If there have been differences of opinion concerning the nature of ice
epochs, as distinct from each other and from ice periods, there has
been a failure to adequately apprehend the nature, the extent, and the
meaning of the real criteria on which the final recognition of separate
ice epochs, if such there were, must be based.
Such criteria are several in number. They are of unequal value. In some
instances a single one of them might be quite sufficient to establish
the fact of two ice epochs. In other cases, single criteria which might
not be in themselves demonstrative, have great corroborative weight,
when found in association with others. In all cases, much discretion
must be used in the interpretation of these criteria. They may be
enumerated under several specific heads.
(1) _Forest Beds._ Beds of vegetal deposits or old soils are frequently
found between layers of glacial drift. This is one of the criteria
most commonly cited, because it is of common occurrence and easy of
recognition. The advocates of the unity of the glacial period maintain
that such beds of organic matter might become interbedded with morainic
debris during minor oscillations of the ice's edge. The phenomena of
existing glaciers make it evident that forest beds or soils might
be enclosed by the deposits of an oscillating ice edge. By repeated
oscillations of the ice's edge during the general retreat of the ice,
such vegetal beds might become interstratified with glacial drift more
or less frequently over all the area once covered by the ice, and from
which it has now disappeared. The mere presence of vegetable material
between beds of drift is therefore no proof of distinct ice epochs.
This does not destroy the value of the vegetal beds as a criterion for
the recognition of distinct ice epochs, but it makes caution necessary
in its application. It does not follow that, since _some_ inter-drift
forest-beds do not prove interglacial epochs, _none_ do. The question
is not how forest-beds might originate, but how existing forest-beds
did originate.
Where the plant-remains found in the relations indicated are so well
preserved as to make identification of the species possible, we have
a means of determining, with some degree of accuracy, the climatic
conditions which must have obtained at the place where the plants grew
during the time of their life. If these interbedded plant-remains are
of such a character as to indicate a temperate climate, we can not
suppose that they grew at the immediate edge of the ice, and therefore
that they were buried beneath its oscillating margin. To be specific,
if the inter-drift plant remains in any given locality of the area
once covered by ice are such as to indicate a climate _as warm as the
present in the same locality_, the ice must have receded so far to the
northward that its re-advance might, in our judgment, appropriately be
regarded as a separate ice epoch.
It has been suggested in opposition that temperate conditions may
obtain even up to the edge of the ice, and that interbedded vegetal
remains indicating temperate climate do not prove any considerable
recession of the ice. The phenomena about existing glaciers have been
appealed to in support of this demurrer. But the objection is not well
taken. The climatic conditions which obtain about the borders of small,
local glaciers, are not a safe guide as to climatic conditions which
obtained about the margin of a continental ice-sheet, any more than the
climatic conditions which obtain about a small inland lake are a safe
criterion as to the climatic conditions about a sea-coast. The general
principles of climatology, as well as specific facts concerning plant
distribution, seem to us to indicate that the climate about the border
of a continental ice-sheet must have been arctic.
It is evident that the greater the distance north of the overlying
drift remains of temperate plants are found, the more conclusive
becomes the evidence. Plant remains indicating temperate climate at
the very margin of the drift sheet which overlies them, would be less
conclusive than similar evidences one hundred miles to the northward.
It might be difficult to prove in any given instance that the ice which
deposited the drift overlying plant remains advanced one hundred miles,
or any other specific distance, south of any particular underlying
forest bed. If the forest bed were continuous for the whole distance,
the case would be clear. It would also be conclusive if the continuity
of the drift overlying a forest bed at any point with that of a
remote point to the south, could be demonstrated. In spite of these
difficulties in its application, the vegetal beds constitute a valuable
criterion in making the discriminations under consideration, when they
are properly applied. Under proper circumstances the criterion may be
conclusive when taken alone, and it may have corroborative significance
when not itself conclusive.
