ANCIENT GLACIERS IN THE WESTERN HEMISPHERE.
_New England._
In North America all the indubitable signs of glacial action are found
over the entire area of New England, the southern coast being bordered
by a double line of terminal moraines. The outermost of these appears in
Nantucket, Martha's Vineyard, No Man's Land, Block Island, and through
the entire length of Long Island--from Montauk Point, through the centre
of the island, to Brooklyn, N. Y., and thence across Staten Island to
Perth Amboy in New Jersey. The interior line is nearly parallel with the
outer, and, beginning at the east end of Cape Cod, runs in a westerly
direction to Falmouth, and thence southwesterly through Wood's Holl, and
the Elizabeth Islands--these being, indeed, but the unsubmerged portions
of the moraine. On the mainland this interior line reappears near Point
Judith, on the south shore of Rhode Island, and, running slightly south
of west, serves to give character to the scenery at Watch Hill, and
thence crops out in the Sound as Fisher and Plum Islands, and farther
west forms the northern shore of Long Island to Port Jefferson.
[Illustration: MAP SHOWING
THE
GLACIAL GEOLOGY
OF THE
UNITED STATES.]
In these accumulations bordering the southern shore of New England, the
characteristic marks of glacial action can readily be detected even by
the casual observer, and prolonged examination will amply confirm the
first impression. The material of which they are composed is, for the
most part, foreign to the localities, and can be traced to outcrops
of rock at the north. The boulders scattered over the surface of Long
Island, for example, consist largely of granite, gneiss, hornblende, mica
slate, and red sandstone, which are easily recognised as fragments from
well-known quarries in Connecticut, Rhode Island, and Massachusetts; yet
they have been transported bodily across Long Island Sound, and deposited
in a heterogeneous mass through the entire length of the island. Not
only do they lie upon the surface, but, in digging into the lines of
hills which constitute the backbone of Long Island, these transported
boulders are found often to make up a large part of the accumulation.
Almost any of the railroad excavations in the city of Brooklyn present
an interesting object-lesson respecting the composition of a terminal
moraine.
All these things are true also of the lines of moraine farther east,
as just described. Professor Shaler has traced to its source a belt
of boulders occurring extensively over southern Rhode Island, and
found that they have spread out pretty evenly over a triangular area
to the southward, in accordance with the natural course to be pursued
by an ice-movement. Nearly all of Plymouth County, in southeastern
Massachusetts, is composed of foreign material, much of which can be
traced to the hills and mountains to the north. Even Plymouth Rock is a
boulder from the direction of Boston, and the "rock-bound" shores upon
which the Pilgrims are poetically conceived to have landed are known, in
scientific prose, as piles of glacial rubbish dumped into the edge of the
sea by the great continental ice-sheet.
The whole area of southeastern Massachusetts is dotted with conical
knolls of sand, gravel, and boulders, separated by circular masses of
peat or ponds of water, whose origin and arrangement can be accounted
for only by the peculiar agency of a decaying ice-front. Indeed, this
whole line of moraines, from the end of Cape Cod to Brooklyn, N. Y.,
consists of a reticulated network of ridges and knolls, so deposited by
the ice as to form innumerable kettle-holes which are filled with water
where other conditions are favourable. Those which are dry are so because
of their elevation above the general level, and of the looseness of the
surrounding soil; while many have been filled with a growth of peat, so
that their original character as lakelets is disguised.
As already described, these depressions, so characteristic of the
glaciated region, are, in the majority of cases, supposed to have
originated by the deposition of a great quantity of earthy material
around and upon the masses of ice belonging to the receding front of
the glacier, so that, when at length the ice melted away, a permanent
depression in the soil was left, without any outlet.
To some extent, however, the kettle-holes may have been formed by the
irregular deposition of streams of water whose courses have crossed each
other, or where eddies of considerable force have been produced in any
way. The ordinary formation of kettle-holes can be observed in progress
on the foot of almost any glacier, or, indeed, on a small scale, during
the melting away of almost any winter's snow. Where, from any cause, a
stratum of dirt has accumulated upon a mass of compact snow or ice, it
will be found to settle down in an irregular manner; furrows will be
formed in various directions by currents of water, so that the melting
will proceed irregularly, and produce upon a miniature scale exactly what
I have seen on a large scale over whole square miles of the decaying foot
of the great Muir Glacier in Alaska. The effects of similar causes and
conditions we can see on a most enormous scale in the ten thousand lakes
and ponds and peat-bogs of the whole glaciated area both in North America
and in Europe.
In addition to these two lines of evidence of glacial action in New
England, we should mention also the innumerable glacial grooves and
scratches upon the rocks which can be found on almost any freshly
uncovered surface. In New England the direction of these grooves is
ordinarily a little east of south. Upon the east coast of Massachusetts
and New Hampshire the scratches trend much more to the east than they
do over most of the interior. This is as it should be on the glacial
theory, since the ice would naturally move outwards in the line of least
resistance, which would, of course, be towards the open sea wherever
that is near. In the interior of New England the scratches upon the
rocks indicate a more southerly movement in the Connecticut Valley than
upon the mountains in the western part of Massachusetts. This also is as
it should be upon the glacial theory. The scratches upon the mountains
were made when the ice was at its greatest depth and when it moved
over the country in comparative disregard of minor irregularities of
surface, while in the valleys, at least in the later portion of the Ice
age, the movement would be obstructed except in one direction. In the
interpretation of the glacial grooves and scratches it should be borne in
mind that they often represent the work done during the closing stages
of the period. Just as the last shove of the carpenter's plane removes
the marks of the previous work, so the last rasping of a glacial movement
wears away the surfaces which have been previously polished and striated.
In various places of New England it is interesting as well as instructive
to trace the direction of the ice-movement by the distribution of
boulders. My own attention was early attracted to numerous fragments
of gneiss in eastern Massachusetts containing beautiful crystals
of feldspar, which proved to be peculiar to the region of Lake
Winnepesaukee, a hundred miles to the north, and to a narrow belt
stretching thence to the southwestward. In ascending almost any of the
lower summits of the White Mountains one's attention can scarcely
fail of being directed to the difference between the material of which
the mountains are composed and that of the numerous boulders which lie
scattered over the surface. The local geologist readily recognises these
boulders as pilgrims that have wandered far from their homes to the
northward.
Trains of boulders, such as those already described in Rhode Island, can
frequently be traced to some prominent outcrop of the rock in a hill or
mountain-peak from which they have been derived. One of the earliest
of these to attract attention occurs in the towns of Richmond, Lenox,
and Stockbridge, in the western part of Massachusetts. Here a belt of
peculiar boulders about four hundred feet wide is found to originate in
the town of Lebanon, N. Y., and to run continuously to the southeast
for a distance of nine miles. West of Fry's Hill, where the outcrop
occurs, no boulders of this variety of rock are to be found, while to
the southeast the boulders gradually diminish in size as their distance
from the outcrop increases. Near the outcrop boulders of thirty feet in
diameter occur, while nine miles away two feet is the largest diameter
observed.
Sir Charles Lyell endeavoured to explain this train of boulders by the
action of icebergs during a period of submergence--supposing that, as
icebergs floated past or away from this hill in Lebanon, N. Y., they
were the means of the regular distribution described. It is needless to
repeat the difficulties arising in connection with such a theory, since
now both by observation and experiment we have become more familiar
with the movement of glacial ice. What we have already said about the
transportation of boulders over Switzerland by the Alpine glaciers, and
what is open to observation at the present time upon the large glaciers
of Alaska, closely agree with the facts concerning this Richmond train of
boulders, and we have no occasion to look further for a cause.
Indeed, trains of boulders ought to appear almost everywhere over
the glaciated area; and so they do where all the circumstances are
favourable. But, readily to identify the train, requires that to furnish
the boulders there should be in the line of the ice-movement a projecting
mass of rock hard enough to offer considerable resistance to the abrading
agency of the ice and characteristic enough in its composition to be
readily recognised. Ship Rock, in Peabody, Mass., weighing about eleven
hundred tons, and Mohegan Rock, in Montville, Conn., weighing about ten
thousand tons, have ordinarily been pointed to as boulders illustrating
the power of ice-action. Their glacial character, however, has been
challenged from the fact that the variety of granite to which they
belong occurs in the neighbourhood, and indeed constitutes the bed-rock
upon which they rest.[AV] Some would therefore consider them, like some
of which we have already spoken, to be boulders which have originated
through the disintegration of great masses of rock, of which these were
harder nuclei that have longer resisted the ravages of the tooth of time.
It must be admitted that possibly this explanation is correct; but it
is scarcely probable that, in a region where there are so many other
evidences of glacial action, these boulders could have remained immovable
in presence of the onward progress of the ice-current that certainly
passed over them.
[Footnote AV: Popular Science Monthly, vol. xxxvii, pp. 196-201.]
However, as already seen, we are not left to doubt as to the movement
of some boulders of great size. That which now claims the reputation
of being the largest in New England is in Madison, N. H., and measures
thirty by forty by seventy-five feet. This can be traced to ledges of
Conway granite, about two miles away.[AW] Many boulders in the vicinity
of New Haven, Conn., can be identified, as from well-known trap-dykes,
sixteen miles or more to the north. The so-called Judge's Cave, on West
Rock, 365 feet above the adjoining valley and weighing a thousand tons,
is one of these. Professor Edward Orton[AX] describes a mass of Clinton
limestone near Freeport, Warren County, Ohio, as covering an area of
three-fourths of an acre, and as sixteen feet in thickness. It overlies
glacial clays and gravels, and must have been transported bodily from the
elevations containing this rock several miles to the northwest.
[Footnote AW: See W. 0. Crosby's paper in Appalachia, vol. vi, pp. 59-70.]
[Footnote AX: Geological Survey of Ohio, vol. iii, p. 385,]
[Illustration: Fig. 26.--Mohegan Rock.]
Portions of New England present the best illustrations anywhere afforded
in America of what are called "drumlins." These are "lenticular-shaped"
hills, composed of till, and containing, interspersed through their mass,
numerous scratched stones of all sizes. They vary in length from a few
hundred feet to a mile, and are usually from half to two-thirds as wide
as they are long. In height they vary from twenty-five to two hundred
feet.
But, according to the description of Mr. Upham, whatever may be their
size and height, they are singularly alike in outline and form, usually
having steep sides, with gently sloping, rounded tops, and presenting a
very smooth and regular contour. From this resemblance in shape to an
elliptical convex lens, Professor Hitchcock has called them lenticular
hills to distinguish these deposits of till from the broadly flattened or
undulating sheets which are common throughout New England.
[Illustration: Fig. 27.--Drumlins in Goffstown, N. H. (Hitchcock).]
