THE CONTINENTAL GLACIERS OF THE “ICE AGE”
=Earlier cycles of glaciation.=—Our study of the rocks composing the
outermost shell of the lithosphere tells us that in at least three
widely separated periods of its history the earth has passed through
cycles of glaciation during which considerable portions of its surface
have been submerged beneath continental glaciers. The latest of these
occurred in the yesterday of geology and has often been referred to as
the “ice age”, because until quite recently it was supposed to be the
only one of which a record was preserved.
[Illustration: FIG. 322.—Map of the globe showing the areas which were
covered by the continental glaciers of the so-called “ice-age” of the
Pleistocene period. The arrows show the directions of the centrifugal
air currents in the fixed anticyclones above the glaciers.]
[Illustration:
FIG. 323.—Glaciated granite bowlder which has weathered out of a
moraine of Permo-Carboniferous age upon which it rests. South Australia
(after Howchin).]
This latest ice age represents four complete cycles of glaciation, for
it is believed that the continental ice developed and then completely
disappeared during a period of mild climate before the next glacier
had formed in its place, and that this alternation of climates was
no less than three times repeated, making four cycles in all. At
nearly or quite the same time ice masses developed in northern North
America and in northern Europe, the embossments of the ice domes being
located in Canada and in Scandinavia respectively (Fig. 322). There
appears to have been at this time no extensive glaciation of the
southern hemisphere, though in the next earlier of the known great
periods of glaciation—the so-called Permo-Carboniferous—it was the
southern hemisphere, and not the northern, that was affected (Fig.
323 and Fig. 304, p. 276). From the still earlier glacial period our
data are naturally much more meager, but it seems probable that it
was characterized by glaciated areas within both the northern and the
southern hemispheres.
[Illustration: FIG. 324.—Map to show the glaciated and nonglaciated
regions of North America (after Salisbury and Atwood).]
=Contrast of the glaciated and nonglaciated regions.=—Since we have
now studied in brief outline the characteristics of the existing
continental glaciers, we are in a position to review the evidences of
former glaciers, the records of which exist in their carvings, their
gravings, and their deposits.
[Illustration:
FIG. 325.—Map of the glaciated and nonglaciated areas of northern
Europe. The strongly marked morainal belts respectively south and north
of the Baltic depression represent halting places in the retreat of the
latest continental glacier (compiled from maps by Penck and Leverett).]
An observant person familiar with the aspects of Nature in both the
northern and southern portions of the central and eastern United States
must have noticed that the general courses of the Ohio and Missouri
rivers define a somewhat marked common border of areas which in most
respects are sharply contrasted (Fig. 324). Hardly less striking is the
contrast between the glaciated and the nonglaciated regions upon the
continent of Europe (Fig. 325).
It is the northern of the two areas which in each case reveals
the characteristic evidences of glaciation, while there is entire
absence of such marks to the southward of the common border. Within
the American glaciated region there is, however, an area surrounded
like an island, and within this district (Fig. 324) none of the marks
characteristic of glaciation are to be found. This area is usually
referred to as the “driftless area”, and occupies portions of the
states of Wisconsin, Illinois, Minnesota, and Iowa. Even better than
the area to the southward of the Ohio and Missouri rivers, it permits
of a comparison of the nonglaciated with the drift-covered region.
[Illustration:
FIG. 326.—“Stand Rock” near the “Dells” of the Wisconsin river, an
unstable erosion remnant characteristic of the driftless area of North
America (after Salisbury and Atwood).]
=The “driftless area.”=—Within this district, then, we have preserved
for our study a landscape which remains largely as it was before
the several ice invasions had so profoundly transformed the general
surface of the surrounding country. Speaking broadly, we may say that
it represents an uplifted and in part dissected plain, which to the
south and east particularly reveals the character of nearly mature
river erosion (Fig. 177, p. 170). The rock surface is here everywhere
mantled by decomposed and disintegrated rock residues of local origin.
The soluble constituents of the rock, such as the carbonates, have
been removed by the process of leaching, so that the clays no longer
effervesce when treated with dilute mineral acid.