The absence of forest beds and of all traces of vegetal deposits
whatsoever between beds of drift, is no proof of the absence of
recurrent ice epochs, since the second advance of the ice might have
destroyed all trace of the preëxistent soil and its vegetal life. It
is always possible, too, that such beds exist, even if they have not
been discovered. It would have been anticipated that they would not be
abundant, or wide spread. The absence of forest beds is therefore at
best no more than negative evidence.
(2) _Remains of Land Animals._ Bones of mammalia or remains of other
land animals, occurring in relations similar to those in which forest
beds occur, may have a like significance. Their value as a criterion of
separate glacial epochs is subject to essentially the same limitations
as forest beds.
(3) _Inorganic Products formed during a time of Ice Recession._
The recession of the ice after a maximum of advance would leave
a land surface more or less affected with marshes and ponds. In
such situations, bog iron ore might accumulate, if conditions were
favorable. Such ore beds, buried by the drift of a later ice advance,
would have a significance comparable to that of forest beds, except
that they would give less definite information as to climate, and would
be correspondingly less trustworthy. Should such ore beds be found in
such relations as to prove that the underlying and overlying bodies
of drift were deposited by ice sheets which extended great distances
further south, their significance would be enhanced. From the thickness
of the ore beds some inference might be drawn as to the length of time
concerned in their accumulation. But because of the variable rate
at which bog ore may accumulate, such inference should be used with
caution.
Concretions of iron oxide might be formed in the marshes or in
ill-drained drift areas where accumulations of greater extent were
not made. A subsequent incursion of the ice might incorporate these
nodules with its drift, wearing and striating them as other stones,
and depositing them as constituent parts of the later drift. Such
iron nodules in the later drift would mean a recession and re-advance
of the ice with some considerable interval between, although not
necessarily an interval sufficiently warm or long to be regarded as
an interglacial epoch.[15] Calcareous concretions, like those of the
loess, would possess a like significance, in like relations. While in
themselves these inorganic products of a time of ice recession might
fail to be conclusive of separate ice epochs, they might have much
corroborative significance when associated with other phenomena. An
inter-till iron ore bed, associated with a forest bed which indicated a
warm climate, would be most significant.
[15] This point concerning iron nodules was suggested to the writer by
Mr. W. J. McGee.
The absence of knowledge of ore beds between sheets of till, and the
absence from an upper bed of till of concretions of iron and lime
carbonate formed during a recession of the ice, would be no proof that
interglacial epochs did not occur. These products were probably formed
in relatively few localities. They stood good chance of destruction at
the hands of the returning ice, and they may exist, where they have not
been discovered, or where their significance has not been understood.
Their absence is at best no more than negative evidence.
(4) _Beds of Marine and Lacustrine Origin._ If between beds of glacial
drift there be found beds of lacustrine or of marine origin, such beds
would indicate a recession of the ice during their time of deposition.
Their position would be a minimum measure of ice recession. If such
lacustrine beds contain organic remains, they will bear testimony
concerning the climatic conditions which existed where they occur,
at the time of their deposition. If the fossils in such beds denote
a temperate climate, or a climate as mild as that of the present day
in the same region, the ice must have receded so far to the northward
as, in our judgment, to constitute its re-advance a distinct ice
epoch. This line of argument may be even stronger than that drawn
from remains of terrestrial life, since the ice would probably affect
the temperature of the sea to greater distances than that of the
land, and affect it to a greater degree within a given distance. The
argument becomes stronger the further north the inter-drift marine and
lacustrine deposits occur, since the ice must always have receded to
a position still further north. If marine or lacustrine beds lying far
north of the later ice limit contain proof of temperate climate, the
argument becomes conclusive.
The absence of marine and lacustrine deposits between beds of drift,
would be no proof that interglacial epochs did not occur. Lacustrine
beds could be made only where there were lakes, and lakes would be the
exception rather than the rule. Marine beds in similar positions would
rarely be known, except where a definite succession of changes of level
has taken place. Both classes of deposits, if once formed, would be
subject to destruction by the over-riding ice of a later epoch, if such
there were. Neither would be likely to be preserved at all points where
formed, and both may exist at many points where their existence is
not known. The absence of these beds is at best no more than negative
evidence.
