It is conceivable that, after the development and extension of a
continental ice-sheet, it might be wholly wasted away. The maximum
extension of such an ice-sheet would mark the culmination of a glacial
epoch. If subsequently another ice-sheet of considerable dimensions
were accumulated, its development and extension would constitute a
second glacial epoch. These successive ice-sheets might be so related
to each other in time, in position, and in the sequence of geological
events, as to be regarded as separate epochs of the same glacial
period.[14] On the other hand they might be so widely separated from
each other in time, in position, and in the sequence of geological
events, as to make their reference to separate glacial periods more
appropriate. In any case their separation would be sufficiently marked
to necessitate their reference to separate ice epochs. So far we
believe there would be no disagreement.
[14] The terms period and epoch are here used in the sense in which
they have been used most commonly in the literature of glacial geology
in the United States.
If, instead of entirely disappearing, the first ice-sheet suffered
great reduction of volume and area, and if this reduction were followed
by a second great expansion of the ice, might the time of such
expansion be regarded as a second glacial epoch of the common glacial
period? To this question, too, as thus stated, we apprehend there would
be but one answer, and that affirmative.
It seems certain that the edge of the continental ice-sheet was subject
to more or less extensive oscillations, as are the ends of glaciers
and the edges of ice-sheets to-day. How much of an oscillation is
necessary, and under what attendant conditions must it take place, in
order that the recession of the ice-edge shall mark an interglacial and
its re-advance a distinct glacial epoch? When the question takes this
specific form, and when inquiry is made concerning the quantitative
value of the different elements entering into the problem, we reach
the battled ground. It is the battled ground, partly because it is
the ground of misunderstanding. It is the ground of misunderstanding,
partly because glacialists are not agreed as to the meaning of certain
terms in common use by them.
Four elements seem to enter into the idea of an ice epoch as distinct
from other ice epochs. These are (1) the distance to which the ice
retreated between successive advances; (2) the duration of the retreat,
or the time which elapsed between successive ice extensions; (3) the
temperature of the region freed from ice during the time between maxima
of advance; and (4) the intervention between successive advances, of
changes interrupting the continuity of geological processes.
(1.) It would be arbitrary to name any definite distance to which
the ice must recede in order to constitute its re-advance a distinct
ice epoch. It would be not so much a question of miles as a question
of proportions. Considering this point alone, we presume it would
be agreed that an ice-sheet should have suffered the loss of a very
considerable proportion of its mass, and that it should have dwindled
to proportions very much less than those subsequently attained, before
its re-advance could properly be called a separate glacial epoch. To be
specific, if the North American ice-sheet, after its maximum extension,
retreated so far as to free the whole of the United States from ice,
we should be inclined to regard a re-advance as marking a distinct
ice epoch of the same glacial period, if in such re-advance the ice
reached an extension comparable with that of the earlier ice-sheet.
Especially should we be inclined to refer the second ice advance to a
second glacial epoch, if it, as well as the preceding retreat, were
accompanied by favoring phases of some or all the other three elements
entering into the notion of a glacial epoch. In this statement we
do not overlook the fact that a northerly region--as Labrador or
Greenland--might be continuously covered with ice throughout the time
of the two glaciations of the more southerly regions. But this is not
regarded as a sufficient reason for discarding the notion of duality.
Greenland has very likely been experiencing continuous glaciation since
a time antedating that of our first glacial deposits. The renewal
to-day of glaciation comparable in extent to that of the glacial period
would certainly be regarded as a distinct glacial epoch, if not a
distinct glacial period, even though Greenland's glaciation may not
have been interrupted. Scandinavia and Switzerland have probably not
been freed from ice since the glacial period. Their snow and ice fields
are probably the direct descendants of the ice fields of the glacial
period. An expansion of the existing bodies of ice in these countries
to their former dimensions, would constitute a new glacial epoch, if
not a new glacial period. Analogous subdivisions in pre-Pleistocene
formations have been frequently recognized.
(2) The application of the time element is hardly susceptible of
quantitative statement. We are inclined to think that it would be
generally agreed that, with a given amount of recession of the ice, its
re-advance would be more properly regarded as a distinct glacial epoch
if the interval which had elapsed since the first advance were long.
Whether a longer time between the separate advances might reduce the
amount of recession necessary in order to constitute the second advance
a second epoch, we are not prepared to assert; but we are inclined to
think it might.
(3) The third element is perhaps somewhat more tangible than the
second. If, during the retreat of the ice, the climate of a region
which was twice glaciated became as temperate as that of the present
day in the same locality, we should be inclined to regard the preceding
and succeeding glaciations as distinct ice epochs, especially if the
intervening recession were great and its duration long.
Unfortunately for simplicity and ease of determination, there are
difficulties in determining with precision how far the ice retreated
between successive maxima of advance, how long the interval during
which it remained in retreat, and the extent to which the climate was
ameliorated, as compared with that which went before and that which
followed.
(4) If changes of any sort which interrupt the continuity of geological
processes intervened between successive maxima of advance of the ice,
the separation of the later advance from the earlier, as a distinct
ice epoch, would be favored. How great the intervening changes should
be in order to constitute the re-advance a distinct ice epoch, is a
point concerning which there might be difference of opinion. But it is
altogether possible that such changes might intervene as alone to give
sufficient basis for the separation. Orographic movements, resulting
either in continental changes of altitude or attitude are among the
events which might come in to separate one ice epoch from another.
Changes of this sort have often furnished the basis for the major
and minor divisions of time in other parts of geological history, so
that there can be no question as to their adequacy, if they were of
sufficient magnitude. We hold that the intervention of orographic or
other important geologic changes might reduce to a minimum the amount
of recession, the duration of the recession, and the warmth of the
intervening climate necessary to constitute the separate ice advances
separate ice epochs. The absence of great orographic or other changes
in glaciated regions between successive advances of the ice would be
no proof that such advances should not be regarded as separate epochs.
Divisions of equal importance have often been made without evidence of
such changes.
From the foregoing discussion, brief as it is, it will be seen that
within certain narrow limits the definition of a glacial epoch, as
distinct from other glacial epochs, must be more or less arbitrary.
It is less important that an arbitrary definition should be accepted,
than that the same meaning should be attached to technical terms in
common use among geologists. In the interest of harmony and of a common
understanding, and without the violation of any truth of science, we
believe it would be well if the conception of a glacial epoch, as
framed by those who are our leaders in position and in fact, were made
the basis for our usage of the term.