Wherever favored by joints and by an alternation of harder and softer
rock layers, picturesque unstable erosion remnants or “chimneys” may
stand out in relief (Fig. 326). Furthermore, the driftless area is
throughout perfectly drained—it is without lakes or swamps—since
all valleys are characterized throughout by forward grades. The side
valleys enter the main valleys as do the branches a tree trunk; in
other words, the drainage is described as arborescent. In so far as any
portions of a plane surface now remain in the landscape, they are found
at the highest levels (plate 16 A). The topography is thus the result
of a partial removal by erosion of an upland and may be described as
_incised topography_. Nowhere within the area are there found rock
masses foreign to the region, but all mantle rock is the weathered
product of the underlying ledges.
┌────────────────────────────────────────────────────────────────────┐
│ PLATE 16. │
│ │
│ [Illustration: _A._ Incised topography within the “driftless area” │
│ (U. S. Geol. Survey).] │
│ │
│ [Illustration: _B._ Built-up topography within glaciated region │
│ (U. S. Geol. Survey).] │
└────────────────────────────────────────────────────────────────────┘
=Characteristics of the glaciated regions.=—The topography of the
driftless area has been described as _incised_, because due to the
partial destruction of an uplifted plain; and this surface is,
moreover, perfectly drained. The characteristic topography of the
“drift” areas is by contrast _built up_; that is to say, the features
of the region instead of being _carved_ out of a plain are the result
of _molding_ by the process of deposition (plate 16 B). In so far as a
plane is recognizable, it is to be found not at the highest, but at the
lowest level—a surface represented largely by swamps and lakes—and
above this plain rise the characteristic rounded hills of various types
which have been _built up_ through deposition. The process by which
this has been accomplished is one easy to comprehend. As it invaded the
region, the glacier planed away beneath its marginal zone all weathered
mantle rock and deposited the planings within the hollows of the
surface (Fig. 327). The effect has been to flatten out the preëxisting
irregularities of the surface, and to yield at first a gently
undulating plain upon which are many undrained areas and a haphazard
system of drainage (Fig. 328). All unstable erosion remnants, such as
now are to be found within the driftless area, were the first to be
toppled over by the invading glacier, and in their place there is left
at best only rounded and polished “shoulders” of hard and unweathered
rock—the well-known _roches moutonnées_.
[Illustration:
FIG. 327.—Diagram showing the manner in which a continental glacier
obliterates existing valleys (after Tarr).]
=The glacier gravings.=—The tools with which the glacier works are
never quite evenly edged, and instead of an in all respects perfect
polish upon the rock pavement, there are left furrowings, gougings, and
scratches. Of whatever sort, these scorings indicate the lines of ice
movement and are thus indubitable records graven upon the rock floor.
When mapped over wide areas, a most interesting picture is presented to
our view, and one which supplements in an important way the studies of
existing continental glaciers (Fig. 334, p. 308, and Fig. 336, p. 312).
[Illustration: FIG. 328.—Lake and marsh district in northern
Wisconsin, the effect of glacial deposition in former valleys (after
Fairbanks).]
It has been customary to think of the glacier as everywhere eroding
its bed, although the only warrant for assuming degradation by flow of
the ice is restricted to the marginal zone, since here only is there
an appreciable surface grade likely to induce flow. Both upon the
advance and again during the retreat of a glacier, all parts of the
area overridden must be subjected to this action. Heretofore pictured
in the imagination as enlarged models of Alpine glaciers, the vast
ice mantles were conceived to have spread out over the country as the
result of a kind of viscous flow like that of molasses poured upon
a flat surface in cold weather. The maximum thickness of the latest
American glacier of the ice age has been assumed to have been perhaps
10,000 feet near the summit of its dome in central Labrador. From this
point it was assumed that the ice traveled southward up the northern
slope of the Laurentian divide in Canada, and thence to the Ohio river,
a distance of over 1300 miles. If such a mantle of ice be represented
in its natural proportions in vertical section, to cover the distance
from center to margin we may use a line six inches in length, and
only 1/100 of an inch thick. Upon a reduced scale these proportions
are given in Fig. 329. Obviously the force of gravity acting within
a viscous mass of such proportions would be incompetent to effect a
transfer of material from the center to the periphery, even though the
thickness should be doubled or trebled. Yet until the fixed glacial
anticyclone above the glacier had been proven and its efficiency as a
broom recognized, no other hypothesis than that of viscous flow had
been offered in explanation. The inherited conception of a universal
plucking and abrasion on the bed of the glacier is thus made untenable
and can be accepted for the marginal portion only.
[Illustration: FIG. 329.—Cross section in approximate natural
proportions of the latest North American continental glacier of
Pleistocene age from its center to its margin.]