The trend, or direction of the longer axis, of these lenticular hills is
nearly the same for all of them comprised within any limited area, and is
approximately like the course of the striæ or glacial furrows marked upon
the neighbouring ledges. In eastern Massachusetts and New Hampshire,
within twenty-five miles of the coast, it is quite uniformly to the
southeast, or east-southeast. Farther inland, in both of these States,
it is generally from north to south, or a few degrees east of south;
while in the valley of the Connecticut River it is frequently a little to
the west of south. In New Hampshire, besides its accumulation in these
hills, the till is frequently amassed in slopes of similar lenticular
form. These have their position almost invariably upon either the south
or north side of the ledgy hills against which they rest, showing a
considerable deflection towards the southeast and northwest in the east
part of the State. It cannot be doubted that the trend of the lenticular
hills, and the direction taken by these slopes, have been determined
by the glacial current, which produced the striæ with which they are
parallel.[AY]
[Footnote AY: Proceedings of the Boston Society of Natural History, vol.
xx, pp. 224, 225.]
Drumlins are abundant in the vicinity of Boston, and constitute nearly
all the islands in Boston Harbour. On the mainland, Beacon Hill, Bunker
Hill, Green Hill, Powderhorn Hill, Tufts College Hill, Winter Hill, Mount
Ida, Corey Hill, Parker Hill, Wollaston Heights, Prospect Hill, and
Telegraph Hill are specimens.
The northeastern corner of Massachusetts and the southeastern corner
of New Hampshire are largely covered with these peculiar-shaped
glacial deposits, while they are numerous as far west as Fitchburg,
in Massachusetts, and Ware, N. H., and in the northeastern part of
Connecticut. A little later, also, we shall refer to an interesting line
of them in central New York. Elsewhere in America, except in a portion of
Wisconsin, they rarely occur in such fine development as in New England.
In Europe they are best developed in portions of Ireland.
One's first impression in examining an exposed section of a drumlin
would lead him to think that the mass was entirely unstratified; but
closer examination shows that there is a coarse stratification, but
evidently not produced by water-action. The accumulation has probably
taken place gradually by successive deposits underneath the glacier
itself. Professor William M. Davis has suggested a plausible explanation
which we will briefly state.
[Illustration: Fig. 28.--Drumlins in the vicinity of Boston (Davis).]
The frequency with which drumlins are found to rest upon a mass of
projecting rock, the general co-ordination of the direction of their
axes with the direction of the scratches upon the underlying rock, and
the abundance of scratched stones in them, all support the theory that
drumlins are formed underneath the ice-sheet, somewhat in the way that
islands and bars of silt are formed in the delta of a great river. The
movement of ice seems to have been concentrated in pretty definite
lines, often determined by the contour of the bottom, leaving a slacker
movement in intervening areas, which were evidently protected in some
cases by projecting masses of rock. In these areas of slower movement
there was naturally an accumulation at the same time that there was
vigorous erosion in the lines of more rapid movement.
There was doubtless a continual transfer of material from the end of the
drumlin which abutted against the moving mass of ice to the lower end,
as there is in the formation of an island in a river. If time enough had
elapsed, the whole accumulation would have been levelled by the glacier
and spread over the broader area where the more rapid lines of movement
became confluent, and where the differential motion was less marked.
Drumlins are thus characteristic of areas in the glaciated region whose
floor was originally only moderately irregular, and where there was an
excessive amount of ground-moraine to be transported, and where the
movement did not continue indefinitely. It has been suggested, also,
that some of the long belts of territory in New England and central New
York covered by drumlins may represent old terminal moraines which were
subsequently surmounted by a readvance of the ice, and partially wrought
over into their present shape.
It is in New England, also, that kames are to be found in better
development than anywhere else in America. These interesting remnants of
the Glacial age are clearly described by Mr. James Geikie. His account
will serve as well for New England as for Scotland.
The sands and gravels have a tendency to shape themselves into mounds
and winding ridges, which give a hummocky and rapidly undulating outline
to the ground. Indeed, so characteristic is this appearance, that by it
alone we are often able to mark out the boundaries of the deposits with
as much precision as we could were all the vegetation and soil stripped
away and the various subsoils laid bare. Occasionally, ridges may be
tracked continuously for several miles, running like great artificial
ramparts across the country. These vary in breadth and height, some of
the more conspicuous ones being upward of four or five hundred feet
broad at the base, and sloping upward at an angle of twenty-five or even
thirty-five degrees, to a height of sixty feet and more above the general
surface of the ground. It is most common, however, to find mounds and
ridges confusedly intermingled, crossing and recrossing each other at all
angles, so as to enclose deep hollows and pits between. Seen from some
dominant point, such an assemblage of kames, as they are called, looks
like a tumbled sea--the ground now swelling into long undulations, now
rising suddenly into beautiful peaks and cones, and anon curving up in
sharp ridges that often wheel suddenly round so as to enclose a lakelet
of bright clear water.[AZ]
[Footnote AZ: The Great Ice Age, pp. 210, 211.]
[Illustration: Fig. 29.--Section of kame near Dover, New Hampshire.
Length, three hundred feet; height, forty feet; base, about forty feet
above the Cocheco River, or seventy-five feet above the sea. _a_, _a_,
gray clay; _b_, fine sand; _c_, _c_, coarse gravel containing pebbles
from six inches to one foot and a half in diameter; _d_, _d_, fine gravel
(Upham).]
[Illustration: Fig. 30.--Kames in Andover Mass.]
In New England attention was first directed to kames in 1842, by
President Edward Hitchcock, in a paper before the American Association
of Geologists and Naturalists, describing the gravel ridges in Andover,
Mass. In the accompanying plate is shown a portion of this kame system,
which has a double interest to me from the fact that it was while living
upon the banks of the Shawshin River, near where the kames and the river
intersect, that I began, in 1874, my special study of glacial deposits.
The Andover ridges are composed of imperfectly stratified water-worn
material, and are very sharply defined, from the town of Chelsea, back
from the coast into New Hampshire, for a distance of twenty-five miles.
The base of the ridges does not maintain a uniform level, but the system
descends into shallow valleys, and rises over elevations of one hundred
to two hundred feet, without interruption. This indifference to slight
changes of level is specially noticeable where the system crosses the
Merrimac River, just above the city of Lawrence. It is also represented
in the accompanying plate, where the base of the ridges in the immediate
valley of the Shawshin is fifty feet lower than the base of those a short
distance to the north, at the points marked _a_, _b_, and _c_. The ridges
here terminate at the surface in a sharp angle, and are above their base
forty-one feet at _a_, forty-nine feet at _b_, and ninety-one feet at
_c_. Between _c_ and _b_ there is an extensive peat-swamp, filling the
depression up to the level of an outlet through which the surplus water
has found a passage.
[Illustration: Fig. 31.--Longitudinal kames near Hingham, Massachusetts.
The parallel ridges of gravel in the foreground run nearly east and west,
and coalesce at each end, near the edges of the picture, to form an
elongated kettle-hole. The ridges from fifty to sixty feet in height. The
kame-stream was here evidently emptying into the ocean a few miles to the
east (Bouvé).]
Several systems of kames approximately parallel to this have been
traced out in Massachusetts and New Hampshire, while the remnants of
a very extensive system are found in the Connecticut Valley above the
Massachusetts line. But they abound in greatest profusion in the State
of Maine, where Professor George H. Stone has plotted them with much
care. The accompanying map gives only an imperfect representation of the
ramifying systems which he has traced out, and of the extent to which
they are independent of the present river-channels. One of the longest
of these extends more than one hundred miles, crossing the Penobscot
River nearly opposite Grand Lake, and terminating in an extensive delta
of gravel and sand in Cherryfield, nearly north of Mount Desert. This
is represented on our map by the shaded portion west of the Machias
River. Locally these ridges are variously designated as "horsebacks,"
"hogbacks," or "whalebacks," but that in Andover, Mass., was for some
reason called "Indian Ridge." Nowhere else in the world are these ridges
better developed than in New England, except it be in southern Sweden,
where they have long been known and carefully mapped.
[Illustration: Fig. 32.--The kames of Maine and southeastern New
Hampshire. (Stone.)]
The investigations of Mr. W. 0. Crosby upon the composition of till in
eastern Massachusetts is sufficiently important in its bearings upon the
question of glacial erosion to merit notice at this point.[BA] The object
of his investigations was to determine how much of the so-called ground
moraine, or till, consisted of material disintegrated by mechanical
action, and how much by chemical action. The "residuary clay," which has
arisen from chemical decomposition, would properly be attributed to the
disintegrating agencies of preglacial times, while the clay, which is
strictly mechanical in its origin, remains to represent the true "grist"
or "rock flour" of the Glacial period.
[Footnote BA: Proceedings of the Boston Society of Natural History, vol.
xxv (1890), pp. 115-140.]
The results of Mr. Crosby's investigations show that "not more than
one-third of the _detritus_ composing the till of the Boston Basin was in
existence before the Ice age, and that the remaining two-thirds must be
attributed to the mechanical action of the ice-sheet and its accompanying
torrents of water. In other words, if we assume the average thickness
of the drift as thirty feet, the amount of glacial erosion can scarcely
fall below twenty feet. After scraping away the residuary clays and
half-decomposed material, the ice-sheet has cut more than an equal depth
into the solid rocks."
Mr. Crosby's investigations also convinced him that the movement of the
till, or ground moraine, underneath the ice was not _en masse_, but that
"it must have experienced differential horizontal movements or flowing,
in which, normally, every particle or fragment slipped or was squeezed
forward with reference to those immediately below it, the velocity
diminishing downward through the friction of the underlying ledges....
The glaciation was not limited to masses which were firmly caught between
the ice and the solid ledges, and it was in every case essentially a
slipping and not a rolling movement.... These differential horizontal
movements mean that the till acted as a lubricant for the ice-sheet;
and the clayey element, especially, co-operating in many cases with the
pent-up subglacial waters, must have greatly facilitated the onward
progress of the ice." He concludes, therefore, that the onward movement
of the vast ice-sheet greatly exceeded that of the main part of the
ground moraine, the ice-sheet slipping over the till, the whole being
in some degree analogous to that of a great land-slip. "In both cases
the progress of a somewhat yielding and mobile mass is facilitated by an
underlying clayey layer saturated with water."
