, is an unnecessary demand upon the forces of Nature, when the
facts are so easily accounted for by the simple supposition of the dam at
Cincinnati.[CJ]
[Footnote CJ: See matter discussed more at length in the lee Age, pp.
326-350, 480-500; Bulletin of the United States Geological Survey, No.
58, pp. 76-100; Popular Science Monthly, vol. xlv, pp. 184-199. _Per
contra_, Mr. Frank Leverett, in American Geologist, vol. x, pp. 18-24.]
[Illustration: Fig. 55.--Map showing the condition of things when the
ice-front had withdrawn about on hundred and twenty miles, and while
it still filled the valley of the Mohawk. The outlet was then through
the Wabash. Niagara was not yet born (Claypole). (Transactions of the
Edinburgh Geological Society.)]
We have already described[CK] the various temporary lakes and lines of
drainage caused by the direct obstruction of the northward outlets to
the basin of the Great Lakes. In connection with the map, it will be
unnecessary to do anything more here than add a list of such temporary
southern outlets from the Erie-Ontario basin.[CL] The first is at Fort
Wayne, Indiana, through a valley connecting the Maumee River basin with
that of the Wabash. The channel here is well defined, and the high-level
gravel terraces down the Wabash River are a marked characteristic of the
valley. The elevation of this col above the sea is 740 feet. Similar
temporary lines of drainage existed from the St. Mary's River to the
Great Miami, at an elevation of 942 feet; from the Sandusky River to
the Scioto, through the Tymochtee Gap, at an elevation of 912 feet;
from Black River to the Killbuck (a tributary of the Muskingum) through
the Harrisville Gap, at 911 feet; from the Cuyahoga into the Tuscarawas
Valley, through the Akron Gap, at 971 feet; from Grand River into the
Mahoning, through the Orwell Gap, 938 feet; from Cattaraugus Creek, N.
Y., into the Alleghany Valley through the Dayton Gap, about 1,300 feet;
between Conneaut Creek and Shenango River, at Summit Station, 1,141 feet;
from the Genesee River, N. Y., into the head-waters of the Canisteo, a
branch of the Susquehanna, at Portageville, 1,314 feet; from Seneca Lake
to Chemung River, at Horseheads, 879 feet; from Cayuga Lake to the valley
of Cayuga Creek, at Spencer, N. Y., 1,000 feet; from Utica, N. Y., into
the Chenango Valley at Hamilton, about 900 feet.
[Footnote CK: See pp. 92 seq., 199 _seq._]
[Footnote CL: See also accompanying map.]
[Illustration: Fig. 56.--Map illustrating a stage in the recession of the
ice in Ohio. For a section of the deposit in the bed of this lakelet, see
page 200. The gravel deposits formed at this stage along the outlet into
the Tuscarawas River are very clearly marked (Claypole). (Transactions of
the Edinburgh Geological Society.)]
Perhaps it would have been best to give this list in the reverse order,
which would be more nearly chronological, since it is clear that the
highest outlets are the oldest. We should then have to mention, after the
Fort Wayne outlet, two others at lower levels which are pretty certainly
marked by distinct beach ridges upon the south side of Lake Erie. The
first was opened when the ice had melted back from the south peninsula of
Michigan to the water-shed across from the Shiawassee and Grand Rivers,
uncovering a pass which is now 729 feet above the sea. This continued to
be the outlet of Lake Erie-Ontario until the ice had further retreated
beyond the Strait of Mackinac, when the water would fall to the level of
the old outlet from Lake Michigan into the Illinois River, which is a
little less than 600 feet, where it would remain until the final opening
of the Mohawk River in New York attracted the water in that direction,
and lowered the level to that of the pass from Lake Ontario to the Mohawk
at Rome.[CM]
[Footnote CM: Mr. Warren Upham, in the Bulletin of the Geological Society
of America, vol. ii, p. 259.]
A study of these lines of temporary drainage during the Glacial period
sheds much light upon the long lines of gravel ridges running parallel
with the shores of Lake Erie and Lake Ontario. South of Lake Erie a
series of four ridges of different elevations can be traced. In Lorain
County, Ohio, the highest of these is 220 feet above the lake; the next
160 feet; the next 118 feet; and the lower one 100 feet, which would make
them respectively 795, 755, 715, and 700 feet above tide.