Not only do the rock scorings show the lines of ice movement, but
the directions as well may often be read upon the rock. Wherever
there are pronounced irregularities of surface still existing on the
pavement, these are generally found to have gradual slopes upon the
side from which the ice came, and relatively steep falls upon the
lee or “pluck” side. If, however, we consider the irregularities of
smaller size, the unsymmetrical slopes of these protruding portions
of the floor are found to be reversed—it is the steep slope which
faces the oncoming ice and the flatter slope which is upon the lee
side. Such minor projections upon the floor usually have their origin
in some harder nodule which deflects the abrading tools and causes
them to pass, some on the one side and some upon the other. By this
process a staple-shaped groove comes to surround the nodule, leaving an
unsymmetrical elevated ridge within, which is steep upon the stoss side
and slopes gently away to leeward.
[Illustration: FIG. 330.—Limestone surface at Sibley, Michigan.]
=Younger records over older—the glacier palimpsest.=—Many important
historical facts have been recovered from the largely effaced writing
upon ancient palimpsests, or parchments upon which an earlier record
has been intentionally erased to make room for another. In the
gravings upon the glacier pavement, earlier records have been likewise
in large part effaced by later, though in favorable localities the
two may be read together. Thus, as an example, at the great limestone
quarries of Sibley, in southeastern Michigan, the glaciated rock
surface wherever stripped of its drift cover is a smoothly polished and
relatively level floor with striæ which are directed west-northwest.
Beneath this general surface there are, however, a number of elliptical
depressions which have their longer axes directed south-southwest, one
being from twenty-five to thirty feet long and some ten feet in depth
(Fig. 330). These boat-shaped depressions are clearly the remnants of
an earlier more undulating surface which the latest glacier has in
large part planed away, since the bottoms of the depressions are no
less perfectly glaciated but have their striæ directed in general near
the longer axis of the troughs. Palimpsest-like there are here also the
records of more than one graving.
[Illustration:
FIG. 331.—Map to show the outcroppings of peculiar rock types in the
region of the Great Lakes, and some of the localities where “float
copper” has been collected (float copper localities after Salisbury).]
=The dispersion of the drift.=—Long before the “ice age” had been
conceived in the minds of Agassiz and his contemporaries, it had been
remarked that scattered over the North German plain were rounded
fragments of rock which could not possibly have been derived from their
own neighborhood but which could be matched with the great masses of
red granite in Sweden well known as the “Swedish granite.” Buckland, an
English geologist, had in 1815 accounted for such “erratic” blocks of
his own country, here of Scotch granite, by calling in the deluge of
Noah; but in the late thirties of the nineteenth century, Sir Charles
Lyell, with the results of English Arctic explorers in mind, claimed
that such traveled blocks had been transported by icebergs emanating
from the polar regions. A relic of Buckland’s earlier view we have
in the word “diluvium” still occasionally used in Germany for glacier
transported materials; while the term “drift” still remains in common
use to recall Lyell’s iceberg hypothesis, even though the original
meaning of the term has been abandoned. Drift is now a generic term
and refers to all deposits directly or indirectly referable to the
continental glaciers.
In general the place of derivation of the glacial drift may be said to
be some point more distant from and within the former ice margin at the
time when it was deposited; in other words, the dispersion of the drift
was centrifugal with reference to the glacier.
[Illustration:
FIG. 332.—Map of the “bowlder train” from Iron Hill, R. I. (based upon
Shaler’s map, but with the directions of glacial striæ added).]
Wherever rocks of unusual and therefore easily recognizable character
can be shown to occur in place and with but limited areas, the
dispersion of such material is easy to trace. The areas of red Swedish
and Scotch granite have been used to follow out in a broad way the
dispersion of drift over northern Europe. Within the region of the
Great Lakes of North America are areas of limited size which are
occupied by well marked rock types, so that the journeyings of their
fragments with the continental glacier can be mapped with some care.
Upon the northern shore of Georgian Bay occurs the beautiful jasper
conglomerate, whose bright red pebbles in their white quartz field
attract such general notice. At Ishpeming in the northern peninsula of
Michigan is found the equally beautiful jaspilite composed of puckered
alternating layers of black hematite and red jasper. On Keweenaw
Peninsula, which protrudes into Lake Superior from its southern shore,
is found that remarkable occurrence of native copper within a series
of igneous rocks of varied types and colors. Fragments of this copper,
some weighing several hundreds of pounds each and masked in a coat
of green malachite, have under the name of “drift” or “float” copper
been collected at many localities within a broad “fan” of dispersal
extending almost to the very limits of glaciation (Fig. 331).