(5) _Beds of Subaërial Gravel, Sand and Silt._ Layers of stratified
drift between layers of ground moraine are of common occurrence in many
regions. Under ordinary conditions their existence is not regarded as
evidence that the underlying and overlying tills are to be referred
to separate ice epochs. But it is conceivable that beds of stratified
drift may, under the proper circumstances and relations, be strong
evidence of separate ice epochs. The last stages of ice work in the
glacial period were accompanied, in many regions, by the deposition
upon adjacent land surfaces, of extensive bodies of gravel and sand,
washed on beyond the ice by waters issuing from it. Except in valleys
through which strong currents coursed, such deposits were apparently
not carried far beyond the edge of the ice. But as the edge of the ice
withdrew to the northward, sand plains may have extended themselves
in the same direction, by additions to their ice-ward faces. It is
conceivable that the process of subaërial plain building at the edge
of a receding phase of ice, might be carried so far under favorable
circumstances, as to result in the construction of plains of great
extent. In this event, a subsequent ice-advance might overspread
such plains in such wise as to bury, without destroying them, though
such a course of events would certainly be exceptional. In order to
constitute the interstratified gravel and sand evidence of separate
ice-epochs, its continuity for great distances between beds of till,
and in the direction of ice movement, would need to be demonstrated.
In themselves, these beds, under the conditions indicated, would
simply be a minimum measure of the amount of ice recession between
the deposition of the underlying and overlying bodies of till. It is
hardly likely, though possible, that the continuity of interbedded
gravel and sand could be proved for a sufficient distance north of the
southern limit of the less extensive bed of ground moraine, to alone
constitute evidence of a recession of ice great enough to make it
necessary to refer its re-advance to a new epoch. Beds of silt in like
relations, deposited by waters beyond the edge of the ice, would have
a like significance so far as the question here under consideration
is concerned. Such beds of stratified drift might sometimes have
corroborative value when their testimony, taken by itself, is
inconclusive. If, for example, their surfaces are marked by forest
beds, and especially by forest beds whose plants denote a warm climate,
the association becomes most significant.
In view of what has been said, it is evident that the absence of beds
of subaërially stratified silt, sand, and gravel, between beds of till
can not be brought in evidence against separate ice epochs. It would
rarely be true that topographic and hydro-*graphic conditions would
make possible the construction of plains of sufficient extent to serve
as criteria for the purpose here indicated, and few of those formed
would escape such a degree of destruction as to leave them demonstrably
continuous. There is also the further possibility that such beds exist,
even though their continuity be not known. To prove the continuity of
a buried bed of stratified and incoherent drift, even if it existed,
would be a most difficult task.
(6) _Differential Weathering._ If, after covering a given region, the
ice retreated, the drift which it left in the area which it previously
covered would be subject to oxidation, leaching and disintegration.
The depth to which this oxidation, leaching and disintegration would
extend, would be dependent upon the length of time during which the
drift was exposed, and upon the climate which affected the region
during its exposure. The longer the exposure and the warmer the
climate, the deeper would the weathering extend. If, subsequently,
the ice extended over the same region, it might, in some places,
override and bury the old surface without destroying it. The earlier
oxidized and leached drift would thus come to be buried by the newer,
unoxidized, unleached drift. If, therefore, beneath the newer drift
of any given locality there be found a lower drift, the surface of
which is oxidized and leached to a considerable depth, the evidence
is strong that the lower drift was exposed for a long period of time
before the upper drift was deposited upon it. Within certain limits a
similar result might be brought about, it is true, if the ice, after
having reached a certain maximum stage of advance, were to retreat for
a short distance only and there remain for a very long period of time.
