of the Mus. Comp. Zoöl. Harvard Univ. vol. xvi. No. 4, p. 82.
[48] J. D. Dana, Amer. Jour. Sci. 3d. ser., vol. xv, p. 506.
[49] J. D. Dana, Amer. Jour. of Sci. 3d ser. vol. xxv, p 446.
[50] J. D. Dana, Amer. Jour. of Sci., 3d ser., vol. xxiii, p. 198.
[51] J. D. Dana. Amer. Jour. Sci., 3d ser., vol. xxv, p. 441.
_STUDIES FOR STUDENTS._
GEOLOGICAL HISTORY OF THE LAURENTIAN BASIN.
The study of the Pleistocene history of the basin drained by the St.
Lawrence has been fragmentary and is still far from being complete.
There is a lack of agreement in the interpretation of observations
already made, due in part to the comparatively limited portion of the
field examined even by those who have given the subject most attention,
and in part to lack of uniformity in the standards of comparison used.
It is with the hope of assisting in reaching more harmonious results
that attention is here invited to methods of study.
In the present treatment of the subject it may be advantageously
subdivided, and the facts and hypotheses relating to each division
separately considered. Of the divisions that may be suggested the
following seem the most important:
1. Character of the sub-morainal or hard-rock topography in the
Laurentian basin.
2. Origin of the basin.
3. Sedimentary deposits.
4. Shore markings left by former water-bodies.
5. Fossils in ancient sediments, shore ridges, terraces, etc.
6. Fauna of the present lakes.
7. Changes in elevations of the land.
8. Former outlets.
9. Probable effects of an ice sheet on drainage.
10. Probable effects of a subsidence which would make the basin an arm
of the sea.
1. _Character of the hard-rock topography._ In order to learn the
character of the Laurentian basin it is necessary to examine the
rock surface beneath the general covering of glacial débris and
stratified sediments which partially fill it. To do this, those areas
in which rock in place forms the surface require to be mapped and
their elevations noted; the records of wells and other excavations
which pass through the superficial deposits should also be obtained
and the character of the underlying rock ascertained, as far as is
practicable. When sufficient data of this nature shall have been
recorded, a contour map of the basin can be drawn that will reveal the
shape of the depression with which the student has to deal. The depth
of the present lakes plus an estimated thickness of clay and morainal
material covering their bottoms, will probably furnish the only means
of sketching contours over the deeper portions of the basin. Even an
approximately accurate map of this character cannot be constructed for
a long time to come, but every advance towards it will serve to make
the problems to be studied more and more definite.
Something of the form of the rock-basin is already known and several
deep channels in its borders, now filled with drift, have been
discovered. The courses of buried channels connecting the basins of
some of the present lakes have also been approximately determined. It
is not necessary at this time to refer specifically to the discoveries
that have been made, but it may be stated that enough is known to
assure us that the basin is a depression in solid rock, the bottom of
which is below sea level.
2. _Origin of the basin._ The rocks in which the Laurentian basin
is situated are, with the exception of the Lake Superior region,
nearly horizontal and belong almost wholly to the Paleozoic. The
basin is essentially a depression in undisturbed strata, and all who
have considered its origin seem agreed that it has been formed by
excavation. A vast mass of horizontal strata has been removed, leaving
an irregular rim of undisturbed rocks on all sides. The form of the
depression is now obscured by drift; the deeper portions contain
stratified sediments which have been deposited within it and it has
been warped somewhat by orographic movement.
The manner in which the excavation was formed has been explained
principally in two ways. One hypothesis is that it owes its origin to
a time of subaërial denudation preceding the Glacial epoch, during
which a valley, or series of valleys, was worn out by stream erosion;
and that the depression thus produced has been but slightly modified
by ice action. The closing of the ancient valley has been referred
to orographic movements and to the filling of its outlet by glacial
débris. Another hypothesis is to the effect that the excavation is
mainly due to ice erosion during the Glacial epoch, without special
reference to previous topographic relief. A warping of the earth’s
crust so as to produce a true orographic basin does not seem to require
consideration, for the same reason as already stated, that the rocks in
which the basin lies have been but little disturbed from their original
horizontal position. Future study of the region must determine which
of the two hypotheses outlined above best suits the facts; or if each
hypothesis has something in its favor, what combination of the two may
be accepted as the final explanation.
It is a suggestive fact in connection with the first of these
hypotheses, that the youngest rocks in the region antedating the
Pleistocene belong to the Carboniferous. This seems to show that the
land has not been submerged since at least the close of the Paleozoic.