Some miles to the north of Providence in Rhode Island there is a hill
known as Iron Hill composed in large part of black magnetite rock,
the so-called Cumberlandite. From this hill as an apex there has
been dispersed a great quantity of the rock distributed as a well
marked “bowlder train” within which the size and the frequency of the
dispersed bowlders is in inverse ratio to the distance from the parent
ledge (Fig. 332). Similar though less perfect trains of bowlders are
found on the lee side of most projecting masses of resistant rocks
within the area of the drift.
Large bowlders when left upon a ledge of notably different appearance
easily attract attention, and have been described as “perched
bowlders.” Resting as they sometimes do upon a relatively small area,
they may be nicely balanced and thus easily given a pendular or rocking
motion. Such “rocking stones” are common enough, especially among the
New England hills (plate 17 B). Many such bowlders have made somewhat
remarkable peregrinations with many interruptions, having been carried
first in one direction by an earlier glacier to be later transported in
wholly different directions at the time of new ice invasions.
┌───────────────────────────────────────────────────────────────────┐
│ PLATE 17. │
│ │
│ [Illustration: _A._ Soled glacial bowlders which show differently │
│ directed striæ upon the same facet.] │
│ │
│ [Illustration: _B._ Perched bowlder upon a striated ledge of │
│ different rock type, Bronx Park, New York (after Lungstedt).] │
│ │
│ [Illustration: _C._ Characteristic knob and basin surface of a │
│ moraine.] │
└───────────────────────────────────────────────────────────────────┘
=The diamonds of the drift.=—Of considerable popular, even if not
economic, interest are the diamonds which have been sown in the drift
after long and interrupted journeyings with the ice from some unknown
home far to the northward in the wilderness of Canada. The first stone
to be discovered was taken by workmen from a well opening near the
little town of Eagle in Wisconsin in the year 1876. Its nature not
being known, it remained where it was found as a curiosity only, and it
was not until 1883 that it was taken to Milwaukee and sold to a jeweler
equally ignorant of its value, and for the merely nominal sum of one
dollar. Later recognized as a diamond of the unusual weight of sixteen
carats, it was sold to the Tiffanys and became the cause of a long
litigation which did not end until the Supreme Court of Wisconsin had
decided that the Milwaukee jeweler, and not the finder, was entitled to
the price of the stone, since he had been ignorant of its value at the
time of purchase.
[Illustration:
FIG. 333.—Shapes and approximate natural sizes of some of the more
important diamonds from the Great Lakes region of the United States. In
order from left to right these figures represent the Eagle diamond of
sixteen carats, the Saukville diamond of six and one half carats, the
Milford diamond of six carats, the Oregon diamond of four carats, and
the Burlington diamond of a little over two carats.]
An even larger diamond, of twenty-one carats weight, was found at
Kohlsville, and smaller ones at Oregon, Saukville, Burlington, and Plum
Creek in the state of Wisconsin; at Dowagiac in Michigan; at Milford in
Ohio, and in Morgan and Brown counties in Indiana. The appearance of
some of the larger stones in their natural size and shape may be seen
in Fig. 333.
While the number of the diamonds sown in the drift is undoubtedly
large, their dispersion is such that it is little likely they can be
profitably recovered. The distribution of the localities at which
stones have thus far been found is set forth upon Fig. 334. Obviously
those that have been found are the ones of larger size, since these
only attract attention. In 1893, when the finding of the Oregon stone
drew attention to these denizens of the drift, the writer prophesied
that other stones would occasionally be discovered under essentially
the same conditions, and such discoveries are certain to continue in
the future.
[Illustration:
FIG. 334.—Glacial map of a portion of the Great Lakes region,
showing the unglaciated area and the areas of older and newer drift.
The driftless area, the moraines of the later ice invasion, and the
distribution of diamond localities upon the latter are also shown.
With the aid of the directions of striæ some attempt has been made to
indicate the probable tracks of more important diamonds, which tracks
converge in the direction of the Labrador peninsula.]
=Tabulated comparison of the glaciated and nonglaciated regions.=—It
will now be profitable to sum up in parallel columns the contrasted
peculiarities of the glaciated and the unglaciated regions.