_New York, New Jersey, and Pennsylvania._
West of New England the glacial phenomena over the northern part of the
United States are equally marked all the way to the Missouri River, and
the boundary-line of the glaciated region can be traced with little
difficulty. It emerges from New York Bay on Staten Island and enters New
Jersey at Perth Amboy. A well-formed moraine covers the northern part
of Staten Island, and upon the mainland marks the boundary from Perth
Amboy, around through Raritan, Plainfield, Chatham, Morris, and Hanover,
to Rockaway, and thence in a southwesterly direction to Belvidere, on
the Delaware River. That portion of New Jersey lying north of this
serpentine line of moraine hills is characterised by the presence of
transported boulders, by numerous lakes of evident glacial origin, and by
every other sign of glacial action, while south of it all these peculiar
characteristics are absent. The observant passenger upon the railroad
trains between New York and Philadelphia can easily recognise the
moraine as it is passed through on the Pennsylvania Railroad at Metuchen
and on the Bound Brook Railroad at Plainfield. Near Drakestown, in
Morris County, there is a mass of blue limestone measuring, as exposed,
thirty-six by thirty feet, and which was quarried for years before
discovering that it was a boulder brought with other drift material from
many miles to the northwest and lodged here a thousand feet above the sea.
Across Pennsylvania the glacial boundary passes through Northampton,
Monroe, Luzerne, Columbia, Sullivan, Lycoming, Tioga, and Potter
Counties, where it enters the State of New York, running still in a
northwest direction through Allegany and Cattaraugus Counties to the
vicinity of Salamanca. Here it turns to the south nearly at a right
angle, running southwestward to Chautauqua County and re-entering
Pennsylvania in Warren County, and thence passing onward in the same
general direction through Crawford, Venango, Mercer, Butler, and Lawrence
Counties to the Ohio line in Columbiana County, about ten miles north of
the Ohio River.
The occurrence of a well-defined terminal moraine to mark the glacial
boundary eastward from Pennsylvania led Professor Lewis and myself, who
made the survey of that State in 1880, to be rather too sanguine in
our expectations of finding an equally well-marked moraine everywhere
along the southern margin of the glaciated area; still, the results are
even more interesting than would have been the exact fulfilment of our
expectations, since they more fully revealed to us the great complexity
of effect which is capable of being brought about by ice-action. Before
proceeding farther with the details, therefore, it will be profitable
at this point to pause in the narrative and briefly record a few
generalisations that have forced themselves into prominence during the
years in which field-work has been in progress.
Previous to our explorations in Pennsylvania it had been thought that the
indications of ice-action would extend much farther south in the valleys
than on the mountains, and this indeed would have been the case if the
glaciers in northern Pennsylvania had been of local origin; but our
experience very soon demonstrated that the great gathering-place of the
snows which produced the glacial movement in northern Pennsylvania could
not have been local, but that over the northern part of that State there
was distinct evidence of a continental movement of ice whose centre was
far beyond the Alleghanies.
For example, we found that the evidences of direct glacial action
extended farther south upon the hills and plateaus than they did in the
narrow valleys, while everywhere on the very southern border of glacial
indications we found boulders that had been brought from the granitic
region of northern New York or central Canada. In eastern Pennsylvania
we found indeed a terminal moraine more or less distinctly marking the
southern border over the highlands. This was more specially true in
Northampton and Monroe Counties.
In Northampton County it was very interesting to see long lines of hills,
a hundred or more feet in height and lying several hundred feet above
the Delaware River, composed entirely of glacial _débris_, much of which
had been brought bodily over the sharp summit of the Blue Ridge, or
Kittatinny Mountain, which rises as a continuous wall to the northwest
and is everywhere several hundred feet higher than the moraine in
Northampton County. The summit of Blue Ridge, also, as far south as the
glacial movement extended, shows evident signs of glacial abrasion, some
hundreds of feet evidently having been removed by that means, leaving a
well-defined shoulder, marking the limits of its southwestern border.
Resting upon the summit of the glaciated portion of the Blue Ridge, there
are also numerous boulders of Helderberg limestone, which must have been
brought from ledges at least five hundred feet lower than the places upon
which they now lie.
In Monroe County the terminal moraine marking there the extreme limit
of the ice-movement is upon an extensive plateau of Pocono sandstone,
about eighteen hundred feet above sea-level, and five or six hundred
feet lower than the crest of the Alleghany Mountains, a short distance
to the north. The moraine hills are here well marked by the occurrence
of circular lakelets and kettle-holes (such as have been described
as characteristic of the shores and islands bordering the south of
New England); by the occurrence of granitic boulders, which must have
been brought from the Adirondacks or Canada; and by the various other
indications referred to on a previous page.
As already intimated, the instructive point in our observations is the
fact that, between Kittatinny Mountain, in Northampton County, and Pocono
plateau, in Monroe County, there is a longitudinal depression, running
northeast by southwest, parallel with the ranges of the mountain system,
which is here about a thousand feet below the respective ridges on either
side. This, therefore, is one of the places where we should have expected
a considerable southern extension of the ice, if it had been largely due
to local causes. Now, while there is indeed a gradual southern trend down
the flanks of the mountain, yet, upon reaching the axis of the valley,
there appears at once a very marked change in the character of the
deposit, and the influence of powerful streams of water becomes manifest,
and it is evident, upon a slight inspection, that we have come upon a
line of drainage which sustained a peculiar relation to the continental
ice-sheet.
From Stroudsburg, near the Delaware Water-Gap, to Weissport, on the
Lehigh River, a distance of about thirty miles, the valley between the
mountains is continuous, and the elevation at each end very nearly the
same. But about half-way between the two places, near Saylorsburg, there
is a river-parting from which the water now runs on the one hand north
to Stroudsburg, and thence to the Delaware River, and on the other hand
south, through Big and Aquonchichola Creeks, to the Lehigh River. The
river-parting is formed by a great accumulation of gravel, whose summit
is about two hundred feet above the level of the valleys into which
the creeks empty at either end; and there are numerous kettle-holes and
lakelets in the vicinity, such as characterize the glacial region in
general.
In short, we are, without doubt, here on a well-marked terminal moraine
much modified by strong water-action in a valley of glacial drainage.
The gravel and boulders are all well water-worn, and the material is of
various kinds, including granite boulders from the far north, such as
characterise the terminal moraine on the highlands; but the pebbles are
not scratched, and the gravel is more or less stratified. It is evident
that we are here where a violent stream of water poured forth from that
portion of the ice-front which filled this valley, and which found its
only outlet in the direction of the Lehigh River. The gravel can be
traced in diminishing quantities to the southward, in accordance with
this theory, while to the northward there extends a series of gravel
ridges, or kames, such as we have shown naturally to owe their origin to
the accumulations taking place in ice-channels formed near the front of a
glacier as it slowly melts away.
From similar occurrences of vast gravel accumulations in other valleys
stretching southward from the glacial margin, we came to expect that,
wherever there was an open, line of drainage from the glaciated region
southward, the point of intersection between the glacial margin and
the drainage valley would be marked by an excessive accumulation of
water-worn gravel, diminishing in coarseness and abundance down the
valleys in proportion to the distance from the glacial margin.
For example, the Delaware River emerges from the glaciated region at
Belvidere, and there are there vast accumulations of gravel rising a
hundred or more feet above the present level of the river, while gravel
terraces, diminishing in height, mark the river below to tide-water at
Trenton. The Lehigh River leaves the glaciated region at Hickory Run, a
few miles above Mauch Chunk, but the gorge is so steep that there was
little opportunity either for the accumulation of gravel there or for its
preservation. Still, the transported gravel and boulders characteristic
of the melting floods pouring forth from a glacier, are found lining the
banks of the Lehigh all along the lower portion of its course. In the
Susquehanna River we have a better example at Beach Haven, in Luzerne
County, where there are very extensive accumulations of gravel resting
on the true glacial deposits of the valley, and extending down the river
in terraces of regularly diminishing height for many miles, and merging
into terraces of moderate elevation which line the Susquehanna Valley
throughout the rest of its course. Above Beach Haven the gravel deposits
in the trough of the river valley are more irregular, and betray the
modifying influence of the slowly decaying masses of ice which belonged
to the enveloping continental glacier.
Westward from the north fork of the Susquehanna, similar extensive
accumulations of gravel occur at the intersection of Fishing Creek in
Columbia County, Muncy, Loyalsock, Lycoming, and Pine Creeks in Lycoming
County, all tributary to the Susquehanna River, and all evidently being
channels through which the melting floods of the ice-sheet brought
vast quantities of gravel down to the main stream. Williamsport, on
the West Branch of the Susquehanna, is built upon an extensive terrace
containing much granitic material, brought down from the glaciated region
by Lycoming Creek, when it was flooded with the waters melted from the
continental ice-sheet which had here surmounted the Alleghanies and
invaded the valley of the Susquehanna.
Analogous deposits of unusual amounts of gravel, occurring in streams
flowing southward from the glaciated region, occur at Great Valley,
Little Valley, and Steamburg in Cattaraugus County, New York, and
at Russelburg and Garland in Warren County, Pennsylvania, also at
Titusville and Franklin in Venango County, and at Wampum in Lawrence
County, of the same State.
As a rule, Professor Lewis and myself found it more difficult to
determine with accuracy the exact point to which the ice extended in
the axis of these south-flowing valleys than we did upon the highlands
upon either side; and, in looking for the positive indications of direct
ice-action in these lines of drainage, we were almost always led up
the valley to a considerable distance inside of the line. This arose
from our inexperience in interpreting the phenomena, or rather from
our inattention to the well-known determining facts in the problem. On
further reflection it readily appeared that this was as it should be. The
ice-front, instead of extending farther down in a narrow valley than on
the adjoining highlands (where they are of only moderate elevation) ought
not to extend so far, for the subglacial streams would not only wear away
the ice of themselves, but would admit the air into the tunnels formed by
them so as to melt the masses both from below and from above, and thus
cause a recession of the front. If we had understood this principle at
the beginning of our survey, it would have saved us much perplexity and
trouble.
A single further illustration of this point will help to an understanding
of many references which will hereafter be made to the water deposits
which accumulated in the lines of drainage running southward from the
glaciated area. At Warren, Pa., Conewango Creek, which is the outlet
from Chautauqua Lake, enters the Alleghany River after flowing for a
number of miles in a deep valley with moderate slopes. In ascending the
creek from Warren, the gravel terraces, which rise twenty-five or thirty
feet above high-water mark, rapidly increase in breadth and height, and
the pebbles become more and more coarse. After a certain distance the
regular terraces begin to give place to irregular accumulations of gravel
in ridges and knobs. In the lower portion of the valley no pebbles
could be found which were scratched. Up the valley a few miles pebbles
were occasionally discovered which showed some slight indications of
having been scratched, but which had been subjected to such an amount of
abrasion by water-action as almost to erase the scratches. On reaching
Ackley's Station, the stream is found to be cutting through a regular
terminal moraine, extending across the valley and full of clearly marked
glaciated stones. Above this terminal moraine the terraces and gravel
ridges which had characterised the valley below disappear, giving place
to long stretches of level and swampy land, which had been subject to
overflow.