A subsequent minor advance might bury the oxidized surface of the drift
beyond the position of the long ice-halt. Under these conditions, the
climate which would have obtained in the area of the drift exposed
during the minor retreat would have been cold, and oxidation, leaching,
and disintegration would have proceeded slowly. If they reached
considerable depths, the time involved must have been very long. If
this surface of oxidized and leached and disintegrated drift were found
to reach far to the northward beneath the layer of newer and upper
drift, it would indicate a great recession of the ice. We maintain that
if it were found sufficiently far north of the margin of the overlying
drift, and if its depth were sufficiently great, extending well down
below any possible accumulation of superglacial till, it might be a
positive criterion of so great a recession of the ice, protracted
through so great an interval of time, as to constitute its new advance
a separate ice epoch.
There is much reason to believe that the soil developed under the
influence of a warm climate differs in some respects from one developed
from similar material under other conditions. The well-known fact that
red and reddish soils are especially characteristic of low latitudes
and warm climates is significant. If therefore a soil developed on
the surface of one sheet of drift and buried by another, be found
to possess, in addition to unmistakable marks of long exposure, the
peculiar marks which seem to be characteristic of soils developed under
high temperatures, the argument gains in strength.
This argument from oxidation and weathering has another application.
If in a later advance, following a protracted recession, the ice-sheet
failed to reach the limit of its earlier advance, there would remain
an area of drift deposited by the first ice-sheet, outside the drift
deposited by the later. Now if the time interval between these two
advances was great, and especially if during this interval the climate
was mild, the oxidation and weathering of the older drift surface would
be markedly different in degree from that of the newer. If, under
these circumstances, the surface of the older sheet were found to be
weathered and oxidized and reddened up to the border of the newer
drift sheet, and if here there were found to be a sudden change in
the character of the surface of the drift so far as depth and degree
of oxidization and weathering is concerned, we should have strong
evidence that the one sheet of drift was much older than the other.
The statement sometimes urged that the drift which was deposited near
the edge of the greatest ice advance would be largely made up of the
residual materials which occupied the surface invaded by the ice,
would not meet the case. For if it be granted that this statement is
qualitatively good, we should find the greatest degree of weathering
and oxidation at the extreme margin of the drift, and it should be
found to be less and less on receding from this margin. There would in
this case be no sudden transition from a deeply weathered and oxidized
surface, to one which is fresh and unoxidized, along a definite line.
We maintain that if the whole of the drift deposits are referable to
one epoch, there should be no sudden transition in the surface of the
drift from that which is deeply weathered to that which is not, the
one surface being separated from the other by a definite and readily
traceable line.
It has been urged against the criterion of differential weathering
that superglacial material is or may be thoroughly oxidized before
its deposition, and that a layer of oxidized drift between layers of
till may be no more than superglacial debris deposited during a minor
recession of the ice.[16] We believe that this attempt to eliminate
the value of this criterion rests partly on an exaggerated idea
concerning the amount of superglacial material, but more especially
on a failure to apprehend the real meaning of the argument for the
validity of the criterion, and upon a failure to note the limitations
imposed upon it by its advocates. It is not affirmed that a layer of
oxidized drift between beds of unoxidized drift is _per se_ proof of
two glacial epochs; but it is affirmed that if such layer of weathered
drift can be shown to extend far below any possible superglacial till,
into the subglacial till below, in such wise as to indicate that it
is the result of subaërial exposure in a warm climate subsequent to
its deposition and prior to the deposition of the overlying till,
it constitutes the best possible evidence of an interglacial epoch,
especially when accompanied by the corroborative testimony of other
criteria. It is further affirmed that if the second sheet of drift
failed to reach the limit of the first, and if the drift which was
deposited by the first and never covered by the second ice-sheet, is
more thoroughly and more deeply weathered than that deposited by the
second, and especially if the two types of drift surface meet along
a definite and readily traceable line, the argument becomes, in our
judgment, irrefragable. In its application, this criterion would be
infallible only in the hands of one who could distinguish between
superglacial and superglacially oxidized material on the one hand, and
material subaërially weathered after its deposition, on the other.
