It is of some importance, both to the practical work of the field
and the theoretical deductions of the study, to determine the nature
and amount of the drift that was carried forward in the body of the
ancient continental glaciers, and brought out on their terminal slopes
and at length deposited at their frontal edges, and to distinguish it
from that which was pushed or dragged or rolled along at the bottom of
the ice.[10] It may be helpful to indulge in a speculative discussion
at the outset to prepare the way for the specific evidence and the
inferences to which it leads.
[10] Debris, which may be imbedded in the basal layer of the ice
during some part of its transportation, but which is brought down to
the bottom and subjected to basal action in the latter part of its
course, and ultimately becomes a part of the basal deposit, is not here
included in the englacial drift.
Whenever a prominence of rock is overridden and enveloped by a
glacier of the free-moving continental type, one of two things takes
place; either that part of the ice which passes over the summit of
the prominence flows down its lee slope, carrying whatever debris it
dislodges down to the rear base, and thence onward along the bottom
of the ice, or else the currents which pass on either side of the
prominence close in behind it before the corresponding current which
passes over the summit reaches the point of their junction, in which
case the summit current is forced to pass off more nearly horizontally
into the body of the ice, carrying with it whatsoever debris it has
dislodged from the summit of the prominence and embodied within its
base. The law of the phenomena appears to be that whenever the height
of the prominence is less than one-half the base, measured transversely
to the movement of the ice, the summit current will follow down the lee
slope; but whenever the height of the prominence is more than one-half
the transverse base, the lateral currents will close in on the lee,
and the summit current will flow off into the body of the ice. This
simple law is, however, subject to very considerable modifications from
several different sources which may be grouped under (1) differences
in the friction arising from basal contact, and (2) differences of
internal friction and mobility. The lateral currents will expose more
surface to the sides and base of the hill and the adjoining plain, and
will be more subject to conflicting currents, while, on the other hand,
being deeper currents, they will presumably be more fluent. These and
other qualifying conditions will go far to vitiate the application
of the law, but its statement may have some value as representing a
general conception of the phenomena. When the height of the prominence
becomes great relative to the total thickness of the ice, the fluency
of the summit current may be much reduced relative to that of the
central parts of the lateral currents. When the prominence reaches the
surface, blocks dislodged from it are borne away on the surface of the
glacier, and constitute superglacial drift. Blocks dislodged from near
the summit, but below the surface of the ice, are presumably carried
onward in the upper zone of the glacier; while other blocks detached
at various but sufficient heights on the side of the prominence are
doubtless borne around into the lee and carried forward in the same
vertical plane as the summit stream, so that there comes to be a
vertical zone set with boulders moving on from the lee side of the
nunatak.
Lofty ledges or plateaus, with vertical or undercut faces, furnish
similar means for the lodgment of debris within the body of the ice.
In these and doubtless in other ways it appears that there came to be
lodged directly within the body of the Pleistocene glaciers at some
considerable distances above their bases, blocks derived from rock
prominences that rose with sufficient steepness above the general
surface of the country over which the ice passed. The lodgment of
debris on the lateral borders of glaciers is neglected here because
it has little or no applicability to the phenomena of the upper
Mississippi basin. It is also doubtful whether any prominences
protruded through the ice except near the thin edge, when advancing and
retreating, and these are too inconsiderable to merit attention.