These gravel ridges are evidently old beach lines, and indicate the
different levels up to which the water was held by ice-obstructions
across the various outlets of the drainage valley. The material in the
ridges is water-worn and well assorted, and in coarseness ranges from
fine sand up to pebbles several inches in diameter. The predominant
material in them is of local origin. Where the rocks over which they run
are sandstone, the material is chiefly sand, and where the outcropping
rock is shale, the ridges consist chiefly of the harder nodules of that
formation which have successfully resisted the attrition of the waves.
Ordinarily these ridges are steepest upon the side facing the lake.
According to Mr. Upham, who has driven over them with me, the Lake Erie
ridges correspond, both in general appearance and in all other important
respects, to those which he has so carefully surveyed around the shores
of the ancient Lake Agassiz in Minnesota and Manitoba, an account of
which will be given a little farther on in this chapter.
[Illustration: Fig. 57.--Section of the lake ridges near Sandusky, Ohio.]
We are not permitted, however, to assume that there have been no changes
of level since the deposition of these beaches surrounding the ancient
glacial Lake Erie-Ontario. On the contrary, there appears to have been
a considerable elevation towards the east and northeast in post-glacial
times. The highest ridge south of Lake Erie, which at Fort Wayne is about
780 feet high, is now about 795 feet in Lorain County. The second of the
ridges above-mentioned, which is about 740 feet above tide at Cleveland,
Ohio, rises to 870 feet where the last traces of it have been discovered
at Hamburg, N. Y. The third ridge, which is 673 feet at Cleveland, has
risen to the height of 860 feet at Crittenden, about one hundred miles to
the east of Buffalo, N. Y.
A similar eastern increase of elevation is discoverable in the main ridge
surrounding Lake Ontario. What Professor Spencer calls the Iroquois
beach, which is 363 feet above tide at Hamilton, Ontario, has risen to a
height of 484 feet near Syracuse, N. Y.; while farther to the northeast,
in the vicinity of Watertown, it is upwards of 800 feet above tide.
There is also a similar northward increase of elevation in the beaches
surrounding the higher lands of Ontario eastward of Lake Huron and
Georgian Bay.
All this indicates that at the close of the Glacial period there
was a subsidence of several hundred feet in the area of greatest
ice-accumulation lying to the east and north of the Great Lake region.
The formation of these ridges occurred during that period of subsidence.
The re-elevation which followed the disappearance of the ice of course
carried with it these ridges, and brought them to their present
position.[CN]
[Footnote CN: See Spencer, in Bulletin of the Geological Society of
America, vol. ii, pp. 465-476.]
In returning to consider more particularly the remarkable gorge joining
the Minnesota with the Red River of the North, we are brought to the
largest of the glacial lakes of this class, and to the typical place in
America in which to study the temporary changes of drainage produced by
the ice itself daring the periods both of its advance and of its retreat.
[Illustration: Fig. 58.--Map showing the stages of recession of the ice
in Minnesota as described in the text (Upham).]
By turning to our general map of the glaciated region of the United
States,[CO] one can readily see the relation of the valley between
Lake Traverse and Big Stone Lake to an area marked as the bed of what
is called Lake Agassiz. During the Glacial period Brown's Valley, the
depression joining these two lakes, was the outlet of an immense body of
water to the north, whose natural drainage was towards Hudson Bay or the
Arctic Ocean, but which was cut off, by the advancing ice, from access to
the ocean-level in that direction, and was compelled to seek an exit to
the south.
[Footnote CO: See page 66.]
Thus for a long period the present Minnesota River Valley was occupied
by a stream of enormous dimensions, and this accounts for the great
size of the trough--the present Minnesota being but an insignificant
stream winding about in this deserted channel of the old "Father of
Waters," and having as much room as a child of tender age would have in
his parent's cast-off garments. This glacial stream has been fittingly
named River Warren, after General Warren, who first suggested and proved
its existence, and so we have designated it on the accompanying map of
Minnesota.
Lake Traverse is fifteen miles long, and the water is nowhere more than
twenty feet deep. Big Stone Lake is twenty-six miles long, and of about
the same depth. Brown's Valley, which connects the two, is five miles
long, and the lakes are so nearly on a level that during floods the water
from Lake Traverse sometimes overflows and runs to the south as well as
to the north.