[16] This point was urged at the reading of the paper at Ottawa, by
Prof. C. H. Hitchcock, Mr. Upham, and others.
In circumstances and relations where the weathering of the drift is
not in itself conclusive, it might still have corroborative value in
association with other lines of evidence.
The absence of an oxidized and disintegrated zone of drift below a
superficial layer which is not oxidized, would be no proof that there
were not distinct ice epochs, since the ice of any later epoch, if such
there were, might have planed off the surface of the drift left by its
predecessor to the depth of the weathering. The preservation of such
surfaces after a second ice invasion must be regarded as the exception
rather than as the rule. There is always the possibility, too, that
an oxidized and weathered zone marking the surface of an older drift
sheet exists, where excavations have not opened full sections of drift
to view. The absence of weathered zones of drift beneath the surface,
or the absence of knowledge of their existence, is therefore at best
no more than negative evidence. The absence of greater weathering of
the drift outside the limit of the drift supposed to belong to a later
epoch, would be positive evidence against the reference of the two
sheets of drift concerned to different epochs.
A specific part of the above line of evidence may be separately
mentioned. One phase of weathering is the disintegration of boulders,
and this is a point which can be readily applied even by those who are
not geologists. If the boulders of one region are much more commonly
disintegrated than those of another, and if the two regions are
separated from each other by a well-marked boundary line, the inference
lies close at hand that the boulders in the one case have been much
longer exposed to disintegrating agencies than in the other. It is
no answer to this argument to say that the materials lying at the
very front of the drift deposits contain boulders which were derived
from the disintegrated rock over which the ice has passed, and that
they were therefore in a less firm state at the outset. In many cases
these boulders have come from great distances, and coming from great
distances they must have come in a firm and solid state, else they
could not have suffered such extensive transportation, except indeed
their position was superglacial throughout their whole journey. This
argument has equal force when applied to the area covered by the two
sheets of drift where two exist. If within the region of drift under
investigation we find a surface layer of greater or less depth,
the boulders of which are hard and fresh, and if beneath this we
find another layer of drift, the stony material of which is largely
disintegrated, at least in its upper parts, we have good evidence
that the surface bearing the disintegrated boulders was exposed for
a considerable length of time before the deposition of the overlying
drift, which carries fresh boulders. Since the disintegration of
boulders is only one phase of weathering, the limitations of this
argument are identical with those already noted in connection with the
general argument from differential weathering.
(7) _Differential Subaërial Erosion._ If the drift deposited by one
ice-sheet were to be exposed for a considerable interval of time, and
if the ice in its subsequent advance failed to reach the limit of
its first invasion, the two areas should show different amounts of
subaërial erosion, since the one has been exposed to the action of air
and water much longer than the other. The line which marks the limit
of the later ice invasion should be the line of more or less sudden
transition from an area without, where stream erosion has been greater,
to an area within, where stream erosion has been less.
The point here made can not be met by the suggestion that the greater
erosion of the outer area was effected by the water issuing from the
ice which had retreated to the position now marked by the border of the
area of the lesser erosion. So far as we know, such waters would be
depositing, not eroding. Furthermore, much of the erosion of the outer
area would have such relation to drainage lines that waters issuing
from the ice could never have reached the localities where it is shown.
If the outer and older drift be found to have suffered ten times as
much stream erosion as the inner and newer, it is fair to assume that
it has been exposed something like ten times as long, if the conditions
for erosion are equally favorable in the two regions. The argument has
especial weight if it can be found that beneath the newer drift the
surface of the older is such as to indicate that it was deeply eroded
before the newer was placed upon it. The argument is stronger the
farther from the margin of the newer drift such erosion on the surface
of the underlying older drift can be proved to have taken place. In
other words, if, in addition to the greater surface erosion of the
older drift sheet as now exposed outside the limit of the newer drift,
we find a notable unconformity between the newer and the older drift,
and especially if this unconformity lie far enough north of the margin
of the newer drift, the argument becomes conclusive.