It is obvious, upon consideration, that blocks detached from summits
or from the sharp angles of out-jutting ledges or plateaus might
suffer some glacial abrasion in the process of their dislodgment and
transposition along the crest or projecting angle, but that in general
such abrasion would be small, and, in most cases, nearly or quite
absent. The debris so incorporated in the body of the ice would be,
for the most part, angular, and, as it was brought forward in the ice,
it would probably suffer very little abrasion. If it continued to move
forward in the plane in which it started, descending only so much as
the bottom wastage of the ice required, it would be brought out to
the terminal slope of the ice sheet by virtue of the melting away of
the ice above, and thence it would be carried on down the terminal
slope as superglacial debris, and dropped at the frontal edge. If this
be the true and full history, there would be no commingling of this
englacial matter with the subglacial debris. It is evident, that the
englacial matter brought forward from the crest of one prominence would
be intermingled with that brought forward from other prominences lying
in a line with it, or lying so near it that the lateral spreading of
the debris would lead to commingling. It is also clear that variations
in the direction of currents would tend to the same result, so that
englacial matter from different prominences of the same general region
might be commingled. So also englacial material, by crevassing and by
the descent of streams from the surface to the base, would be carried
down to the bottom and mingled with the subglacial debris. So also
blocks broken away from the base of the prominence which yielded the
englacial erratics might be moved forward along the bottom parallel
with the englacial material above, and lodged at any point along the
line. It is therefore to be expected that the basal deposits will
contain the same rock species as the englacial, but if there be no
process by which the basal material is carried upward the reverse will
not be the case, and there will be a clear distinction between the
englacial deposit and the subglacial deposit, in composition as well as
physical state.
Not a few glacialists, however, advocate in somewhat differing forms
and phases the doctrine that basal material is carried upward into
the body of the glacier and at length reaches the surface, and that
at the extremity of the ice this is commingled with any erratics that
may be englacial or superglacial by original derivation. This doctrine
appears to have had its origin in the endeavor to explain the very
common fact that glacial drift has been carried from lower to higher
altitudes. Erratics are often found lodged several hundred feet higher
than the outcrop from which they were derived. It has never seemed to
me, however, that this phenomenon necessarily was different in kind
from that which takes place in the bottom of every stream; at least I
have not come in contact with any instances that seemed to require a
different explanation, except those connected with kames and eskers
that require a special explanation in any case. We are so accustomed
to view streams from above, and so accustomed to study the extinct
glaciers from the bottom, that we are liable to overlook the community
of some of the simpler processes involved alike in both phenomena. The
dictum that water never runs up hill is measurably true of the surface
currents of the ice as well as water, but it altogether fails when
applied to the basal currents of either. It is probable that there is
no natural stream of any length in which, at some part of its course,
basal debris is not carried from lower to higher altitudes and lodged
there. If the bed of any stream were made dry and the debris in it
critically examined, it would be found that at numerous points the
silts or sands or gravels had been carried from the bottom of some
basin in its bed to the higher rim or bar or reef that bordered it
on the downstream side. So I conceive that, on a grander scale, the
natural result of the flow of the basal ice of a continental glacier
over the inequalities of the country was the lifting of material from
some of the lower horizons and its lodgment on the crests of ridges or
the slopes or summits of mountains that lay athwart its course.
So again, it is certain that a considerable part of the peripheral
drainage of glaciers takes place through tunnels beneath the ice.
It is reasonable to suppose that during the winter season, when the
drainage is slack, these tunnels tend to collapse in greater or less
degree, under the continued pressure of the ice and the "fattening" of
the glacier, so that in the early part of the next melting season the
contracted tunnels may be over-flooded by glacial waters. To the extent
that these tunnels become incompetent the water would become ponded
back in the crevasses and moulins by which the surface-water gains
access to them. They thus come to have something of the force of water
flowing in tubes, and may be presumed to be capable of forcing rounded
material to some considerable height, and of carrying ice-imbedded
boulders to any point reached by the stream. These tunnels probably
undulate with the bottom, and lodgment along them takes place wherever
enlargement permits.