[Illustration: Fig. 59.--Glacial terrace near the boundary of the
glaciated area, on Raccoon Creek, a tributary of the Licking River, in
Granville, Licking County, Ohio. Height about fifty feet.]
The trough occupied by these lakes and valley is from one mile to one
mile and a half in width and about 120 feet in depth. If we had been
permitted to stand upon the bluffs overlooking it during the latter
part of the Glacial period, we should have seen the whole drainage of
the north passing by our feet on its way to the Gulf of Mexico. As lie
follows down the valley of the Minnesota River, the observant traveller,
even now, cannot fail to see in the numerous well-preserved gravel
terraces the high-water marks of that stream when flooded with the joint
product of the annual precipitation over the vast area to the north, and
of the still more enormous quantities set free by the melting of the
western part of the great Laurentide Glacier.
Numerous other deserted water-ways in the northwestern part of the
valley of the Mississippi have been brought to light in the more
recent geological surveys, both in the United States and in Canada.
During a considerable portion of the Glacial period the Saskatchewan,
the Assiniboine, the Pembina, and the Cheyenne Rivers, whose present
drainage is into the Red River of the North, were all turned to the
south, and their temporary channels can be distinctly traced by deserted
water-courses marked by lines of gravel deposits.[CP]
[Footnote CP: For further particulars, see Ice Age, pp. 293 _et seq._]
In Dakota, Professor J. E. Todd has discovered large deserted channels
on the southwestern border of the glaciated region near the Missouri
River, where evidently streams must have flowed for a long distance in
ice-channels when the ice still continued to occupy the valley of the
James River. From these channels of ice in which the water was held up
to the level of the Missouri Coteau the water debouched directly into
channels with sides and bottom of earthy material, which still show every
mark of their former occupation by great streams.[CQ]
[Footnote CQ: For particulars, see Ice Age, p. 292.]
In Minnesota, also, there is abundant evidence that while the
northeastern part of the valley from Mankato to St. Paul was occupied by
ice, the drainage was temporarily turned directly southward across the
country through Union Slough and Blue Earth River into the head-waters of
the Des Moines River in Iowa.
_Ancient River Terraces._
The interest of the whole inquiry respecting the relation of man to
the Glacial period in America concentrates upon these temporary lines
of southern drainage. Wherever they existed, the swollen floods of the
Glacial period have left their permanent marks in the deposition of
extensive gravel terraces. The material thus distributed is derived
largely from the glacial deposits through which they run and out of which
they emerge. While the height of the terraces depended upon various
conditions which must be studied in detail, in general it may be said
that it corresponds pretty closely with the extent of the area whose
drainage was turned through the channel during the prevalence of the ice.
The height of the terraces and the coarseness of the material seem also
to have been somewhat dependent upon the proximity of their valleys to
the areas of most vigorous ice-action, and this, in turn, seems to lie in
the rear of the moraines which President Chamberlin has attributed to the
second Glacial epoch. Southward from this belt of moraines the terraces
uniformly and gradually diminish both in height and in the coarseness of
their gravel, until they finally disappear in the present flood-plain of
the Mississippi River.
[Illustration: Fig. 60.--Ideal section across a river-bed in drift
region: _b b b_, old river-bed; _R_, the present river; _t t_, upper or
older terraces; _t' t'_, lower terraces.]
An interesting illustration of this principle is to be observed in the
continuous valley of the Alleghany and Ohio Rivers. The trough of this
valley was reached by the continental glacier at only a few points,
the ice barely touching it at Salamanca, N. Y., Franklin, Pa., and
Cincinnati, Ohio. But throughout its whole length the ice-front was
approximately parallel to the valley, and occupied the head-waters of
nearly all its tributaries. Now, wherever tributaries which could be fed
by glacial floods, enter the trough of the main stream, they brought down
an excessive amount of gravel, and greatly increased the size of the
terrace in the trough itself, and from the mouth of each such tributary
to that of the next one below there is a gradual decrease in the height
of the terrace and in the coarseness of the material.