When differential erosion and drift unconformities are not in
themselves conclusive, they may have great corroborative value in
conjunction with differential weathering, forest beds, or other
indications of separate ice epochs.
The absence of observable unconformity between sheets of drift would
be no proof that there were not distinct and widely separated ice
epochs, since the later ice invasion might have so far modified the
surface which it transgressed, as to destroy all patent evidences
of unconformity. It would have been anticipated that distinct
unconformities in the drift would be rare, even if there were distinct
ice epochs, for the same reason that weathered zones and forest beds
would be rare. But if the drift which lies outside a line supposed to
mark the limit of a sheet of drift belonging to a later ice epoch, be
not more eroded than that which lies within such line, the absence of
greater erosion in the outer drift is positive evidence against the
reference of the drift of the two areas to distinct ice epochs, if
conditions for erosion in the two areas are equally favorable.
(8) _Valleys Excavated Between Successive Depositions of Drift._ A
closely related, but not identical, point may be found in the extent of
the valley excavations which can be proved to have taken place between
the deposition of the earlier and later drift. We do not refer to
valleys excavated in the drift especially, but to those excavated in
other formations as well. If it can be shown, for example, that after
the deposition of an earlier drift sheet, and before the deposition
of a later, valleys were excavated which extended not merely into the
drift itself, but far beneath the drift into the underlying rock,
these valleys would be conclusive evidence of a long interval between
the deposition of the two bodies of drift. The argument is of especial
force when such excavations in the rock beneath the drift can be shown
to have taken place at great distances within the margin of the newer
drift. For valleys in such situations imply that the ice had receded at
least as far to the north as they lie, during the interval between the
two drift depositions, and may be so situated as to show that the ice
had wholly left the drainage basin where they occur.
The absence of evidences of deep valley excavations in any given region
during a supposed interglacial epoch, is no proof that such interval
did not exist. The conditions may not have been everywhere favorable
for erosion within the limits of any narrowly circumscribed area, and
the absence of interglacial valleys would be only negative evidence
against an interglacial epoch. The absence of such evidence everywhere
would bear against the existence of an interglacial epoch of much
duration in such wise as to be more than negative evidence.
(9) _Different Directions of Movement._ If, after its maximum advance,
the ice suffered merely a minor recession and then remained stationary,
or nearly so, for a time, the general direction of its movement in a
subsequent advance would probably be essentially the same as in the
earlier. But if, after its maximum advance, the ice receded to a great
distance, and especially if it entirely disappeared, a subsequent
ice-sheet might have a very different direction of movement, since
its center of accumulation and dispersion might be very different.
It is conceivable that this center might shift during the history of
a single ice-sheet. In this case there should be a gradual change in
the direction of ice movement, not an abrupt one. If, therefore, there
be found one sheet of drift made by an ice movement in one direction,
overlaid by another sheet of drift deposited by ice moving in a very
different direction, with an abrupt transition between them, such
drift sheets would be presumptive evidence of distinct ice epochs. An
exception would need to be made in the case of drift sheets along the
margins of confluent or proximate ice lobes. In such cases, if the
one lobe temporarily secured the advantage of the other, drift beds
formed by movements from opposite directions might be found in vertical
succession, without being evidence of separate ice epochs.
It is no part of the purpose of this essay to point out the
difficulties which might arise in the application of this criterion
of diverse directions of ice movements. It is possible that gradual
changes in the direction of movement might leave records which would
seem to indicate abrupt changes instead. This possibility makes care
necessary in the application of the criterion, but does not destroy its
value. When not itself conclusive, this criterion may be so associated
with differential weathering, differential erosion, forest beds, etc.,
that their combined testimony makes but one conclusion possible.