Something similar to this so often appears, that there can be no
question as to its meaning, which is, that during the farthest extent
of the ice the front rested for a considerable period of time along the
line marked by the terminal moraine. During this period there occurred
both the accumulation of the moraine and of the gravel terraces in the
valley below, due to the vast flow of water emerging from the ice-front,
especially during the period when it was most rapidly melting away. Upon
the retreat of the ice, the moraine constituted a dam which has not yet
been wholly worn away. For a while the water was so effectually ponded
back by this as to form a lake, which has since become filled up with
sediment and accumulations of peat. From this it is evident, also, that
when the ice began to retreat, the retreat was so continuous and rapid
that no parallel terminal moraines were formed for many miles.
Before leaving this section we will summarise the leading facts
concerning the glacial phenomena north of Pennsylvania and New Jersey.
From the observations of Professor Smock, it appears that, from the
southern margin the ascent to the summit of the ice-sheet was pretty
rapid; the depth one mile back from the margin being not much less
than a thousand feet. "Northward the angle of the slope diminished, and
the glacier surface approximated to a great level plain. The distance
between the high southwestern peaks of the Catskills and Pocono Knob
in Pennsylvania is sixty miles. The difference in the elevation of the
glacier could not have exceeded a thousand feet,"[BB] that is, the slope
of the surface was about seventeen feet to the mile.
[Footnote BB: American Journal of Science, vol. cxxv, 1883, p. 339 _et
seq._]
Professor Dana estimates the thickness of the ice in southern Connecticut
to have been between fifteen hundred and two thousand feet. Attempts to
calculate the thickness of the ice farther north, except from actual
discovery of glacial action on the summits of the mountains, are based
upon uncertain data with reference to the slope necessary to secure
glacial movement. In the Alps the lowest mean slopes down which glaciers
move are about two hundred and fifty feet to a mile; but in Greenland,
Jensen found the slope of the Frederickshaab Glacier to be only
seventy-five feet to the mile, while Helland found that of the Jakobshavn
Glacier to be only forty-five feet.
It is doubtful if even that amount is necessary to secure a continental
movement of ice, since, as already remarked, it is unsafe to draw
inferences concerning the movements of large masses of ice from those of
smaller masses in more constricted areas. We have seen, from the glacial
deposits on the top of Mount Washington, that over the northern part of
New England the ice was more than a mile in depth. We have no direct
evidence of the depth of the stream which surrounded the Adirondack
Mountains. Nor, on the other hand, are we certain that the Catskills
were not completely enveloped in ice, though most observers, reasoning
from negative evidence, have supposed that to be the case. But from the
facts stated concerning the boulders along the glacial boundary in
Pennsylvania, it is certain that the ice was deep enough to surmount
the ridge of the Alleghanies where they are two thousand and more feet
in height. At the least calculation the ice must have been five hundred
feet thick, in order to secure the movement of which there is evidence
across the Appalachian range. Supposing this to be the height of the ice
above the sea on the crest of the Alleghanies, and that the slope of the
surface of the ice-sheet was as moderate as Professor Smock has estimated
it (namely seventeen feet to the mile), the ice would be upwards of
six thousand feet in thickness in the latitude of the Adirondacks,
which corresponds closely with the positive evidence Ave have from the
mountains in New England.
A study of the map of New York will make it easy to understand the
distribution of some interesting glacial marks over the State. The
distance along the Hudson from the glacial boundary in the vicinity of
New York to the valley of the Mohawk is about one hundred and sixty
miles. Prom the glacial boundary at Salamanca, N. Y., to the same valley,
is not over eighty miles. It is easy to see, therefore, that when, in
advancing, the ice moved southward past the Adirondacks, the east end
of the valley of the Mohawk was reached and closed by the ice, while at
the west end of Lake Ontario the ice-front was still in Canada. Thus the
drainage, which naturally followed the course of the St. Lawrence, would
first be turned through the Mohawk. Afterwards, when the Mohawk had been
closed by ice, the vast amount of ponded water was compelled to seek a
temporary outlet over the lower passages leading into the Susquehanna or
into the Alleghany.
A number of such passages exist. One can be traced along the line of the
old canal from Utica to Binghamton, whose highest level is not far from
eleven hundred feet. Another lies in a valley leading south of Cayuga
Lake, whose highest point, at Wilseyville, is nine hundred and forty
feet above tide. Another leads south to the Chemung River from Seneca
Lake, whose highest point, at Horseheads, is less than nine hundred feet
above tide. The cols farther west are somewhat more elevated; the one at
Portage, leading from the Genesee River into the Canisteo, being upwards
of thirteen hundred feet, and that of Dayton, leading from Cattaraugus
Creek into the Conewango, being about the same. Of other southern outlets
farther west we will speak later on.
Fixing our minds now upon the region under consideration, in the southern
part of the State of New York, we can readily see that a glacial lake
must have existed in front of the ice while it was advancing, until it
had reached the river-partings between the Mohawk and the St. Lawrence
Rivers on the north and the Susquehanna and Alleghany Rivers on the
south. After the ice had attained its maximum extension, and was in
process of retreat, there would be a repetition of the phenomena, only
they would occur in the reverse order. The glacial markings which we see
are, of course, mainly those produced during the general retreat of the
ice.
The Susquehanna River stretching out its arms--the Chenango and Chemung
Rivers--to the east and the west, evidently serves as a line of drainage
for the vast glacial floods. These floods have left, along their courses,
extensive elevated gravel terraces, with much material in them which
is not local, but which has been washed out of the direct glacial
deposits from the far north. The east-and-west line of the water-parting
throughout the State is characterised by excessive accumulations of
glaciated material, forming something like a terminal moraine, and is
designated by President Chamberlin as "the terminal moraine of the second
Glacial epoch," corresponding, as he thinks, to the interior line already
described as characterising the south shore of New England.
In the central part of New York the remarkable series of "Finger Lakes,"
tributary to Lake Ontario and emptying into it through the Oswego and
Genesee Rivers, all have a glacial origin. Probably, however, they are
not due in any great degree to glacial erosion, but they seem to occupy
north-and-south valleys which had been largely formed by streams running
towards the St. Lawrence when there was, by some means (probably through
the Mohawk River), a much deeper outlet than now exists, but which has
been filled up and obliterated by glacial _débris_. The ice-movement
naturally centred itself more or less in these north-and-south valleys,
and hence somewhat enlarged them, but probably did not deepen them. The
ice, however, did prevent them from becoming filled with sediment, and on
its final retreat gave place to water.
Between these lakes and Lake Ontario, also, and extending east and west
nearly all the way from Syracuse to Rochester, there is a remarkable
series of hills, from one hundred to two or three hundred feet in height,
composed of glacial _débris_. But while the range extends east and west,
the axis of the individual hills lies nearly north and south. These are
probably remnants of a morainic accumulation which were made during
a pause in the first advance of the ice, and were finally sculptured
into their present shape by the onward movement of the ice. These are
really "drumlins," similar to those already described in northeastern
Massachusetts and southeastern New Hampshire. In the valley of central
New York these have determined the lines of drainage of the "Finger
Lakes," and formed dams across the natural outlets of nearly all of them.
North of the State of New York the innumerable lakes in Canada are all
of glacial origin, being mostly due to depressions of the nature of
kettle-holes, or to the damming up of old outlets by glacial deposits. A
pretty well-marked line of moraine hills, formed probably as terminal
deposits in the later stages of the Ice age, runs from near the eastern
end of Lake Ontario to the Georgian Bay, passing south of Lake Simcoe.
_The Mississippi Basin._
The physical geography of the glaciated region north of the Ohio River is
so much simpler than that of New England and the Middle States, that its
characteristics can be briefly stated. Ohio, Indiana, and Illinois are
covered with nearly parallel strata of rock mostly of the Carboniferous
age. In general, the surface slopes gently to the west; the average
elevation of Ohio being about a thousand feet above tide, while that of
the Great Lakes to the north and of the middle portion of the Mississippi
Valley is less than six hundred feet. The glacial deposits are spread in
a pretty even sheet over the area which was reached by the ice in these
States, and the lines of moraine, of which a dozen or more have been
partially traced in receding order, are much less clearly marked than
they are in New England, or in Michigan, and the States farther to the
northwest.
The line marking the southern limit attained by the ice of the Glacial
period in these three States is as follows: Entering Ohio in Columbiana
County, about ten miles north of the Ohio River, the glacial boundary
runs westward through New Lisbon to Canton in Stark County, and thence
to Millersburg in Holmes County. A few miles west of this place it
turns abruptly south, passing through Danville in Knox County, Newark
in Licking County, Lancaster in Fairfield County, to Adelphi in Ross
County. Thence bearing more westward it passes through Chillicothe
to southeastern Highland County and northwestern Adams, reaching the
Ohio River near Ripley, in Clermont County. Thence, following the
north bank of the Ohio River to Cincinnati, it crosses the river, and
after extending through the northern part of Boone County, Kentucky,
and recrossing the river to Indiana, not far from Rising Sun, it again
follows approximately the north bank of the river to within about ten
miles of Louisville, Ky., where it bends northward running through
Clarke, Scott, Jackson, Bartholomew, and Brown Counties to Martinsville,
in Morgan County, where it turns again west and south and follows
approximately the West Branch of the White River through Owen, Greene,
and Knox Counties, where it crosses the main stream of White River, and,
continuing through Gibson and Posey Counties, crosses the Wabash River
near New Harmony.
In Illinois the line still continues southwesterly through White,
Gallatin, Saline, and Williamson Counties, where it reaches its southern
limit near Carbondale, in latitude 37° 40', and from this point trends
northwestward, approximately following the northeastern bluff of the
Mississippi River, to the vicinity of Carondelet, Mo., a short distance
south of St. Louis.
Beyond the Mississippi the line follows approximately the course of the
Missouri River across Missouri, and continues westward to the vicinity of
Manhattan, in Kansas, where it turns northward, keeping about a hundred
miles west of the Missouri River, through eastern Kansas and Nebraska,
and striking the river near the mouth of the Niobrara, in South Dakota.
From there the line follows approximately the course of the Missouri
River to the vicinity of Fort Benton, in northwestern Montana, where the
line again bears more northward, running into British America.