Without, therefore, appealing to any upward cross currents within
the ice itself, it is possible to explain the transportation of the
drift from lower to higher altitudes. I have never seen phenomena of
this kind that seemed to call for any other explanation than these. I
am not prepared to say that there are no such phenomena. One of the
purposes of this article will have been accomplished, if it shall call
forth a critical statement of phenomena that require the assumption
of internal upward movements of the ice to account for them, and of
the criteria which distinguish such phenomena from those that may be
referred to upward basal movements such as are common to all streams
or to the exceptionally conditioned subglacial streams. That there are
upward internal movements in most streams is as much beyond question
as the existence of upward basal currents in rivers and glaciers, but
they are dependent chiefly upon the velocity of the current and the
irregularity of the bottom. Theoretically, as I understand, a stream
moving in a straight course on a perfectly smooth bottom would not
develop an upward cross current. Each lower layer would move slower
than that above it by reason of basal friction, but they would move
on in parallel lines. But if irregularity of bottom be introduced the
parallelism is obviously destroyed, and if the velocity be high so that
the momentum of the particles becomes great relative to their cohesion,
irregular internal movements will result, and these will often be of a
rotary nature in vertical planes bringing the basal parts of the fluid
to the surface or the reverse. For this reason rapid streams abound in
rotary currents, while slow streams do not.
Now it is quite obvious that a stream of water moving at a rate of
three or four feet per day, or even fifty or sixty feet per day, would
not develop perceptible upward currents, and certainly would not
lift the lightest silt from its bottom. I do not think there are any
theoretical grounds for believing that internal glacial currents are
developed, which flow from base to surface, carrying bottom debris to
the top.
One of the most remarkable expressions of the drift phenomena of the
Upper Mississippi region consists of belts of boulders stretching
for great distances over the face of the country, and disposing
themselves in great loops after the fashion of the terminal moraines
of the region with which they are intimately connected. Besides this,
there are numerous patches of boulders of more or less irregular form
and uncertain relations. The whole of these have not been studied
in detail, but a sufficient portion of them have received careful
examination to justify the drawing of certain conclusions from them.
Those which have been most studied lie in Ohio, Indiana, Illinois,
Michigan, Wisconsin, Iowa and Dakota. Those of the first three States
have been most carefully traced and their constitution is such as to
give them the greatest discriminative value. To these our discussion
will be limited chiefly.[11]
[11] Parts of these tracts were long since described by Bradley of
the Illinois Survey. (Geol. Surv. Ill., Vol. IV. p. 227). Collet of
the Indiana Survey (An. Rep. 1875, p. 404) and Orton and Hussey of
the Ohio Survey (Geol. Surv. Ohio, Vol. III., pp. 412, 414 and 475).
The relationship of these tracts to morainic lines and to each other
I worked out some years since (Third An. Rep. U. S. G. S. pp. 331,
332, 334) but I owe many details and some important additions to my
associate, Mr. Leverett.
Emerging from the dunes at a point north of the Iroquois river in
Jasper county, northwestern Indiana, a well characterized belt of
surface boulders stretches westward to the State line, just beyond
which it curves about to the south and then to the east, and re-enters
Indiana a little south of the northwest corner of Benton county. It
soon turns abruptly to the south and reaches the Wabash river near the
centre of Warren county. The immediate valley of the Wabash is thickly
strewn with boulders from the point where the belt reaches it to the
vicinity of West Point on the western line of Tippecanoe county. The
uplands, however, do not give any clear indication of the continuity
of the belt, and the connection is not altogether certain. There is an
inner well-marked belt that branches away from this in the central part
of Benton county and runs southeasterly into the northwestern quarter
of Tippecanoe county, beyond which only scattered boulders occur, which
leaves its precise connections also in doubt. But starting from West
Point, which is less than a dozen miles from the point where the two
belts cease to be traceable with certainty, a well-defined belt, one
or two miles wide, runs southeasterly across the southwestern corner
of Tippecanoe county and the northeastern quarter of Montgomery county
to the vicinity of Darlington, beyond which its connection is again
obscure, although boulders occur frequently between this point and the
northwestern corner of Brown county, where boulders are very abundant.
So also, patches of exceptionally abundant boulders occur in the west
central part of Clinton county. These may be entitled to be regarded
as a connecting link between the train which enters northwestern
Tippecanoe county and that of northwestern Boone county, as scattered
boulders of the surface type, but of not very exceptionally frequent
occurrence, lie between them. However this may be, a belt of much
more than usually frequent surface boulders stretches southeasterly
to the vicinity of Indianapolis, and probably connects with a very
well-marked belt lying near the south line of the southeast quarter
of Marion county and in the northeastern part of Johnson county.