This law is illustrated with special clearness in Pennsylvania between
Franklin and Beaver. Franklin is upon the Alleghany River, at the last
point where it was reached directly by the ice. Below this point no
tributary reaches it from the glaciated region, and none such reaches the
Ohio after its junction with the Alleghany until we come to the mouth of
Beaver Creek, about twenty-five miles below Pittsburg.
But at this point the Ohio is joined by a line of drainage which emerges
from the glaciated area only ten or twelve miles to the north, and whose
branches occupy an exceptionally large glaciated area. Accordingly, there
is at Beaver a remarkable increase in the size of the glacial terrace
on the Ohio. In the angle down-stream between the Beaver and the Ohio
there is an enormous accumulation of granitic pebbles, many of them
almost large enough to be called boulders, forming the delta terrace,
upon which the city is built and rising to a height of 135 feet above
the low-water mark in the Ohio. In striking confirmation of our theory,
also, the terrace in the Ohio Valley upon the upper side of Beaver Creek
is composed of fine material, largely derived from local rocks and
containing but few granitic pebbles.
From the mouth of Beaver Creek, down the Ohio, the terrace is constant
(sometimes upon one side of the river and sometimes upon the other),
but, according to rule, the material of which it is composed gradually
grows finer, and the elevation of the terrace decreases. According to
rule, also, there is a notable increase in the height of the terrace
below each affluent which enters the river from the glaciated region.
This is specially noticeable below Marietta, at the mouth of the
Muskingum, whose head-waters drain an extensive portion of the glaciated
area. From the mouth of the Little Beaver to this point the tributaries
of the Ohio are all small, and none of them rise within the glacial
limit. Hence they could contribute nothing of the granitic material which
enters so largely into the formation of the river terrace; but below the
mouth of the Muskingum the terrace suddenly ascends to a height of nearly
one hundred feet above low-water mark.
Again, at the mouth of the Scioto at Portsmouth, there is a marked
increase in the size of the terrace, which is readily accounted for by
the floods which came down the Scioto Valley from the glaciated region.
The next marked increase is at Cincinnati, just below the mouth of the
Little Miami, whose whole course lay in the glaciated region, and whose
margin is lined by very pronounced terraces. At Cincinnati the upper
terrace upon which the city is built is 120 feet above the flood-plain.
Twenty-five miles farther down the river, near Lawrenceburg, these
glacial terraces are even more extensive, the valley being there between
three and four miles wide, and being nearly filled with gravel deposits
to a height of 112 feet above the flood-plain. Below this point the
terraces gradually diminish in height, and the material becomes finer
and more water-worn, until it merges at last in the flood-plain of the
Mississippi. The course of the Wabash River is too long to permit it to
add materially to the size of the terraces which characterise the broader
valley of the Ohio below the Illinois line.
It is in terraces such as these just described that we find the imbedded
relics of man which definitely connect him with the great Ice age. These
have now been found in the glacial terraces of the Delaware River at
Trenton, N. J.; in similar terraces in the valley of the Tuscarawas River
at New Comerstown, and in the valley of the Little Miami at Loveland and
Madisonville, in Ohio; on the East Fork of White River, at Medora, Ind.;
and still, again, at Little Falls, in the trough of the Mississippi, some
distance above Minneapolis, Minn.
I append a list of the points at which various streams from the Atlantic
Ocean to the Mississippi River emerge from the glacial boundary, and
below which the terraces are specially prominent. Of course, with the
retreat of the ice, the formation of the terraces continued northward
in the glaciated area to a greater or less distance, according to the
extent of the valley or to the length of time during which the drainage
was temporarily turned into it. These points of emergence are: In the
Delaware Valley, at Belvidere, N. J.; in the Susquehanna, at Beach Haven,
Pa.; in the Conewango, at Ackley, Warren County; in Oil Creek, above
Titusville: in French Creek, a little above Franklin; in Beaver Creek,
at Chewtown, Lawrence County; on the Middle Fork of Little Beaver, near
New Lisbon, Ohio; on the east branch of Sandy Creek, at East Rochester,
Columbiana County; on the Nimishillin, at Canton, Stark County; on the
Tuscarawas, at Bolivar; on Sugar Creek, at Beech City; on the Killbuck,
at Millersburg, Holmes County; on the Mohican, near the northeast corner
of Knox County; on the Licking River, at Newark; on Jonathan Creek, Perry
County; on the Hocking, at Lancaster; on the Scioto, at Hopetown, just
above Chillicothe; on Paint Creek, and its various tributaries, between
Chillicothe and Bainbridge; and on the Wabash, above New Harmony, Ind.;
to which may be added the Ohio River itself, at its junction with the
Miami, near Lawrenceburg.