If not a region of sedimentation during this vast interval, it must
have been subjected to erosion. The erosion of an ancient land surface
might result in the production of topographic forms of diverse
character, depending on its altitude, on the length of time it was
exposed to atmospheric agencies during various stages of elevation, and
on climatic and other conditions. The study of topographic forms is
now sufficiently advanced to enable one to predict somewhat definitely
what features would appear under certain conditions. We also know the
characteristics of topographic forms due to glacial erosion. It seems
evident, therefore, that a knowledge of the hard-rock topography in
the Laurentian basin, would enable one to draw definite conclusions in
reference to the part that ice and water each had in shaping the forms
now found there.
The conclusion that the region under consideration has been glaciated
is well established; it remains, therefore, to determine what
topographic forms, if any, due to pre-glacial stream erosion can be
recognized. As an example of this kind of evidence desired, attention
may be directed to the northward facing rock escarpments which follow
the southern shores of lakes Erie and Ontario for a large part of their
courses and at varying distances up to several miles. These escarpments
are composed of the edges of nearly horizontal strata, mostly of
Paleozoic limestone, and their bases are buried beneath glacial débris
and stratified clays so deeply that in some instances, at least, they
do not reveal half of their actual height. These escarpments not only
have Pleistocene deposits banked against them, but their faces and
summits are polished and grooved, showing how stubbornly they resisted
the invasion of the ice which impinged against them from the north.
South of lake Ontario especially, the trend of the escarpment referred
to is directly athwart the course of the ancient glaciers. The entire
history of these escarpments cannot be discussed here, as my desire
is simply to call attention to the fact that they existed before the
Glacial epoch, and are relics of a strongly accented pre-glacial
topography. They are within the southern border of the Laurentian
basin, and hence afford means of determining, in part, what was the
form of that basin before it was modified by ice action. Other similar
escarpments exist in the northern and western portions of the same
great basin, and as this study progresses it is to be expected that
still other features of the pre-glacial land will be revealed. It is
perhaps too early to decide what were the special topographic forms
which gave character and expression to the St. Lawrence basin before
the ice invasion, but the Erie and Ontario escarpments and some other
similar features now recognized, suggest that in Tertiary times it
resembled the present condition of the upper portion of the Mississippi
valley, where bold, rock escarpments border wide stream-worn
depressions.
Deep drift-filled channels are known to cut across the Erie and Ontario
escarpments. These seem to have been formed by streams tributary
to the main drainage line to the north. If this conclusion is well
founded, a study of the hard-rock topography should reveal other
similar channels and finally indicate a well matured drainage system.
If even the broader and stronger features of the pre-glacial surface
can be determined, then the modifications due to glacial abrasion will
become conspicuous, and the amount that glaciers have broadened and
deepened the basin be determinable.
A study of the lithological character of the drift south of the present
lakes should show, at least in a rough way, what portion of it was
derived from the waste of rocks within the Laurentian basin. This
inquiry has already been undertaken by at least two geologists, and
estimates of the quantity of material removed from the basins of lakes
Michigan and Erie respectively, have been made. This method may be
extended so as to embrace a larger area, or some special portion of
the great depression best suited for the trial may be selected. If the
material removed from the basin or re-distributed within it by glacial
action can be shown to be approximately equivalent in volume to the
amount of rock excavated in order to form the depression, it would
evidently tend to support the hypothesis of glacial erosion. If, on
the contrary, the amount of débris derived from the basin should fall
far short of what would be requisite to refill it, no very definite
conclusion would seem to be indicated unless account could also be
taken of the fine material carried away by glacial streams.
As the case stands at present it appears that there is evidence of a
pre-glacial valley or series of valleys as has been claimed by several
geologists, and that all but the boldest features of the old topography
have been obliterated or greatly modified by glacial erosion followed
by glacial and other sedimentation. Additional observations should
show somewhat definitely the amount of work assignable to particular
portions of the history. How far the results of subaërial and of
glacial erosion have been modified by other agencies, more especially
by orographic movements, has also to be considered.
If the St. Lawrence basin shall be shown to be largely the result of
subaërial erosion it will follow, unless it is found that the deeper
portions are the result of glacial action, that the land at the time
the streams did their work, must have stood higher than at present, for
the reason that the bottom of the depression is now below sea level.
Some idea of the smallest amount of elevation necessitated by this
hypothesis might be obtained by estimating the gradients of the ancient
streams and the amount of elevation required to bring the bottom of the
depression up to sea level.
A study of the hard-rock topography in the valleys of the Ottawa and
St. Lawrence and of the present submerged Atlantic border of the
continent would also be instructive in this connection. The strict
correlation of the topographic history of the interior and of the
continent’s margin may be difficult, but as the two regions are
directly connected, valuable results should follow their comparative
study.