There is also a well-defined tract in southeastern Hendricks county,
running east and west, without evident connection with the foregoing
tracts, though it may be the equivalent of the Darlington belt. There
is also a somewhat unusual aggregation in the form of irregular belts
in southeastern Johnson county, in the vicinity of Nineveh, and in
southern Shelby county. The belt south of Indianapolis is probably to
be correlated by scattered boulders only slightly more abundant than
those of the adjacent region, but of the surface type, stretching
northeasterly to near the center of the west half of Henry county,
where a well-marked belt again sets in. From this point the tract runs
northeasterly nearly to the north limit of the county, where it turns
easterly and runs in the vicinity of the line between Randolph and
Wayne counties to near the Ohio line, where it curves to the southeast
entering Ohio near the northwest corner of Preble county. In its
southeasterly course across that county it is phenomenally developed as
has been well shown by the descriptions of Professor Orton. Soon after
entering Montgomery county it curves about to a northeasterly course,
and crossing the great Miami river, a few miles above Dayton, holds
its northeast course across the southeastern part of Miami county,
the northwestern part of Champaign county, and thence on to about the
center of Logan county, where it curves about and runs in a direction a
little east of south to near the southeast corner of Champaign county,
beyond which it ceases to be a specially notable phenomenon.
In the region between the Wabash and Kankakee rivers, in northern
Indiana, there are numerous tracts of irregular form over which surface
boulders in phenomenal abundance are scattered. These are particularly
noticeable in southern Jasper county; in the vicinity of Wolcott, Monon
and Chalmers in White county; near Star City in Pulaski county; in
the southeastern corner of Stark county, and very generally along the
great interlobate moraines, lying parallel with the Eel river, and
some others of the Saginaw glacial lobe. These are so associated with
the inter-tangled morainic phenomena of that region as not to admit of
convenient and brief description in their genetic relationships.
The well-defined tracts have a most significant distribution. The first
part described is associated with the terminal moraine that marked
the margin of a lobe of ice that moved westward along the axis of the
Iroquois basin to a point a few miles beyond the Indiana-Illinois line.
The portion that runs southward to the Wabash is associated with the
moraine that follows the same course, and runs at right angles over the
older moraines of the Lake Michigan lobe. The tract in Tippecanoe and
Montgomery counties, that in south Marion county, and that in Henry and
Randolph counties, in the eastern part of the state, are associated
with the terminal moraines that form a broad loop with the West White
river basin lying in its axis. In western Ohio the belt is intimately
associated with a moraine that bordered the Miami lobe of the ice
sheet, and the south-trending portion in eastern Logan and Champaign
counties lies on the western margin of the Scioto lobe.
The relationship of these tracts to terminal moraines is very clear and
specific. They constitute marginal phenomena of the ancient ice sheet.
Their distribution completely excludes their reference to floating ice,
for they not only undulate over the surface utterly negligent of any
horizontal distribution, but they are disposed in loops in crossing the
basins of the region, and the convexities of these loops are turned
down stream. These basins for the most part open out in southerly or
westerly directions which makes it improbable that ice-bearing bodies
of water occupied them. But if this were not fatal, certainly the
fact that the convexities of the boulder belts are turned down stream
and cross the centers of the basins is precisely contrary to the
distribution they must have assumed if they were due to floating ice
in bodies of water occupying the basins. I hold it, therefore, to be
beyond rational question that these tracts were deposited as we find
them by the margins of the glacial lobes that invaded the region.
If these boulder belts were of the same nature as the average boulders
of the till-sheets beneath them, then the simple fact of unusual
aggregation might be plausibly referred to the accidents of gathering
and deposition. But they are very clearly distinguished from the
average boulders of the till by several characteristics.