The absence of evidence of radically diverse directions of movement
during the time of deposition of the various sheets of drift, would
be no proof that there were not distinct epochs. In the first place,
the movements of different epochs might be harmonious--a condition of
things more probable than any other if the more common views of the
causes of glaciation be correct. In the second place, if the movements
were diverse, the deposits might still be so similar that their
differentiation, when the one is buried, might not be easily made. In
the third place, the later ice might have so far incorporated the older
drift material with that which belonged more properly to it, as to have
destroyed all definition between them.
(10) _The Superposition of Beds of Till of Different Physical
Constitution._ After the retreat of an ice-sheet, the surface of the
country thus discovered would be largely mantled with drift. This drift
would serve to protect the underlying rock from disintegration. But
where there was little or no drift, the rock surface would be subject
to all the disrupting agencies which affect surface rocks. The same
would be true of all rock surfaces bared by subaërial erosion after the
disappearance of the ice. Under these conditions, if a second sheet
of ice invaded the region in question after it had been long exposed,
it would find a surface prepared to yield large bowlders. The result
would be the deposition of a new sheet of drift containing bowlders
much larger than those which would have been proper to an ice-sheet
overspreading a surface but recently abandoned. If, therefore, in
the upper of two layers of subglacial till, bowlders of great size
predominate, as compared with those of a lower homologous layer, they
may be indicative of a great interval of time between the deposition
of the upper and lower beds of drift. If the home of these bowlders
be far north of the limit of the lesser sheet of drift, the distance,
as well as the duration, of the ice retreat must have been great, and
the reference of the two beds of till to distinct ice epochs would be
favored. The case might be so strong as to make no other interpretation
possible. Where in itself inconclusive, this criterion would have
corroborative significance. In its application, the discrimination of
subglacial and superglacial till would be imperative.
The absence of physical dissimilarity between superposed layers of
subglacial till would not be proof of the absence of separate glacial
epochs. The phenomena constituting the criterion could hardly be
expected to be of common occurrence. They would never be obtrusive, and
may easily have escaped attention where they exist.[17]
[17] The 10th criterion, in the order here named, was suggested by Mr.
McGee in the discussion which followed the reading of the paper at
Ottawa.
(11) _Varying Altitudes and Attitudes of the Land._ Another line of
argument has to do with the altitude and attitude of the land during
the deposition of various members of the drift complex. If during
the deposition of one part of the drift that part of the continent
covered by the outer part of the ice was low, the drainage from it
would be sluggish. If the deposits of this drainage persist to the
present time, we may find in their character evidence of the nature
of the drainage, and therefore of the attitude of the land. If at a
later time of drift deposition the glacial drainage in the same region
was more vigorous, the deposits made by the glacial streams would be
correspondingly coarser. In these deposits, if they persist to the
present day, we should find conclusive evidence of the swiftness of the
streams. If it can be shown that during the deposition of one sheet of
drift drainage was sluggish, and that during the deposition of a later
body of drift the drainage was vigorous, these facts are evidence of an
interval between the two times of drift deposition, sufficiently long
to accomplish the corresponding changes in elevation or attitude. Since
such changes of altitude and attitude are generally believed to have
been accomplished slowly, the interval must be believed to have been of
considerable duration.
It is true that continental altitudes and attitudes might change during
a single epoch of glaciation. If the change thus brought about resulted
in increased slope, the more sluggish drainage of the earlier part
of the epoch would be gradually transformed into the more vigorous
drainage of the later part. In this case, if the evidence of both the
earlier sluggish drainage and of the later vigorous drainage remain,
there should also remain the evidence of the intermediate stages. If
the deposits representing the intermediate condition of drainage do not
exist, while those representing both extremes do, there would be the
best of reason for believing that the intermediate phases of drainage
did not exist during a glacial epoch, but during an interglacial
epoch, when streams were not handling glacial debris, and when they
were eroding rather than depositing. The deposits of the slow and of
the swift drainage might occur in such relations as to prove, beyond
peradventure, that intermediate stages of _glacial_ drainage never
existed.