Another class of terraces having most interesting connection with the
Glacial period is found in the arid basins west of the Rocky Mountains.
Over wide areas in Utah and Nevada the evaporation now just balances
the precipitation, and all the streams disappear in shallow bodies of
salt water of moderate dimensions, of which Great Salt Lake in Utah, and
Mono, Pyramid, and North Carson Lakes in Nevada, are the most familiar
examples. These occupy the lowest sinks of enclosed basins of great depth.
But there is abundant evidence that in consequence of the increased
precipitation and diminished evaporation of the Glacial period one of
these basins was filled to the brim and the other to a depth of several
hundred feet. These former enlargements have been named after the first
explorers of the region, Captains Lahontan and Bonneville, and are shown
on the accompanying sketch map by the shading surrounding the existing
lakes.
Lake Lahontan has been carefully studied by Mr. I. C. Russell, and has
been found to extend from the boundary of Oregon to latitude 38° 30'
south, a distance of two hundred and sixty miles. The Central Pacific
Railroad runs through its dried-up bed from Golconda to Wadsworth, a
distance of one hundred and sixty-five miles. The terraces of the former
lake are distinctly traceable at a height of 700 feet above the present
level of Lake Mono.
Lake Bonneville, whose present representative is Great Salt Lake, is the
subject of a recent monograph by Mr. G. K. Gilbert, from which it appears
that this ancient body of water occupied 19,750 square miles--an area
about ten times that of the present lake. At the time of its maximum
extension its depth was 1,000 feet, while Great Salt Lake ranges only
from fifteen to fifty feet in depth.
The pass through which the discharge finally took place is at Red Rock,
on the Utah and Northern Railroad, at the head of Cache Valley on the
south and the lower part of Marsh Creek Valley on the north. During
the long period preceding and accompanying the gradual rise of water in
the Utah basin to the level of the highest terrace, Marsh Creek (the
upper portion of which comes from the mountains on the east and turns
at right angles) had been at work depositing a delta of loose material
in the col which separates the two valleys. This deposit rested upon
a stratum of limestone at the bottom of the pass, and covered it with
sand, clay, and gravel to a depth of 375 feet. Thus, when the water was
approaching its upper level, the only barrier to prevent its escape was
this unstable accumulation of loose material upon top of the rock. It
would have required, therefore, no prophet's eye to predict that the way
was preparing for a tremendous _débâcle_.
[Illustration: Fig. 61.--Map of the Quaternary Lakes. Bonneville and
Lahontan (after Gilbert and Russell).]
The critical point at length was reached. After remaining nearly at
the elevation of the pass for a considerable period, during which the
1,000-foot shore-line was formed, the crisis came when the water began
to flow northward towards Snake River. Once begun in such loose material,
the channel rapidly enlarged until soon a stream equal to Niagara, and
at times probably much larger, was pouring northward through the valley
heretofore occupied by the insignificant rivulets of Marsh Creek and
the Port Neuf. It is impossible to tell how rapidly the loose barrier
wore away, but there is abundant evidence in the valley below that not
only the present channel of the lower part of Marsh Creek, but the whole
bottom of the valley for a mile or more in width, was for a considerable
time covered by a rapid stream from ten to twenty feet in depth, and
descending at the rate of thirteen feet to the mile.
The continuance of this flood was dependent upon the amount of water to
be discharged, which, as we have seen, was that contained in an area of
20,000 square miles, with a depth of 375 feet. A stream of the size of
Niagara would occupy about twenty-five years in the discharge of such a
mass, and this may fairly be taken as a measure of the time through which
it lasted. When the loose material lying above the strata of limestone
in Red Rock Pass had been washed away, the lake then continued at that
level for an indefinite period, with an overflow regulated by the annual
precipitation of the drainage basin. This stage of the lake, during which
it occupied 13,000 square miles and was 625 feet above its present level,
is also marked by an extensive and persistent shore-line all around
the basin. But, finally, the balance again turned when the evaporation
exceeded the precipitation, and the vast body of water has since dwindled
to its present insignificant dimensions.