UNGLACIATED REGION GLACIATED REGION
TOPOGRAPHY
The topography is _destructional_; The topography is _constructional_;
the remnants of a plain are found the remnants of a plain are found
at the highest levels or upon the at the lowest levels in lakes and
hill tops; hills are _carved_ out swamps; hills are _molded_ above a
of a high plain; unstable erosion plain in characteristic forms; no
remnants are characteristic. unstable erosion remnants, but only
rounded shoulders of rock.
DRAINAGE
The area is completely drained, The area includes undrained
and the drainage network is areas,—lakes and swamps,—and
_arborescent_. the drainage system is _haphazard_.
ROCK MANTLE
The exposed rock is decomposed No decomposed or disintegrated
and disintegrated to a rock is “in place”, but only
considerable depth; it is all of hard, fresh surface; loose rock
local derivation and hence of few material is all foreign and of many
types—_homogeneous_; the fragments sizes and types—_heterogeneous_;
are angular; soils are leached and rock bowlders and pebbles are
hence do not contain carbonates. faceted and polished as well as
striated, usually in several
directions upon each facet; soils
are rock flour—the grist of the
glacial mill.
ROCK SURFACE
Rock surface is rough and Rock surface is planed or grooved,
irregular. and polished. Shows glacial striæ.
=Unassorted and assorted drift.=—The drift is of two distinct types;
namely, that deposited directly by the glacier, which is without
stratification, or unassorted; and that deposited by water flowing
either beneath or from the ice, and this like most fluid deposited
material is assorted or stratified. The unassorted material is
described as _till_, or sometimes as “bowlder clay”; the assorted
is sand or gravel, sometimes with small included bowlders, and is
described as _kame gravel_. To recall the parts which both the glacier
and the streams have played in its deposition, all water-deposited
materials in connection with glaciers are called _fluvio-glacial_.
[Illustration: FIG. 335.—Section in coarse till. Note the range in
size of the materials, the lack of stratification, and the “soled” form
of the bowlders.]
Till is, then, characterized by a noteworthy lack of homogeneity, both
as regards the size and the composition of its constituent parts. As
many as twenty different rock types of varied textures and colors may
sometimes be found in a single exposure of this material, and the
entire gamut is run from the finest rock flour upon the one hand to
bowlders whose diameter may be measured in feet (Fig. 335).
In contrast with those derived by ordinary stream action, the pebbles
and bowlders of the till are faceted or “soled”, and usually show
striations upon their faces. If a number of pebbles are examined,
some at least are sure to be found with striations in more than one
direction upon a single facet. As a criterion for the discrimination
of the material this may be an important mark to be made use of to
distinguish in special cases from rock fragments derived by brecciation
and slickensiding and distributed by the torrents of arid and semiarid
regions.
Inasmuch as the capacity of ice for handling large masses is greater
than that of water, assorted drift is in general less coarse, and, as
its name implies, it is also stratified. From ordinary stream gravels,
the kame gravels are distinguished by the form of their pebbles, which
are generally faceted and in some cases striated. In proportion,
however, as the materials are much worked over by the water, the
angles between pebble faces become rounded and the original shapes
considerably masked.
=Features into which the drift is molded.=—Though the preëxisting
valleys were first filled in by drift materials, thus reducing the
accent of the relief, a continuation of the same process resulted in
the superimposition of features of characteristic shapes upon the
imperfectly evened surface of the earlier stages. These features belong
to several different types, according as they were built up outside
of, at and upon, or within the glacier margin. The extra-marginal
deposits are described as _outwash plains_ or _aprons_, or sometimes as
_valley trains_; the marginal are either _moraines_ or _kames_; while
within the border were formed the _till plain_ or _ground moraine_,
and, locally also, the _drumlin_ and the _esker_ or _os_. These
characteristic features are with few exceptions to be found only within
the area covered by the latest of the ice invasions. For the earlier
ones, so much time has now elapsed that the effect of weathering,
wash, and stream erosion has been such that few of the features are
recognizable.
Marginal and extra-marginal features are extended in the direction
of the margin or, in other words, perpendicular to the local ice
movement; while the intra-marginal deposits are as noteworthy for being
perpendicular to the margin, or in correspondence with the direction
of local ice movement. Each of these features possesses characteristic
marks in its form, its size, proportions, surface molding and
orientation, as well as in its constituent materials. It should perhaps
be pointed out that the existing continental glaciers, being in high
latitudes, work upon rock materials which have been subjected to
different weathering processes from those characteristic of temperate
latitudes. Moreover, the melting of the Pleistocene glaciers having
taken place in relatively low latitudes, larger quantities of rock
débris were probably released from the ice during the time of definite
climatic changes, and hence heavier drift accumulations have for both
of these reasons resulted.