If the sluggish drainage accompanied the maximum ice invasion, while
the vigorous accompanied a lesser, the evidence of the swift streams
might be found far north of the southern limit of the earlier drift.
The farther north of the outer border of the older drift the gravel
representing the vigorous drainage of the later and minor ice-sheet
occurs, the further the ice must have retreated before the change from
the one type of drainage to the other was effected. On the other
hand, the farther north of the limit of the later ice advance the
sluggish drainage accompanying the earlier ice-sheet may be traced,
the farther must the ice have receded before the changes resulting in
vigorous drainage occurred. Under certain relations, the retreat of the
ice might be shown to have been great enough, before the orographic
movements which altered the nature of the drainage, to constitute
in our judgment, a re-advance a distinct ice epoch. If for example
throughout the course of a long river whose basin was largely covered
with ice, there be evidence that sluggish drainage obtained during the
maximum ice advance, and during all stages of the ice retreat until the
basin was free from ice, and if there be evidence of a vigorous glacial
drainage in the same valley at a later time, with no gradations between
the two types, we have proof positive of at least a great recession,
and of a considerable elevation of the land after the ice had receded
beyond the limits of the drainage basin and before it again reached
it in its re-advance. We hold that these phases of glacial drainage
deposits may be so related to each other, to the valleys in which
they occur, and to more or less distinct bodies of glacier drift, as
to prove so great a recession of ice between the diverse phases of
drainage deposition, as to constitute the second advance a distinct ice
epoch.
The absence of evidence that the land stood at different elevations
during different parts of the period of drift deposition, does not
in any way militate against the theory of recurrent and distinct ice
epochs. A constant attitude of the land is the thing to be assumed,
until positive evidence to the contrary is adduced.
(12) _Vigor and Sluggishness of Ice Action._ If it can be shown that
during one epoch of glaciation, we will say the epoch of maximum ice
extension, the ice action was relatively sluggish, while during a later
and minor advance its action was vigorous, the difference of action
might be regarded as presumptive evidence of distinct ice epochs.
Evidence of the two phases of ice action here referred to are difficult
of definition, but they have been independently noted by more than
one glacialist. It is true that a forward oscillation of the ice edge
might be more forceful than an earlier forward movement which might
have reached a greater extension. In itself, therefore, this line of
evidence can not be regarded as possessing great value.
It has been indicated that under certain circumstances, and in certain
relations, some of the foregoing criteria, taken singly, may be
conclusive of glaciations so distinct from each other, as to make their
reference to separate epochs proper. But where the facts and relations
which constitute one of the criteria are found, the facts and relations
constituting one or more of the others are likely to be found as
well. Where two of the foregoing criteria are found to be coexistent,
their joint force is greater than that of either one. If neither one
be absolutely conclusive, the two may still be, since the one may
exactly meet the deficiency of the other. If three or more concurrent
lines of evidence exist in any locality, the case is still further
strengthened. We maintain that several of the foregoing criteria may be
so related to each other and to the formations concerned, as not only
to make the recognition of separate ice epochs proper, but to make the
failure of such recognition altogether unscientific. Even when a single
line of evidence, or when double, or triple, or quadruple lines of
evidence are not absolutely conclusive in ruling out every conceivable
technical escape from the conclusion that there were separate ice
epochs, their cumulative and corroborative force may still be such as
to carry conviction scarcely less positive than that which mathematical
demonstration would afford. In the nature of the case not all of these
various lines of evidence could be expected to be found in any one
locality, or perhaps in any one limited geographic area, but where one
occurs, some or all of the others are liable to be found under favoring
circumstance. The number of criteria, and the great extent of area
where they may hope for application, afford great possibilities.
From the foregoing discussion, it will be readily seen that the nature
of the criteria and the limitations imposed upon their application by
the difficulty of proving stratigraphic continuity in such a formation
as the drift, necessitate the greatest care in their use, and reduce
the value of hasty and inexpert conclusions to a minimum.