My own interest in this discovery of Mr. Gilbert is enhanced by the
explanation it gives of a phenomenon in the Snake River Valley which I
was unable to solve when on the ground in 1890. The present railroad
town of Pocatello is situated just where this flood emerged from the
narrower valley of Marsh Creek and the Port Neuf, and spread itself out
upon the broad plain of the Snake River basin. The southern edge of the
plain upon which the city is built is a vast boulder-bed covered with a
thin stratum of sand and gravel. Everywhere, in sinking wells and digging
ditches on the vacant lots and in the streets of the city, water-worn
boulders of a great variety of material and sometimes three or four feet
in diameter are encountered. I was debarred from regarding this as a
terminal moraine, both by the water-worn character of the boulders and by
the absence of any sign of ice-action in the surrounding mountains, and I
was equally debarred from attributing it to any ordinary stream of water,
both by the size of the boulders and the fact that for a mile or more up
the Port Neuf Valley there is an intervale, forty or fifty feet below the
surface at Pocatello, and occupying the whole width of the valley, in
which there is only gravel and fine sand, through which the present Port
Neuf pursues a meandering course. The upper end of this short intervale
is bounded by the terminus of a basaltic stream which had flowed down the
valley and filled it to a considerable depth, but had subsequently been
much eroded by violent water-action.
In the light of Mr. Gilbert's discoveries, however, everything is clear.
The tremendous _débâcle_ which he has brought within the range of
scientific vision would naturally produce just the condition of things
which is so puzzling at Pocatello. Coming down through the restricted
channel with sufficient force to roll along boulders of great size and
to clear them all out from the upper portion of the valley, the torrent
would naturally deposit them where the current was first checked, a mile
below the lava cliffs. The plunge of the water over these cliffs would
keep a short space below clear from boulders, and the more moderate
stream of subsequent times would fill in the depression with the sand
and gravel now occupying it.
What other effects of this remarkable outburst may be traced farther down
in the Snake River Valley I cannot say, but it will be surprising if
they do not come to light and help to solve some of the many geological
problems yet awaiting us in this interesting region.
It should have been said that during the formation of the 625-foot, or
so-called Provo shore-line, glaciers descended from the cañons on the
west flank of the Wahsatch Mountains, and left terminal moraines to mark
the coincidence of the Glacial period with that stage of the enlargement
of the lake. Evidences of a similar coincidence are to be found on the
high-level terraces surrounding Lake Mono, to which glaciers formerly
descended from the western flanks of the Sierra Nevada.
The ancient shore-lines surrounding Lakes Bonneville and Lahontan bear
evidence also of various other episodes in the Glacial period. Evidently
there were two periods of marked increase in the size of the lakes, with
an arid period intervening. During the first rise the level of Bonneville
attained to within ninety feet of the second, and numerous beaches were
formed, and a large amount of yellow clay deposited. Then it seems
to have been wholly evaporated, while its soluble mineral matter was
precipitated, and so mingled with silt that it did not readily redissolve
during the second great rise of water. Partly on this account, and partly
through the influence of the outlet into the Snake River, the lake was
nearly fresh during its second enlargement.
_European Facts._
In Chapter VI it came in place to mention many of the facts connected
with the influence of the Glacial period upon the drainage systems
of Europe. We there discussed briefly the probable influence of the
ice-obstructions that extended across the mouths of the Dwina, the
Vistula, the Oder, the Elbe, the Weser, and the Rhine. The drainage of
the obstructed rivers in Russia was perhaps turned southward into the
Caspian and Black Seas, and then assisted in forming the fertile soil of
the plains in the southern part of that empire.