=Marginal or “kettle” moraines.=—Wherever for a protracted period the
margin of the glacier was halted, considerable deposits of drift were
built up at the ice margin. These accumulations form, however, not only
about the margin, but upon the ice surface as well; in part due to
materials collected from melting down of the surface, and in part by
the upturning of ice layers near the margin (see _ante_, p. 277).
[Illustration:
FIG. 336.—Sketch map of portions of Michigan, Ohio, and Indiana,
showing the festooned outlines of the moraines about the former ice
lobes, and the directions of ice movement as determined by the striæ
upon the rock pavement (after Leverett).]
An important rôle is played by the thaw water which emerges at the
ice margin, especially within the reëntrants or recesses of the
outline. The materials of moraines are, therefore, till with large
local deposits of kame gravel, and these form in a series of ridges
corresponding to the temporary positions of the ice front. Their width
may range from a few rods to a few miles, their height may reach a
hundred feet or more, and they stretch across the country for distances
of hundreds or even thousands of miles, looped in arcs or scallops
which are always convex outward and which meet in sharp cusps that in
a general way point toward the embossment of the former glacier (Fig.
334, p. 308, and Fig. 336). These festoons of the moraines outline
the ice lobes of the latest ice invasion, which in North America were
centered over the depressions now occupied by the Laurentian lakes.
There was, thus, a Lake Superior lobe, a Lake Michigan lobe, etc. With
the aid of these moraine maps we may thus in imagination picture in
broad lines the frontal contours of the earlier glaciers. At specially
favorable localities where the ice front has crossed a deep valley at
the edge of the Driftless Area, we may, even in a rough way, measure
the slope of the ice face. Thus near Devils Lake in southern Wisconsin
the terminal moraine crosses the former valley of the Wisconsin River,
and in so doing has dropped a distance of about four hundred feet
within the distance of a half mile or thereabouts (Fig. 337).
The characteristic surface of the marginal moraine is responsible for
the name “kettle” moraine so generally applied to it. The “kettles” are
roughly circular, undrained basins which lie among hummocks or knobs,
so that the surface has often been referred to as “knob and basin”
topography (plate 17 C).
[Illustration:
FIG. 337.—Map of the vicinity of Devils Lake, Wisconsin, located
within a reëntrant of the “kettle” moraine upon the margin of the
Driftless Area. The lake lies within an earlier channel of the
Wisconsin River which has been blocked at both ends, first by the
glacier and later by its moraine. The stippled area upon the heights
and next the moraine represents the clay deposits of a former lake
(based on map by Salisbury and Atwood).]
[Illustration: FIG. 338.—Moraine with outwash apron in front, the
latter in part eroded by a river. Westergötland, Sweden (after H.
Munthe).]
[Illustration:
FIG. 339.—Fosse between an outwash plain (in the foreground) and the
moraine, which rises to the left in the middle distance. Ann Arbor,
Michigan.]
=Kames.=—Within reëntrants or recesses of the ice margin the drift
deposits were especially heavy, so that high hills of hummocky surface
have been built up, which are described as _kames_. Most of the higher
drift hills have this origin. They rarely have any principal extension
along a single direction, but are composed in large part of assorted
materials. In contrast with other portions of the morainal ridges they
lack the prominent basins known as kettles. Other _kames_ are high
hills of assorted materials not in direct association with moraines and
believed to have been built up beneath glacier wells or mills (p. 278).
=Outwash plains.=—Upon the outer margin of the moraine is generally
to be found a plain of glacial “outwash” composed of sand or gravel
deposited by the braided streams (Fig. 308, p. 280) flowing from the
glacier margin. Such plains, while notably flat (Fig. 338), slope
gently away from the moraine. Between the outwash plain and the moraine
there is sometimes found a pit, or _fosse_ (Fig. 309, p. 281), where a
part of the ice front was in part buried in its own outwash (Fig. 339).
[Illustration: FIG. 340.—View looking along an esker in southern Maine
(after Stone).]