It is still in dispute whether the ice extended from the eastern centre
far enough west to join the ice-movement from the Rocky Mountain
plateau. Dr. George M. Dawson[BC] is of the opinion that it did not, but
that there was a belt of a hundred miles or more, east of the Rocky
Mountains, which was never covered by true glacial ice. Mr. Upham[BD]
is equally confident that the two ice-movements became confluent,
and united upon the western plateau of Manitoba. The opportunity
for such a difference of opinion arises in the difficulty sometimes
encountered of distinguishing between a direct glacial deposit and a
deposit taking place in water containing boulder-laden icebergs. Where
Mr. Upham supposes the ice-fields of the east and of the west to have
been confluent in western Manitoba, Dr. Dawson supposes there was an
extensive subsidence of the land sufficient to admit the waters of the
ocean. Leaving this question for the present undetermined, we will now
rapidly summarise the glacial phenomena west of the third meridian
from Washington (which corresponds nearly with the western boundary of
Pennsylvania), and east of the Rocky Mountains.
[Footnote BC: Transactions of the Royal Society of Canada, vol. viii,
sec. iv, pp. 54-74.]
[Footnote BD: American Geologist, vol. vi, September, 1890; Bulletin of
the Geological Society of America, vol. ii, pp. 243-276.]
That the glacial movement extended to the southern boundary just
delineated is established by the presence down to that line of all the
signs of glacial action, and their absence beyond. Glacial groovings are
found upon the freshly uncovered rock surfaces at frequent intervals in
close proximity to the line all along its course, while granitic boulders
from the far north are scattered, with more or less regularity, over the
whole intervening space between this line and the Canadian highlands.
I have already referred to a boulder of jasper conglomerate found in
Boone County, Kentucky, which must have come from unique outcroppings of
this rock north of Lake Huron. Granitic boulders from the Lake Superior
region are also found in great abundance at the extreme margin mentioned
in southern Illinois. West of the Missouri River it is somewhat more
difficult to delineate the boundary with accuracy, on account of an
enveloping deposit of fine loam, technically called "loess." Loess is
very abundant in the whole valley of the Missouri River below Yankton,
South Dakota, being for a long distance in the vicinity of the river
a hundred feet or more in depth. Over northern Missouri and southern
Illinois the deposit is nearly continuous, but less in depth, and
everywhere in that region tends to hide from view the unstratified
glacial deposit continuously underlying it.
A single instance of personal experience will illustrate the condition of
things. While going south from Chicago, in search of the southern limit
of glacial action, I stopped off from the train at Du Quoin, about forty
miles north of where I subsequently found the boundary. Here the whole
surface was covered with loess, two or three feet in depth. Below this
was a gravelly soil, three or four feet in thickness, which contained
many scratched pebbles of granite. A well which had recently been dug,
reached the rock at a depth of twenty feet, and revealed a beautifully
polished and scratched surface, betraying, beyond question, the action
of glacial ice. As we shall show a little later, it is probable that,
about the time the ice of the Glacial period had reached its maximum
development, this area, which is covered with loess, was depressed in
level, and remained under water during a considerable portion of the
period when the ice-front was retreating.
[Illustration: Fig. 33.--Western face of the kettle-moraine, near Eagle,
Waukesha County, Wisconsin. (From a photograph by President T. C.
Chamberlain, United States Geological Survey.)]
To such an extent is this portion of the area included in southern
Iowa, northern Missouri, southern Illinois, and the extreme southern
portions of Indiana and Ohio covered with loess, that it has been
difficult to determine the relation of its underlying glacial deposits
to the more irregular deposits found farther north. At an early period
of recent investigations, while making a geological survey of the State
of Wisconsin, President T. C. Chamberlin fixed upon the line of moraine
hills, which can be seen upon our map, running southward between Green
Bay and Lake Michigan, and sweeping around in a curve to the right,
passing south of Madison and northward along the line of Wisconsin
River, and in another curve to the left, around the southern end of Lake
Michigan, as the "terminal moraine of the second Glacial epoch." In
Wisconsin the character of this line of moraine hills had been discovered
and described by Colonel Charles Whittlesey, in 1866. It was first
named the "kettle-moraine," because of the frequent occurrence in it of
"kettle-holes." This line of moraine hills has been traced with a great
degree of confidence across the entire glaciated area, as shown upon our
map, but it is not everywhere equally distinct, and, as will be observed,
follows a very irregular course.
Beginning in Ohio we find it coinciding nearly with the extreme glacial
boundary until it reaches the valley of the Scioto River, on the sixth
meridian west from Washington, where it begins to bear northward and
continues in that direction for a distance of sixty or seventy miles,
and then turns southward again in the valley of the Miami, having formed
between these two valleys a sort of medial moraine.[BE] A similar medial
moraine had also been noted by President Chamberlin between the valleys
of the Grand and Cuyahoga Rivers, in the eastern part of Ohio. Indeed,
for the whole distance across Ohio and Indiana, this moraine occurs in a
series of loops pointing to the south, corresponding in general to the
five gentle valleys which mark the territory, namely, those of the Grand
and Mahoning Rivers; the Sandusky and Scioto Rivers; the Great Miami
River; the White River; and the Maumee and Wabash Rivers. Everywhere,
however, over this area these morainic accumulations approximate pretty
closely to the extreme boundary of the glaciated region.
[Footnote BE: See map at the beginning of the chapter.]
In Illinois President Chamberlin's original determination of the moraine
fixed it near the southern end of Lake Michigan, as shown upon our map,
but Mr. Frank Leverett has subsequently demonstrated that there is a
concentric series of moraines south of this, reaching across the State,
(but somewhat obscured by superficial accumulations of loess referred to)
and extending nearly to the latitude of St. Louis.
West of Wisconsin President Chamberlin's "terminal moraine of the second
Glacial epoch" bends southward through eastern Minnesota, and, sweeping
down through central Iowa, forms, near the middle of the northern part
of that State, a loop, having its southern extremity in the vicinity of
Des Moines. The western arm of this loop runs through Minnesota in a
northwesterly direction nearly parallel with the upper portion of the
valley of the Minnesota, until reaching the latitude of the head-waters
of that river, where, in the vicinity of the Sisseton Agency, in Dakota,
it turns to the south by an acute angle, and makes a loop in that State,
extending to the vicinity of Yankton, and with the valley of the James
River as its axis. The western arm of this loop follows pretty closely
the line of the eastern edge of the trough of the Missouri River,
constituting what is called the "Missouri Coteau," which continues on as
a well-marked line of hills running in a northwesterly direction far up
into the Dominion of Canada.
One of the most puzzling glacial phenomena in the Mississippi Valley is
the driftless area which occupies the southeastern portion of Minnesota,
the southwestern part of Wisconsin, and the northwestern corner of Iowa,
as delineated upon our map. This is an area which, while being surrounded
on every side by all the characteristic marks of glaciation, is itself
conspicuous for their entire absence. Its rocks preserve no glacial
scratches and are covered by no deposits of till, while northern boulders
avoided it in their journey to more southern latitudes.
The reason for all this is not evident in the topography of the region.
The land is not higher than that to the north of it, nor is there any
manifest protection to it by the highlands south of Lake Superior. Nor
yet is there any reason to suppose that any extensive changes of level in
former times seriously affected its relations to the surrounding country.
Professor Dana, however, has called attention to the fact that even now
it is in a region of comparatively light precipitation, suggesting that
the snow-fall over it may always have been insignificant in amount. But
this could scarcely account for the failure of the great ice-wave of the
north to overrun it. We are indebted again to the sagacity of President
Chamberlin in suggesting the true explanation.
By referring to the map it will be noticed that this area sustains a
peculiar relation to the troughs of Lake Michigan and Lake Superior,
while from the arrangements of the moraines in front of these lakes it
will be seen that these lake basins were prominent factors in determining
the direction of the movement of the surplus ice from the north. It is
the more natural that they should do so because of their great depth,
their bottoms being in both cases several hundred feet below the present
water-level, reaching even below the level of the sea.
These broad, deep channels seem to have furnished the readiest outlet for
the surplus ice of the North, and so to have carried both currents of ice
beyond this driftless area before they became again confluent. The slight
elevation south of Lake Superior served to protect the area on account of
the feebleness of direct movement made possible by the strength of these
diverging lateral ice-currents. The phenomenon is almost exactly what
occurs where a slight obstruction in a river causes an eddy and preserves
a low portion of land below it from submergence. A glance at the map
will make it easily credible that an ice-movement south of Manitoba,
becoming confluent with one from Lake Superior, pushed far down into the
Missouri Valley and spread eastward to the Mississippi River, south of
the unglaciated driftless area, and there became confluent with a similar
movement which had been directed by the valleys of Lake Michigan and Lake
Erie. There can be little doubt that President Chamberlin's explanation
is in the main correct, and we have in this another illustration of the
analogy between the behaviour of moving ice and that of moving water.
[Illustration: Fig. 34.--Section of the east-and-west glacial furrows,
on Kelly's Island, preserved by Mr. Younglove. Fine sediment rests
immediately on the rock, with washed pebbles at the surface.]
The accompanying illustrations will give a better idea than words can
do of the celebrated glacial grooves on the hard limestone islands
near Sandusky, in the western part of Lake Erie. Through the interest
aroused in them by an excursion of the American Association for the
Advancement of Science, while meeting in Cleveland, Ohio, in 1888, the
Kelly Island Lime and Transport Company, of which Mr. M. C. Younglove is
the president, has been induced to deed to the Western Reserve Historical
Society for preservation a portion of one of the most remarkable of the
grooves still remaining.
The portion of the groove preserved is thirty-three feet across, and
the depth of the cut in the rock is seventeen feet below the line,
extending from rim to rim. Originally there was probably here a small
depression formed by preglacial water erosion, into which the ice crowded
the material, which became its graving-tool, and so the rasping and
polishing went on in increasing degree until this enormous furrow is
the result. The groove, however, is by no means simple, but presents a
series of corrugations merging into each other by beautiful curves. When
exposed for a considerable length it will resemble nothing else so much
as a collection of prostrate Corinthian columns lying side by side on a
concave surface.
The direction of these grooves is a little south of west, corresponding
to that of the axis of the lake. This is nearly at right angles to the
course of the ice-scratches on the summit of the water-shed south of
this, between the lake and the Ohio River. The reason for this change
of direction can readily be seen by a little attention to the physical
geography. The highlands to the south of the lake rise about seven
hundred feet above it. When the Ice period was at its climax and overran
these highlands, the ice took its natural course at right angles to
the terminal moraine and flowed southeast according to the direction
indicated by the scratches on the summit; but when the supply of ice was
not sufficient to overrun the highlands, the obstruction in front turned
the course and the resultant was a motion towards Toledo and the Maumee
Valley, where in the vicinity of Fort Wayne an extensive terminal moraine
was formed.