1. They are superficial. Sometimes they rest completely on the surface,
sometimes they are very slightly imbedded, sometimes half buried,
sometimes they protrude but a slight portion, and sometimes they are
entirely concealed, but lie immediately at the surface. In all cases
the aggregation is distinctly superficial. Where they are buried, the
burying material is usually of different texture and composition from
the subjacent till, and appears to be distinct in origin from it. The
superficiality of the tract is very obvious almost everywhere, and is
especially so in regions where the subjacent till is of the pebble-clay
rather than boulder-clay order, for the comparative absence of boulders
below emphasizes the contrast. Throughout most of the region the
subjacent till is not of a very bouldery type, so that the distinction
is generally a marked one.
2. The boulders of the belts are almost without exception derivatives
from the crystalline terranes of Canada. Those of the great tract
especially under consideration were derived from the typical Huronian
rocks of the region north of Lake Huron, and from granitic and
gneissoid rocks referable to the Laurentian series of the same region.
These last, however, cannot be sharply distinguished from the granitic
rocks derived from other parts of the Laurentian terrane. The Huronian
rocks are very easily identified because of the peculiarities of some
of the species. Among these the one most conspicuously characterized
is a quartz-and-jasper conglomerate. The matrix is usually a whitish
quartzite. This is studded with pebbles of typical red jasper and of
duller rocks of jasperoid nature, which grade thence into typical
quartzite pebbles. With these are mingled crystalline pebbles of other
varieties. Another peculiar erratic comes from the "slate conglomerate"
of Logan. It consists of a slaty matrix through which are scattered
rather distantly pebbles of granitic, quartzitic and other crystalline
rocks. This is one of the forms of the "basal conglomerate" of Irving.
Other varieties of this "basal conglomerate" are present. In addition
to these very peculiar rocks, a quartzite of a very light greenish
semi-translucent hue has a wide distribution along the tract. It is
readily distinguishable from the numerous other quartzites of the drift
of the interior. Some years since, on returning from my first field
examination of a portion of this belt, I sent a typical series of chips
from the characteristic erratics to Professor Irving, who had recently
returned from the study of the original Huronian region. He returned a
suite of chippings that matched them perfectly throughout, all of which
were taken _in situ_ in the region north of Lake Huron.
Among the boulders of the belt are occasionally found specimens of
impure limestone or of limy sandstone that might perhaps be referred
doubtfully to some member of the paleozoic series; but on the other
hand, might with equal or greater probability perhaps be referred to
the similar rocks of the Huronian series. These are quite rare, never
forming, so far as my observations go, as much as one per cent. of
the series. In the several definite enumerations made to determine
the percentage of the doubtful specimens, the result never exceeded
a fraction of one per cent. In the most extensive enumeration the
result was about one-half of one per cent. Aside from these doubtful
specimens there are practically no boulders in the belts that can be
referred to any of the paleozoic rocks that intervene in the 500 miles
between the parent series north of Lake Huron and the tract over which
the boulders are now strewn. Occasionally there may be seen erratics
from the paleozoic series at or near the surface, but they are not
usually so disposed on the surface as to appear to be true members of
the superficial boulder tract. There is, therefore, the amplest ground
for the assertion that these boulder tracts are of distant derivation,
and that they are essentially uncommingled with derivatives from the
intermediate region.
3. The boulders of this series are much more angular than those of
the typical till sheets. Some of them, indeed, are rounded, but the
rounding is generally of the type which boulders derived by surface
degradation and exfoliation present. They rarely have the forms that
are distinctively glacial. Quite a large percentage are notably
angular, and have neither suffered glacial rounding nor spherical
exfoliation. Some few are glacially worn and scratched, but the
percentage of these is small.
The tracts therefore present these four salient characteristics: (1)
the boulders are derived from distant crystalline terranes (400 to 500
miles) and are essentially uncommingled with rock from the intervening
paleozoic terranes; (2) they are essentially superficial, and the
associated earthy material has a texture differing from that of the
subglacial tills; (3) they are notably angular and free from glacial
abrasion, except in minor degree; (4) the tracts are so associated with
terminal moraines and so related to the topography of the region, that
there is no rational ground for doubt that the boulders were borne to
their present places by the glaciers that produced the correlative
moraines.