The obstructed drainage of the German rivers was probably turned westward
in front of the ice through the Straits of Dover or across the southern
part of England. This was during the climax of the Glacial period; but
later, according to Dawkins, during a period in which the land of the
British Isles stood about 600 feet above its present level, the streams
of the eastern coast--namely, "the Thames, Medway, Humber, Tyne, and
others, joined the Rhine, the Weser, and the Elbe, to form a river
flowing through the valley of the ocean. In like manner, the rivers of
the south of England and of the north of France formed a great river
flowing past the Channel Islands due west into the Atlantic, and the
Severn united with the rivers of the south of Ireland; while those to the
east of Ireland joined the Dee, Mersey Ribble, and Lune, as well as those
of western Scotland, ultimately reaching the Atlantic to the west of the
Hebrides. The water-shed between the valleys of the British Channel and
the North Sea is represented by a ridge passing due south from Folkestone
to Dieppe, and that between the drainage area and the Severn and its
tributaries on the one hand, and of the Irish Channel on the other, by a
ridge from Holyhead westward to Dublin.
"This tract of low, undulating land which surrounded Britain and Ireland
on every side consisted not merely of rich hill, valley, and plain, but
also of marsh-land studded with lakes, like the meres of Norfolk, now
indicated by the deeper soundings. These lakes were very numerous to the
south of the Isle of Wight and off the coast of Norfolk and Suffolk."[CR]
[Footnote CR: Early Man in Britain, p. 151.]
The evidence first regarded by scientific men to be demonstrative of the
formation of extensive lakes during the Glacial period by the direct
influence of ice-dams exists in the Parallel Roads of Glen Roy in
Scotland.
[Illustration: Fig. 62.--Parallel roads of Glen Roy.]
According to the description of Sir Charles Lyell, "Glen Roy is situated
in the western Highlands, about ten miles north of Fort William, near the
western end of the great glen of Scotland, or Caledonian Canal, and near
the foot of the highest of the Grampians, Ben Nevis. Throughout nearly
its whole length, a distance of more than ten miles, three parallel
roads or shelves are traced along the steep sides of the mountains, each
maintaining a perfect horizontality, and continuing at exactly the same
level on the opposite sides of the glen. Seen at a distance they appear
like ledges, or roads, cut artificially out of the sides of the hills;
but when we are upon them, we can scarcely recognize their existence, so
uneven is their surface and so covered with boulders. They are from ten
to sixty feet broad, and merely differ from the side of the mountain by
being somewhat less steep.
"On closer inspection, we find that these terraces are stratified in
the ordinary manner of alluvial or littoral deposits, as may be seen at
those points where ravines have been excavated by torrents. The parallel
shelves, therefore, have not been caused by denudation, but by the
deposition of detritus, precisely similar to that which is dispersed in
smaller quantities over the declivities of the hills above. These hills
consist of clay-slate, mica-schist, and granite, which rocks have been
worn away and laid bare at a few points immediately above the parallel
roads. The lowest of these roads is about 850 feet above the level of the
sea, and the next about 212 feet higher, and the third 82 feet above the
second. There is a fourth shelf, which occurs only in a contiguous valley
called Glen Gluoy, which is twelve feet above the highest of all the Glen
Roy roads, and consequently about 1,156 feet above the level of the sea.
One only, the lowest of the three roads of Glen Roy, is continued through
Glen Spean, a large valley with which Glen Roy unites. As the shelves,
having no slope towards the sea like ordinary river terraces, are always
at the same absolute height, they become continually more elevated above
the river in proportion as we descend each valley; and they at length
terminate very abruptly, without any obvious cause, or any change either
in the shape of the ground or in the composition or hardness of the
rocks."[CS]
[Footnote CS: Antiquity of Man, pp. 252, 253.]
Early in his career Charles Darwin studied these ancient beaches, and
ascribed them to the action of the sea during a period of continental
subsidence. In this view he was supported by the majority of geologists
until the region was visited by Agassiz, who saw at once the true
explanation. If these were really sea-beaches, similar deposits should
be found at the same elevation on other mountains than those surrounding
Glen Roy. Their absence elsewhere points, therefore, to some local cause,
which was readily suggested to the trained eye of one like Agassiz, then
fresh from the study of Alpine glaciers, who saw that these beaches were
formed upon the margin of temporary lakes, held back during the Glacial
period (as the Merjelen See now is) by a glacier which came out of one
glen and projected itself directly across the course of another, and thus
obstructed its drainage. The glacier of Glen Spean had pushed itself
across Glen Roy, as the great Aletsch Glacier in Switzerland now pushes
itself across the little valley behind the Eggishorn.