=Pitted plains and interlobate moraines.=—Where glacial outwash is
concentrated within a long and narrow reëntrant, separating glacial
lobes, strips of high plain are sometimes built up which overtop the
other glacial deposits of the district. The sand and gravel which
compose such plains have a surface which is pitted by numerous deep and
more or less circular lakes, so that the term “pitted plain” has been
applied to them. The surface of such a plain steadily rises toward its
highest point in the angle between the ice lobes. Though consisting
almost entirely of assorted materials, and built up largely without
the ice margins, such gently sloping pitted platforms are described as
_interlobate moraines_. Upon a topographic map the course of such an
interlobate moraine may often be followed by the belts of small pit
lakes (see Fig. 336).
[Illustration:
FIG. 341.—Outline map showing the eskers of Finland trending
southeasterly toward the festooned moraines at the margin of the ice.
The characteristic lakes of a glaciated region appear behind the
moraines (after J. J. Sederholm).]
[Illustration:
FIG. 342.—Small sketch maps showing the relationships in size,
proportions, and orientation of drumlins and eskers in southern
Wisconsin. The eskers are in solid black (after Alden).]
=Eskers.=—Intra-morainal features, or those developed beneath the
glacier but relatively near its margin, include the “serpentine kame”,
_esker_, or, as it is called in Scandinavia, the _os_ (plural _osar_)
(Fig. 340). These diminutive ridges have a width seldom exceeding a
few rods, and a height a few tens of feet at most, but with slightly
sinuous undulations they may be followed for tens or even hundreds of
miles in the general direction of the local ice movement (Fig. 341).
They are composed of poorly stratified, thick-bedded sands, gravels,
and “worked over” materials, and are believed to have been formed by
subglacial rivers which flowed in tunnels beneath the ice. Inasmuch as
the deposits were piled against the ice walls, the beds were disturbed
at the sides when these walls disappeared, and the stratification,
which was somewhat arched in the beginning, has been altered by
sliding at both margins. As already stated, eskers have not a general
distribution within the glaciated area, but are often found in great
numbers at specially favored localities. Formed as they are beneath
the ice, it is believed that many have their materials redistributed
so soon as uncovered at the glacier margin, because of the vigorous
drainage there. They are thus to be found only at those favored
localities where for some reason border drainage is less active, or
where the ice ended in a body of water.
=Drumlins.=—A peculiar type of small hill likewise found behind the
marginal moraine in certain favored districts has the form of an
inverted boat or canoe, the long axis of which is parallel to the
direction of ice movement, as is that of the esker (Fig. 342). Unlike
the esker, this type of hill is composed of till, and from being
found in Ireland it is called a _drumlin_, the Irish word meaning a
little hill (Fig. 343). Drumlins are usually found in groups more or
less radial and not far behind the outermost moraine, to which their
radiating axes are perpendicular. The manner of their formation is
involved in some uncertainty, but it is clear that they have been
formed beneath the margin of the glacier, and have been given their
shape by the last glacier which occupied the district.
The mutual relationships of nearly all the molded features resulting
from continental glaciation may be read from Fig. 344.
[Illustration: FIG. 343.—View of a drumlin, showing an opening in the
till. Near Boston, Massachusetts (after Shaler and Davis).]
=The shelf ice of the ice age.=—Shelf ice, such as we have become
familiar with in Antarctica as a marginal snow-ice terrace floating
upon the sea, no doubt existed during the ice age above the Gulf of
Maine (see Fig. 324, p. 298), and perhaps also over the deep sea to the
westward of Scotland. Though the inland ice probably covered the North
Sea, and upon the American side of the Atlantic the Long Island Sound,
both these basins are so shallow that the ice must have rested upon the
bottom, for neither is of sufficient depth to entirely submerge one of
the higher European cathedrals.
[Illustration:
FIG. 344.—Outline map of the front of the Green Bay lobe of the latest
continental glacier of the United States. Drumlins in solid black,
moraines with diagonal hachure, outwash plains and the till plain or
ground moraine in white (after Alden).]
=Character profiles.=—All surface features referable to continental
glaciers, whether carved in rock or molded from loose materials,
present gently flowing outlines which are convex upward (Fig. 345). The
only definite features carved from rock are the _roches moutonnées_,
with their flattened shoulders, while the hillocks upon moraines and
kames, and the drumlins as well, approximate to the same profile. The
esker in its cross sections is much the same, though its serpentine
extension may offer some variety of curvature when viewed from higher
levels.
[Illustration: FIG. 345.—Character profiles referable to continental
glacier.]
READING REFERENCES FOR CHAPTER XXII
General:—
JAMES GEIKIE. The Great Ice Age. 3d ed. London, 1894, pp. 850, maps 18.