[Illustration: Fig. 35.--Same as the preceding. (Courtesy of M. C.
Younglove.)]
The much-mooted question of a succession of glacial epochs finds the
most of its supporting facts in the portion of the glaciated area lying
west of Pennsylvania. That there have been frequent oscillations of the
glacial front over this area is certain. But it is a question whether
the glacial deposits south of this distinct line of moraine hills are so
different from those to the north of it as to necessitate the supposition
of two entirely distinct glacial epochs. This can be considered most
profitably here.
The following are among the points with reference to which the phenomena
south of the moraine just delineated differ from those north of the line:
1. The glacial deposits to the south appear to be distributed more
uniformly than those to the north. To the north the drift is often
accumulated in hills, and is dotted over with kettle-holes, while to the
south these are pretty generally absent. Any one travelling upon a line
of railroad which traverses these two portions of the glaciated area as
indicated upon our map can easily verify these statements.
2. The amount of glacial erosion seems to be much less south of the line
of moraine hills delineated than north of them. Still, glacial striæ
are found, almost everywhere, close down to the extreme margin of the
glaciated area.
3. The gravel deposits connected with the drainage of the Glacial period
are much less abundant south of the so-called "terminal moraine of the
second Glacial period" than they are north of it. South of this moraine
the water deposits attributed to the Glacial period are of such fine
silt as to indicate slow-moving currents over a gentle low slope of the
surface.
4. The glacial deposits to the south are more deeply coloured than those
to the north, showing that they have been longer exposed to oxidising
agencies. Even the granitic boulders show the marks of greater age south
of this line, being disintegrated to a greater extent than those to the
north.
5. And, finally, there occur, over a wide belt bordering the so-called
moraine hills of the second Glacial epoch, extensive intercalated beds
of vegetal deposits. Among the earliest of these to be discovered were
those of Montgomery County, Ohio, where, in 1870, Professor Orton, of the
Ohio Survey, found at Germantown a deposit of peat fourteen feet thick
underneath ninety-five feet of till, and there seem also to be glacial
deposits underneath the peat as well as over it. The upper portion of the
peat contains "much undecomposed sphagnous mosses, grasses, and sedges,
and both the peat and the clayey till above it" contain many fragments
of coniferous wood which can be identified as red cedar (_Juniperus
Virginianus_). In numerous other places in that portion of Ohio
fresh-appearing logs, branches, and twigs of wood are found underneath
the till, or mingled with it, much as boulders are. Near Darrtown, in
Butler County, Ohio, red cedar logs were found under a covering of
sixty-five feet of till, and so fresh that the perfume of the wood is
apparently as strong as ever. Similar facts occur in several other
counties in the glaciated area of southern Ohio and southern Indiana.
Professor Collett reports that all over southwestern Indiana peat, muck,
rotted stumps, branches, and leaves of trees are found from sixty to one
hundred and twenty feet below the surface, and that these accumulations
sometimes occur to a thickness of from two to twenty feet.
[Illustration: Fig. 36.--Section of till near Germantown, Ohio, overlying
thick bed of peat. The man in the picture stands upon a shelf of peat
from which the till has been eroded by the stream. The dark spot at
the right hand of the picture, just above the water, is an exposure of
the peat. The thickness of the till is ninety-five feet. The partial
stratification spoken of in the text can be seen about the middle of the
picture. The furrows up and down had been made by recent rains. (United
States Geological Survey.) (Wright.)]
Farther to the northwest similar phenomena occur. Professor N. H.
Winchell has described them most particularly in Fillmore and Mower
Counties, Minnesota, from which they extend through a considerable
portion of Iowa. In the above counties of Minnesota a stratum of peat
from eighteen inches to six or eight feet in thickness, with much wood,
is pretty uniformly encountered in digging wells, the depth varying from
twenty to fifty feet. This county is near the highest divide in the State
of Minnesota, and from it "flow the sources of the streams to the north,
south, and east." The wood encountered in this stratum indicates the
prevalence f coniferous trees, and the peat mosses indicate a cool and
moist climate.
Nor are intercalated vegetable deposits absent from the vast region
farther north over the area that drains into Hudson Bay. At Barnesville,
in Clay County, Minnesota, which lies in the valley of the Red River of
the North, and also in Wilkin County in the same valley, tamarack wood
and sandy black mud containing many snail-shells have been found from
eight to twelve feet below a surface of till; and Dr. Robert Bell reports
the occurrence of limited deposits of lignite between layers of till, far
to the northwest, in Canada, and even upon the southern part of Hudson
Bay; while Mr. J. B. Tyrrell reports[BF] many indications of successive
periods of glaciation near the northern end of the Duck Mountain. The
most characteristic indications which he had witnessed consisted of
stratified beds of silt, containing fresh-water shells, with fragments of
plants and fish similar to those living in the lakes of the region at the
present time.
[Footnote BF: Bulletin of the Geological Society of America, vol. i, pp.
395-410.]
Reviewing these facts with reference to their bearing upon the point
under consideration, we grant, at the outset, that they do indicate a
successive retreat and readvance of the ice over extensive areas. This
is specially clear with respect to the vegetal deposits interstratified
with beds of glacial origin. But the question at issue concerning the
interpretation of these phenomena is, Do they necessarily indicate
absolutely distinct glacial epochs separated by a period in which the
ice had wholly disappeared from the glaciated area to the north? That
they do, is maintained by President Chamberlin and many others who have
wide acquaintance with the facts. That they do not certainly indicate a
complete disappearance of the ice during an extensive interglacial epoch,
is capable, however, of being maintained, without forfeiting one's rights
to the respect of his fellow-geologists. The opposite theory is thus
stated by Dr. Robert Bell: "It appears as if all the phenomena might be
referred to one general Glacial period, which was long continued, and
consequently accompanied by varying conditions of temperature, regional
oscillations of the surface, and changes in the distributions of sea and
land, and in the currents in the ocean. These changes would necessarily
give rise to local variations in the climate, and might permit of
vegetation for a time in regions which need not have been far removed
from extensive glaciers."[BG]
[Footnote BG: Bulletin of the Geological Society of America, vol. i, pp.
287-310.]
At my request, Professor J. E. Todd, of Iowa, whose acquaintance with the
region is extensive, has kindly written out for me his conclusions upon
this subject, which I am permitted to give in his own words:
"I am not prepared to write as I would like concerning the forest-beds
and old soils. I will, however, offer the following as a partial report.
I have come to think that there is considerable confusion on the subject.
I believe there are five or six different things classed under one head.
"1. _Recent Much and Soils._--The finest example I have found in the
whole Missouri Valley was twenty feet below silt and clay, in a basin
inside the outer moraine, near Grand View, South Dakota. From my
examination of the reported old soil near Albia, Iowa, I think the most
rational way of reconciling the conflicting statements concerning it is
that it also belongs to this class.
"2. _Peat or Soil under Loess._--This does not signify much if the loess
was formed in a lake subject to orographic oscillations, or if, as I am
coming to believe, it is a fluviatile deposit of an oscillating river
like the Hoang-Ho on the great Chinese plain. It at least does not mean
an interglacial epoch.
"3. _Wood and Dirt rearranged, not in situ._--This occurs either in
subaqueous or in subglacial deposits. I have found drift-wood in the
lower layers of the loess here, but not _in situ_. I have frequently
found traces of wood in till in Dakota, but always in an isolated
way. I think, from reading statements about the deposits in eastern
Iowa, that most if not all of the cases are of this sort. Two things
have conspired to lead to this error: one, the influence of Croll's
speculation; and the other, the easy inference of many well-diggers,
and especially well-borers, that what they pass through are always in
layers. In this way a log becomes a forest-bed. Scattered logs and muck
fragments occurring frequently in a region, though at different levels,
are readily imagined by an amateur geologist to be one continuous stratum
antedating the glacier or floods (as the case may be in that particular
region), when, in fact, it has been washed down from the margin of
the transporting agent and is contemporaneous with it. I suspect the
prevalence of wood in eastern Iowa may be traced to a depression of the
driftless region during the advance of the glacier, so as to bring the
western side of that area more into the grasp of glacial agencies.
"4. _Peat between Subglacial Tills._--If cases of this sort are found,
they are in Illinois, Indiana, and Ohio. Professor Worthen insisted that
there were no interglacial soils or forest-beds in Illinois; and in the
cases mentioned in the State reports he repeatedly explains the sections
given by his assistants, so as to harmonize them with that statement. I
think he usually makes his explanations plausible. He was very confident
in referring most of them, to preglacial times. His views, I suppose,
will be published in the long-delayed volume, now about to be issued.
"5. _Vegetable Matter between Glacial Till and Underlying Berg Till or
other Drift Deposits._--When one remembers that the front of the great
ice-sheet may have been as long in reaching its southern boundary as in
receding from it, and with as many advance and retrograde movements, we
can easily believe that much drift material would have outrun the ice and
have formed deposits so far ahead of it that vegetation would have grown
before the ice arrived to bury it.
"6. _Preglacial Soils, etc._--I believe that this will be found to
include most in southern Ohio, if not in Illinois, as Worthen claimed."
The phenomena of the Glacial period are too vast either to have appeared
or to have disappeared suddenly. By whatever cause the great accumulation
of ice was produced, the advance to the southward must have been slow
and its disappearance must have been gradual, though, as we shall show
a little later, the final retreat of the ice-front occupied but a short
time relatively to the whole period which has elapsed since. As we
shall show also, the advent of the Ice period was probably preceded and
accompanied by a considerable elevation of the northern part of the
continent Whether this elevation was contemporaneous upon both sides of
the continent is perhaps an open question; but with reference to the area
east of the Rocky Mountains, which is now under consideration, the centre
of elevation was somewhere south of Hudson Bay. Putting together what
we know, from the nature of the case, concerning the accumulation and
movement of glacial ice, and what we know from the relics of the great
glacial invasion, which have enabled us to determine its extent and the
vigour of its action, the course of events seems to have been about as
follows:
Throughout the Tertiary period a warm climate had prevailed over British
America, Greenland, and indeed over all the lands in proximity to the
north pole, so far as explorers have been able to penetrate them. The
vegetation characterizing these regions during the Tertiary period
indicates a temperature about like that which now prevails in North
Carolina and Virginia. Whatever may be said in support of the theory
that the Glacial period was produced by astronomical causes, in view of
present facts those causes cannot be regarded as predominant; at most
they were only co-operative. The predominant cause of the Glacial period
was probably a late Tertiary or post-Tertiary elevation of the northern
part of the continents, accompanied with a subsidence in the central
portion. Of such a subsidence in the Isthmus of Panama indications are
thought to be afforded by the occurrence of late Tertiary or, more
probably, post-Tertiary sea-shells at a considerable elevation on the
divide along the Isthmus of Panama, between the Atlantic and Pacific
Oceans. Of this we shall speak more fully in a later chapter.