In contrast to these superficial boulder formations, the till sheets
below are made up of a very large percentage of glacial clay whose
constitution shows that it was produced in part by the grinding down
of the paleozoic series. In this are imbedded boulders and pebbles
that were derived from the paleozoic series as indicated by their
petrological character, and, in many instances, demonstrated by
contained fossils. While a small part of the boulders contained in the
till are angular or but slightly worn, the larger part are blunted,
bruised, scratched and polished by typical glacial action. This obvious
grinding of the boulders, taken in connection with the clay product
resulting from the grinding, affords a clear demonstration that the
deposit was produced at the base of the ice by its pushing, dragging,
rolling action.
The two formations, therefore, stand in sharp contrast; the one
indicating the passive transporting action of the ice in bearing from
their distant homes north of the lakes the crystalline boulders and
dropping them quietly on the surface, the other indicating the active
dynamic function of the ice in rubbing, bruising and scoring the
material at its base. The one seems to me a clear instance of englacial
and superglacial transportation; the other an equally clear example of
subglacial push, drag and kneading.
Now if it were the habit of an ice-sheet of this kind to carry material
from its bottom to the surface by internal movement, it would seem
that the distance of 400 to 500 miles which intervened between the
source of the crystallines and the place of their deposit would have
furnished ample opportunity for its exercise, and that there would
have been commingled with the englacial and superglacial material many
derivatives from the intermediate region, and these derivatives should
have borne the characteristic markings received by them while at the
base of the ice. The very conspicuous absence of such commingling,
and the absence or phenomenal rarity of anything that even looks like
such a commingling, appears to me to testify in quite unmistakable
terms to the distinctness of the methods of transportation. In view
of the great territory over which this particular belt is spread,
and the greater territory which is embraced in the other tracts not
here specially considered, there is left little ground for doubt
that this distinctness of englacial from basal transportation was
a prevailing fact and not an exceptional one. This is supported
by concurrent evidence derived from the territory west of Lake
Michigan. This territory unfortunately does not bear erratics that
have equally distinct characteristics, but, so far as my observation
goes, the phenomena are alike throughout. I am therefore brought to
the conclusion that, in the interior at least, there was no habitual
lifting of boulders from the base of the ice sheets to the surface,
nor any habitual commingling of basal with englacial and superglacial
material, except, of course, as it took place by virtue of the falling
of the latter through crevasses to the base, and by mechanical
intermixture of the two at the edge of the ice.
The amount of englacial till under this view is little more than that
which was lodged in the body of the ice in its passage over the knobs
and ridges of the hilly and semi-mountainous regions of the north. To
this is perhaps to be added occasional derivatives from the more abrupt
prominences of the paleozoic region and the superficial dust blown upon
the ice from the surrounding land, which was probably the chief source
of the silty material intermingled with the superficial boulders. The
total amount is thus quite small, though important in its significance.
The eskers and kames of the region are made up of derivatives from
the basal material as shown by (1) the local origin of the material
in large part, (2) the mechanical origin of the sands and silts, (3)
the not infrequent glacial markings of the pebbles and boulders, and
(4) the disturbed stratification of the beds.[12] If I am correct
in respect to the kind and amount of the englacial and superglacial
material, it is obvious that eskers and kames, such as are found in the
interior, could not be derived from englacial or superglacial sources.
The term englacial as here used does not include such materials as
may be lodged in the basal stratum of the ice and brought down to the
actual bottom by basal melting.
[12] See "Hillocks of Angular Gravel and Disturbed Stratification," Am.
Jour. Sci. Vol. XXVII., May 1884, pp. 378-390.
The conclusions drawn from the phenomena of the plains of the interior
are not necessarily applicable to more hilly or mountainous regions.
=T. C. Chamberlin.=
=_Studies for Students._=