CHAMBERLIN and SALISBURY. Geology, vol. 3, 1906, pp. 327-516.
FRANK LEVERETT. The Illinois Glacial Lobe, Mon. 38, U. S. Geol. Surv.,
1899, pp. 817, pls. 34; Glacial formations and Drainage Features of
the Erie and Ohio Basins, Mon. 41, _ibid._, 1902, pp. 802, pls. 25;
Comparison of North American and European Glacial Deposits, Zeit. f.
Gletscherk., vol. 4, 1910, pp. 241-315, pls. 1-5.
Former glaciations previous to Ice Age:—
A. STRAHAN. The Glacial Phenomena of Paleozoic Age in the Varanger
Fjord, Quart. Jour. Geol. Soc., London, vol. 53, 1897, pp. 137-146,
pls. 8-10.
BAILEY WILLIS and ELIOT BLACKWELDER. Research in China, Pub. 54,
Carnegie Inst. Washington, vol. 1, 1907, pp. 267-269, pls. 37-38.
A. P. COLEMAN. A Lower Huronian Ice Age, Am. Jour. Sci. (4), vol. 23,
1907, pp. 187-192.
W. M. DAVIS. Observations in South Africa, Bull. Geol. Soc. Am., vol.
17, 1906, pp. 377-450, pls. 47-54.
DAVID WHITE. Permo-Carboniferous Climatic Changes in South America,
Jour. Geol., vol. 15, 1907, pp. 615-633.
Driftless and drift areas:—
T. C. CHAMBERLIN and R. D. SALISBURY. Preliminary Paper on the
Driftless Areas of the Upper Mississippi Valley, 6th Ann. Rept. U. S.
Geol. Surv., 1885, pp. 199-322, pls. 23-29.
R. D. SALISBURY. The Drift, its Characteristics and Relationships,
Jour. Geol., vol. 2, 1894, pp. 708-724, 837-851.
R. H. WHITBECK. Contrasts between the Glaciated and the Driftless
Portions of Wisconsin, Bull. Geogr. Soc., Philadelphia, vol. 9, 1911,
pp. 114-123.
Glacier gravings:—
T. C. CHAMBERLIN. The Rock Scorings of the Great Ice Invasions, 7th
Ann. Rept. U. S. Geol. Surv., 1888, pp. 147-248, pl. 8.
The dispersion of the drift:—
R. D. SALISBURY. Notes on the Dispersion of Drift Copper, Trans. Wis.
Acad. Sci., etc., vol. 6, 1886, pp. 42-50, pl.
N. S. SHALER. The Conditions of Erosion beneath Deep Glaciers, based
upon a Study of the Bowlder Train from Iron Hill, Cumberland, Rhode
Island, Bull. Mus. Comp. Zoöl. Harv. Coll., vol. 16, No. 11, 1893, pp.
185-225, pls. 1-4 and map.
WILLIAM H. HOBBS. The Diamond Field of the Great Lakes, Jour. Geol.,
vol. 7, 1899, pp. 375-388, pls. 2 (also Rept. Smithson. Inst., 1901,
pp. 359-366, pls. 1-3).
Glacial features:—
T. C. CHAMBERLIN. Preliminary Paper on the Terminal Moraine of the
Second Glacial Epoch, 3d Ann. Rept. U. S. Geol. Surv., 1883, pp.
291-402, pls. 26-35.
G. H. STONE. Glacial Gravels of Maine and their Associated Deposits,
Mon. 34, U. S. Geol. Surv., 1899, pp. 489, pls. 52.
W. C. ALDEN. The Delaven Lobe of the Lake Michigan Glacier of the
Wisconsin Stage of Glaciation and Associated Phenomena. Prof. Pap.
No. 34, U. S. Geol. Surv., 1904, pp. 106, pls. 15; The Drumlins of
Southeastern Wisconsin, Bull. 273, U. S. Geol. Surv., 1905, pp. 46,
pls. 9.
W. M. DAVIS. Structure and Origin of Glacial Sand Plains, Bull. Geol.
Soc. Am., vol. 1, 1890, pp. 196-202, pl. 3; The Subglacial Origin
of Certain Eskers, Proc. Bost. Soc. Nat. Hist., vol. 35, 1892, pp.
477-499.
F. P. GULLIVER. The Newtonville Sand Plain, Jour. Geol., vol. 1, 1893,
pp. 803-812.