Fixing our thoughts upon what is known as the Laurentian plateau, which,
though now in the neighbourhood of but two thousand feet above the sea,
was then much higher, we can easily depict in imagination the beginnings
of the great "Laurentide Glacier," which eventually extended to the
margin already delineated on the south and southwest in the United
States, and spread northward and eastward over an undetermined area.
Year after year and century after century the accumulating snows over
this elevated region consolidated into glacial ice and slowly pushed
outward the surplus reservoirs of cold. For a long time this process of
ice-accumulation may have been accompanied by the continued elevation of
the land, which, together with the natural effect of the enlarging area
of ice and snow, would tend to lower the temperature around the margin
and to increase still more the central area of accumulation.
The vigour of movement in any direction was determined partly by the
shape of the valleys opening southward in which the ice-streams would
naturally concentrate, and partly by those meteorological conditions
which determine the extent of snow-fall over the local centres of glacial
dispersion. For example, the general map of North America in the Ice
period indicates that there were two marked subcentres of dispersion
for the great Laurentide Glacier, the eastern one being in Labrador and
the western one north of Lake Superior. In a general way the southern
boundary of the glaciated region in the United States presents the
appearance of portions of two circumferences of circles intersecting each
other near the eastern end of Lake Erie. These circles, I am inclined
to believe, represent the areas over which a semi-fluid (or a substance
like ice, which flows like a semi-fluid) would disperse itself from the
subcentres above mentioned.
A study of the contour of the country shows that that also, in a general
way, probably had something to do with the lines of dispersion. The
western lobe of this glaciated area corresponds in its boundary pretty
closely with the Mississippi Valley, having the Ohio River approximately
as its eastern arm and the Missouri as its western, with the Mississippi
River nearly in its north and south axis. The eastern lobe has its
farthest extension in the axis of the Champlain and Hudson River Valleys,
its western boundary being thrown more and more northward as the line
proceeds to the west over the Alleghany Mountains until reaching the
longitude of the eastern end of Lake Erie; but this southern boundary is
by no means a water-level, nor is the contour of the country such that
it could ever have been a water-level. But it conforms in nearly every
particular to what would be the resultant arising from a pretty general
southward flow of a semi-fluid from the two subcentres mentioned, meeting
with the obstructions of the Adirondacks in northern New York and of the
broader Appalachian uplift in northern Pennsylvania.
How far south the area of glacial accumulation may have extended cannot
be definitely ascertained, but doubtless at an early period of the great
Ice age the northern portions of the Appalachian range in New York, New
England, New Brunswick, and Nova Scotia became themselves centres of
dispersion, while only at the height of the period did all their glaciers
become confluent, so that there was one continuous ice-sheet.
In the western portion of the area covered by the Laurentide Glacier, the
depression occupied by the Great Lakes, especially Lakes Michigan and
Superior, evidently had a marked influence in directing the flow of ice
during the stages which were midway between the culmination of the Ice
period and both its beginning and its end. This would follow from the
great depth of these lakes, the bottom of Lake Michigan being 286 feet
below sea-level, and that of Lake Superior 375 feet, making a total depth
of water of about 900 and 1,000 feet respectively. Into these oblong
depressions the ice would naturally gravitate until they were filled,
and they would become the natural channels of subsequent movement in the
direction of their longest diameters, while the great thickness of ice in
them would make them the conservative centres of glacial accumulation and
action after the ice had begun to retreat.
These deductions from the known nature of ice and the known topography of
the region are amply sustained by a study of the detailed map showing the
glacial geology in the United States. But on this we can represent indeed
only the marks left by the ice at various stages of its retreat, since,
as already remarked, the marks of each stage of earlier advance would be
obliterated by later forward movements. We may presume, however, that
in general the marks left by the retreating ice correspond closely with
those actually made and obliterated by the advancing movement.
From observations upon the glaciers of Switzerland and of Alaska, it
is found that neither the advance nor the retreat of these glaciers is
constant, but that, in obedience to meteorologic agencies not fully
understood, they advance and retreat in alternate periods, at one time
receding for a considerable distance, and at other times regaining the
lost ground and advancing over the area which has been uncovered by their
retreat.
"M. Forel reports, from the data which he has collected with much care,
that there have been in this century five periods in the Alpine glaciers:
of enlargement, from 1800 (?) to 1815; of diminution, from 1815 to 1830;
of enlargement, from 1830 to 1845; of diminution, from 1845 to 1875; and
of enlargement again, from 1875 onward. He remarks further that these
periods correspond with those deduced by Mr. C. Lang for the variations
for the precipitations and temperature of the air; and, consequently,
that the enlargement of the glaciers has gone forward in the cold and
rainy period, and the diminution in the warm and the dry."[BH]
[Footnote BH: American Journal of Science, vol. cxxxii, 1886, p. 77.]
When, now, we attentively consider the combination of causes necessary to
produce the climatic conditions of the great Ice age of North America, we
shall be prepared to find far more extensive variations in the progress
of the continental glacier, both during its advance and during its
retreat, than are to be observed in any existing local glaciers.
With respect to the arguments adduced in favor of a succession of glacial
epochs in America the following criticisms are pertinent:
1. So far as we can estimate, a temporary retreat of the front, lasting
a few centuries, would be sufficient to account for the vegetable
accumulations that are found buried beneath the glacial deposits in
southern Ohio, Indiana, central Illinois, and Iowa, while a temporary
readvance of the ice would be sufficient to bury the vegetable remains
beneath a freshly accumulated mass of till. Thus, as Dr. Bell suggested,
the interglacial vegetal deposits do not necessarily indicate anything
more than a temporary oscillation of the ice-front, and do not carry
with them the necessity of supposing a disappearance of the ice from the
whole glaciated area. Thus the introduction of a whole Glacial period to
account for such limited phenomena is a violation of the well-known law
of parsimony, which requires us in our explanations of phenomena to be
content with the least cause which is sufficient to produce them. In the
present instance a series of comparatively slight oscillations of the
ice-front during a single glacial period would seem to be sufficient to
account for all the buried forests and masses of vegetal _débris_ that
occur either in the United States or in the Dominion of Canada.
2. Another argument for the existence of two absolutely distinct glacial
periods in North America has been drawn from the greater oxidation of
the clays and the more extensive disintegration of certain classes of
the boulders found over the southern part of the glaciated area of the
Mississippi Valley, than has taken place in the more northerly regions.
Without questioning this statement of fact (which, however, I believe to
be somewhat exaggerated), it is not difficult to see that the effects
probably are just what would result from a single long glacial period
brought about by such causes as we have seen to be probably in operation
in America. For if one reflects upon the conditions existing when the
Glacial period began, he will see that, during the long ages of warm
climate which characterised the preceding period, the rocks must have
been extensively disintegrated through the action of subaërial agencies.
The extent to which this disintegration takes place can be appreciated
now only by those who reside outside of the glaciated area, where these
agencies have been in uninterrupted action. In the Appalachian range
south of the glaciated region the granitic masses and strata of gneiss
are sometimes found to be completely disintegrated to a depth of fifty or
sixty feet; and what seem to be beds of gravel often prove in fact to be
horizontal strata of gneiss from which the cementing material has been
removed by the slow action of acids brought down by the percolating water.
Now, there can be no question that this process of disintegration had
proceeded to a vast extent before the Glacial period, so that, when the
ice began to advance, there was an enormous amount of partially oxidised
and disintegrated material ready to be scraped off with the first advance
of ice, and this is the material which would naturally be transported
farthest to the south; and thus, on the theory of a single glacial
period, we can readily account for the greater apparent age of the
glacial _débris_ near the margin. This _débris_ was old when the Glacial
period began.
3. With reference to the argument for two distinct glacial periods drawn
from the smaller apparent amount of glacial erosion over the southern
part of the glaciated area, we have to remark that that would occur
in case of a single ice-invasion as well as in case of two distinct
ice-invasions, in which the later did not extend so far as the former.
From the very necessity of the case, glacial erosion diminishes as the
limit of the extent of the glaciation is approached. At the very margin
of the glacier, motion has ceased altogether. Back one mile from the
margin only one mile of ice-motion has been active in erosion, while ten
miles back from its front there has been ten times as much moving ice
actually engaged in erosion, and in the extreme north several hundred
times as much ice, Thus it is evident that we do not need to resort to
two glacial periods to account for the relatively small amount of erosion
exhibited over the southern portion of our glaciated area.
At the same time, it should be said that the indications of active
glacial erosion near the margin are by no means few or small. In Lawrence
County, Pennsylvania, on the very margin of the glaciated area, Mr. Max
Foshay[BI] has discovered very extensive glacial grooves, indicating much
vigour of ice-action even beyond the more extensive glacial deposits
which Professor Lewis and myself had fixed upon as the terminal moraine.
In Highland and Butler Counties, Ohio, and in southwestern Indiana and
southern Illinois, near the glacial margin, glacial grooves and striæ
are as clear and distinct in many cases as can anywhere be found; while
upon the surface of the limestone rocks within the limits of the city of
St. Louis, where the glacial covering is thin, and where disintegrating
agencies had had special opportunities to work, I found very clear
evidences of a powerful ice-movement, which had planed and scratched
the rock surface; and at Du Quoin, Illinois, as already related, the
fragments thrown up from the surface of the rock, fifty or sixty feet
below the top of the soil, were most beautifully planed and striated.
It should be observed, also, that this whole area is so deeply covered
with _débris_ that the extent of glacial erosion underneath is pretty
generally hid from view.
[Footnote BI: Bulletin of the Geological Society, vol. ii, pp. 457-464.]
4. The uniformity of the distribution of the glacial deposits over the
southern portion of the glaciated area in the Mississippi Valley is
partly an illusion, due to the fact that there was a vast amount of
deposition by water over that area during the earlier stages of the
ice-retreat. This has been due partly to the gentler slope which would
naturally characterise the borders of an area of elevation, and partly to
an extensive subsidence which seems to have begun soon after the ice had
reached its farthest extent of motion.
It should be borne in mind that at all times a glacier is accompanied
by the issue of vast streams of water from its front, and that these
of course increase in volume when the climax has been reached and the
ameliorating influences begin to melt away the accumulated mass of ice
and to add the volume of its water to that produced by ordinary agencies.
As these subglacial streams of water poured out upon the more gentle
slopes of the area in front of the ice, they would distribute a vast
amount of fine material, which would settle into the hollow places and
tend to obscure the irregularities of the previous direct glacial deposit.
Such an instance came clearly under my own observation in the vicinity of
Yankton, in South Dakota, where, upon visiting a locality some miles from
any river, and to which workmen were resorting for sand, I found that the
deposit occupied a kettle-hole, filling it to its brim, and had evidently
been superimposed by a temporary stream of water flowing over the region
while the ice was still in partial occupation of it. Thus, no doubt, in
many cases, the original irregularities of the direct glacial deposits
have been obliterated, even where there has been no general subsidence.
But, in the area under consideration, the loess, or loam, is so extensive
that it is perhaps necessary to suppose that the central portions of the
Mississippi Valley were subjected to a subsidence amounting to about five
hundred feet; so that the glacial streams from the retreating ice-front
met the waters of the ocean in southern Illinois and Indiana; thus
accounting for the extensive fine silt which has done so much over that
region to obscure the glacial phenomena.
_West of the Rocky Mountains._
The glacial phenomena in the United States west of the Rocky Mountains
must be treated separately, since American geologists have ceased to
speak of an all-pervading ice-cap extending from the north pole. But,
as already said, the glaciation of North America has proceeded from
two definite centres of ice-accumulation, one of which we have been
considering in the pages immediately preceding. The great centre of
glacial dispersion east of the Rocky Mountains is the region south of
Hudson Bay, and the vast ice-field spreading out from that centre is
appropriately named the Laurentide Glacier. The movement of ice in this
glacial system was outward in all directions from the Laurentian hills,
and extended west several hundred miles, well on towards the eastern foot
of the Rocky Mountains.
The second great centre of glacial dispersion occupies the vast
Cordilleran region of British Columbia, reaching from the Rocky Mountains
on the northeast to the Coast Range of the Pacific on the southwest, a
width of four hundred miles. The length is estimated by Dr. Dawson to
be twelve hundred miles. The principal centre of ice-accumulation lies
between the fifty-fifth and the fifty-ninth parallel. From this centre
the movement was in all directions, but chiefly to the northwest and to
the south. The movement of the Cordilleran glaciers extended northwest
to a distance of three hundred and fifty miles, leaving their moraines
far down in the Yukon Valley on the Lewes and Pelly Rivers.[BJ] Southward
the Cordilleran Glacier moved to a distance of six hundred miles,
extending to the Columbia River, in the eastern part of the State of
Washington.
[Footnote BJ: See George M. Dawson, in Science, vol. xi, 1888, p. 186,
and American Geologist, September, 1890, pp. 153-162.]
From this centre, also, the ice descended to the sea-level upon the
west, and filled all the channels between Vancouver's Island and the
mainland, as well as those in the Alexander Archipelago of Alaska. South
of Vancouver's Island a glacier pushed out through the straits of Juan de
Fuca to an unknown distance. All the islands in Puget Sound are composed
of glacial _débris_, resembling in every respect the terminal moraines
which have been described as constituting many of the islands south of
the New England coast. The ice-movement in Puget Sound, however, was
probably northward, resulting from glaciers which are now represented by
their diminutive descendants on the flanks of Mount Rainier.
South of the Columbia River the country was never completely enveloped
by the ice, but glaciers extended far down in the valleys from all the
lofty mountain-peaks. In Idaho there are glacial signs from the summit
of the Rocky Mountains down to the westward of Lake Pend d'Oreille. In
the Yellowstone Park there are clear indications that the whole area was
enveloped in glacial ice. An immense boulder of granite, resting upon
volcanic deposits, may be found a little west of Inspiration Point, on
the Yellowstone Cañon. Abundant evidences of glacial action are also
visible down the Yellowstone River to the vicinity of Livingston, showing
that that valley must have been filled with glacial ice to a depth of
sixteen hundred feet. To the west the glaciers from the Yellowstone
Park extended to the border of Idaho, where a clearly marked terminal
moraine is to be found in the Tyghee Pass, leading over from the western
fork of the Madison River into Lewis Fork of the Snake River. South of
Yellowstone Park the Teton Mountains were an important centre for the
dispersion of local glaciers, but they did not descend upon the western
side much below the 6,000-foot level, and only barely came to the edge
of the great Snake River lava plains. To the east the movement from the
Teton Mountains joined that from various other lofty mountains, where
altogether they have left a most intricate system of glacial deposits, in
whose reticulations Jackson's Lake is held in place.
[Illustration: Fig. 37.--Moraines of Grape Creek, Sangre del Cristo
Mountains, Colorado (after Stevenson).]
In Utah extensive glaciers filled all the northern valleys of the Uintah
Mountains, and extended westward in the Wahsatch range to the vicinity of
Salt Lake City. The mountain region of Colorado, also, had its glaciers,
occupying the head-waters of the Arkansas, the Platte, the Gunnison, and
the Grand Rivers. The most southern point in the Rocky Mountains at which
signs of local glaciers have been noted is near the summits of the San
Juan range, in southwestern Colorado. Here a surface of about twenty-five
square miles, extending from an elevation of 12,000 feet down to 8,000
feet, shows every sign of the former presence of moving ice. The greater
part of the glaciation in Colorado is confined to elevations above 10,000
feet.
The whole range of the Sierra Nevada through Oregon, and as far south as
the Yosemite Valley in California, formerly sustained glaciers of far
greater size than any which are now found in those mountains. In general
these glaciers were much longer on the western side of the Sierra Nevada
than on the eastern. On the eastern side glaciers barely came down to
Lake Tahoe and Lake Mono in California. The State of Nevada seems to
have been entirely free from glaciers, although it contains numerous
mountain-peaks more than ten thousand feet high. In the Yosemite Cañon
glaciers extended down the Merced River to the mouth of the cañon; while
in the Tuolumne River, a few miles to the north, the glaciers which still
linger about the peaks of Mount Dana filled the valley for a distance of
forty miles.
It is a question among geologists whether or not the glaciation west
of the Rocky Mountains was contemporaneous with that of the eastern
part of the continent. The more prevalent opinion among those who have
made special study of the phenomena is that the development of the
Cordilleran glaciers was independent of that of the Laurentide system.
At any rate, the intense glaciation of the Pacific coast seems to have
been considerably later than that of the Atlantic region. Of this we will
speak more particularly in discussing the questions of the date and the
cause of the Glacial period. It is sufficient for us here simply to say
that, from his extensive field observations throughout the Cordilleran
region, Dr. George M. Dawson infers that there have been several
successive alternations of level on the Pacific coast corresponding to
successive glacial and interglacial epochs, and that when there was a
period of elevation west of the Rocky Mountains there was a period of
subsidence to the east, and _vice versa_. In short, he supposes that the
east and west for a long time played a game of seesaw, with the Rocky
Mountains as the fulcrum. We give his scheme in tabulated form.
_Scheme of Correlation of the Phenomena of the Glacial Period in the
Cordilleran Region and in the Region of the Great Plains._
CORDILLERAN REGION. REGION OF THE GREAT PLAINS.
Cordilleran zone at a high Correlative subsidence and
elevation. Period of most severe submergence of the great plains,
glaciation and maximum development with possible contemporaneous
of the great Cordilleran Glacier. increased elevation of the
Laurentian axis and maximum
development of ice upon it.
Deposition of the lower
boulder-clay of the plains.
Gradual subsidence of the Correlative elevation of the
Cordilleran region and decay of the western part, at least, of the
great glacier, with deposition of great plains, which was probably
the boulder-clay of the interior more or less irregular and led to
plateau and the Yukon basin, of the the production of extensive lakes
lower boulder-clay of the littoral in which interglacial deposits,
and probably also, at a later stage including peat, were formed.
(and with greater submergence), of
the interglacial silts of the same
region.
Re-elevation of the Cordilleran Correlative subsidence of the
region to a level probably as high plains, which (at least in the
as or somewhat higher than the western part of the region)
present. Maximum of second period exceeded the first subsidence and
of glaciation. extended submergence to the base
of the Rocky Mountains near the
forty-ninth parallel. Formation of
second boulder-clay, and (at a
later stage) dispersion of large
erratics.
Partial subsidence of the Correlative elevation of the
Cordilleran region, to a level plains, or at least of their
about 2,500 feet lower than the western portion, resulting in a
present. Long stage of stability. condition of equilibrium as
Glaciers of the second period between the plains and the
considerably reduced. Upper Cordillera, their _relative_
boulder-clay of the coast probably levels becoming nearly as at
formed at this time, though perhaps present. Probable formation of the
in part during the second maximum Missouri coteau along a shore-line
of glaciation. during this period of rest.
Renewed elevation of the Simultaneous elevation of the great
Cordilleran region, with one plains to about their present
well-marked pause, during which the level, with final exclusion of
littoral stood about 200 feet lower waters in connection with the sea.
than at present. Glaciers much Lake Agassiz formed and eventually
reduced, and diminishing in drained towards the close of this
consequence of general amelioration period. This simultaneous movement
of climate towards the close of the in elevation of both great areas
Glacial period. may probably have been connected
with a more general northern
elevation of land at the close of
the Glacial period.
In New Zealand the marks of the Glacial period are unequivocal The
glaciers which now come down from the lofty mountains upon the South
Island of New Zealand to within a few hundred feet of the sea then
descended to the sea-level. The longest existing glacier in New Zealand
is sixteen miles, but formerly one of them had a length of seventy-eight
miles. One of the ancient moraines contains a boulder from thirty to
forty feet in diameter, and the amount of glacial _débris_ covering the
mountain-sides is said to be enormous. Reports have also been recently
brought of signs of ancient glaciers in Australia.
[Illustration: Fig. 38.--Generalised view of the whole glaciated region
of North America. The area of motionless ground-ice is shown by the white
lines in northern part of Alaska.]
According to Darwin, there are distinct signs of glaciation upon the
plains of Patagonia sixty or seventy miles east of the foot of the
mountains, and in the Straits of Magellan he found great masses of
unstratified glacial material containing boulders which were at least
one hundred and thirty miles away from their parent rock; while upon
the island of Chiloe he found embedded in "hardened mud" boulders which
must have come from the mountain-chains of the continent. Agassiz also
observed unquestionable glacial phenomena on various parts of the Fuegian
coast, and indeed everywhere on the continent south of latitude 37°.
Between Concepcion and Arauco, in latitude 37°, Agassiz observed, near
the sea-level, a glacial surface well marked with furrows and scratches,
and as well preserved, he says, "as any he had seen under the glaciers of
the present day."
[Illustration: Fig. 39.--Quartzite boulder of 45 cubic metres, on Mont
Lachat, 800 metres above the valley of the Belley, in Ain, France
(Falsan).]