The hypothesis that the Laurentian basin is due largely to pre-glacial
erosion, necessitates also that the ancient system of river valleys
should have been closed in some way so as to form the basins of the
present and of former lakes. The closing has been referred to several
agencies. An unequal subsidence following the period of stream erosion
has been postulated. During the Glacial epoch the entire region was
ice-covered and only glacial streams of one kind or another could
have existed. On the retreat of the ice, when portions of the basin
were abandoned, the drainage is supposed to have been obstructed by
the ice itself, as will be noticed below. When the glaciers melted,
a vast sheet of débris was left which in many instances filled or
obstructed previous drainage lines. Old channels, now deeply buried,
have been reported to connect the basins of the various existing lakes,
as has already been mentioned, but no similar channel which could
have afforded an escape for the waters of the entire basin has been
discovered. Here again an acquaintance with the hard-rock topography
should give assistance and indicate either that such a channel existed
or that orographic movements have taken place which have obstructed the
former drainage system. The glacial hypothesis assumes that the basin
was excavated mainly by glacial abrasion and does not require that
the land should be either higher or lower than at present. The study
in this direction merges with that of the general glaciation of the
northeastern part of the continent, and cannot be treated at this time.
3. _Sediments._--Regularly stratified deposits of clay and sand occur
along many portions of the borders of the present Laurentian lakes.
These were clearly formed in water bodies which formerly existed within
the Laurentian basin, and which in certain directions, at least, were
of wider extent than the present lakes. The areas occupied by these
deposits have been partially mapped, but much remains to be done in
this direction. Fresh sections, particularly of the stratified clays,
are exposed from time to time by artificial excavations, in which much
of their history may be learned. Not only should records be made of the
facts noted at special excavations, but the extent and character of the
stratified deposits in one area should be determined and compared with
similar data obtained in other areas. For example: the clays covering
large tracts on the west shore of Lake Michigan and on the southern and
western border of Lake Superior are of a red color, while other areas
bordering Lake Erie are covered with blue clay. These two deposits
have been supposed to have been laid down at the same time and in the
same lake. The definite correlation of the clays of these two areas by
direct contact, however, does not seem to have been made, and there
are reasons for thinking that they may be quite distinct and that they
originated in separate lakes.
The outer limits of the deposits of clay and sand here referred to
are known in some instances to be determined by ancient beaches and
terraces. Such associations of deep and of shallow water deposits
require special attention, as the study of one may assist in
interpreting the significance of the other. The fine, evenly stratified
clays frequently contain large angular bowlders, which appear to have
been dropped from floating ice and to show an intimate connection
between the ancient lakes and neighboring glaciers. The possibility,
however, of the bowlders having been brought into the ancient water
bodies by rivers, or floated outwards from the shore by lake ice,
should also be considered. Huge angular masses of limestone have been
reported as occurring in southern Michigan especially, which rest on
superficial deposits and are thought to have been carried northward by
lake ice. The relations of these masses to well defined shore lines
have never been determined. If it should be found that they are above
all former shores, it is evident that they must have been carried by
some other agency than the one mentioned.
A chemical examination of the clays, or of their contained water, may
indicate whether or not the basin was formerly in direct communication
with the ocean. Analyses of the clays of the Champlain valley and of
the similar clays in the Ontario and Erie basins might indicate whether
or not they were deposited under similar conditions.
4. _Shore records._ Beaches and terraces have been studied at many
localities about the borders of the present lakes, sometimes at a
distance of more than twenty miles from their margins and at various
elevations up to several hundred feet above their surfaces. In some
instances these ancient shore records have been followed continuously
for scores of miles. The tracing and mapping of individual beaches is
one of the most important parts of the study here outlined, and is
already well advanced. Confusion has unfortunately arisen, however, for
the reason that topographic features, due to shore action, have, in
some instances, been confounded with somewhat similar features due to
other causes. Moraines and gravel ridges, formed by glacial streams,
have been mistaken for beach ridges, and terraces of various origin
have not been clearly discriminated.
In order not to be led astray by topographic forms that simulate shore
phenomena, the student should examine the shores of existing lakes and
learn what records are there being made. In the study of topography,
“the present is the key to the past,” just as definitely as in any
other branch of geology. The topography of lake shores has already
received attention from one skilled in reading geological history in
the relief of the land[52] and the study of existing shores in the
light of what has already been done in that direction should enable
even the beginner to avoid falling into serious error in interpreting
ancient records of the same nature.
To be able to discriminate clearly between shore features and somewhat
similar glacial phenomena, it is necessary to become familiar also with
the topography of glacial deposits. Fortunately in this study also a
guide is at hand[53] which, in connection with field observations,
should soon train the eye to discriminate the shapes assumed by
moraines and the deposits of glacial streams from all other topographic
forms.
In examining the records of former lakes it will soon be observed that,
in many instances, where the highest of a series of ancient beaches
is obscure and indefinite, the topographic expression above and below
a certain horizon, and also the character of the surface material,
whether of the nature of lacustral clays and sands or of glacial
débris, residual clay, etc., above and below the same level, are
significant, and enable one to map the outline of a former water body
with considerable accuracy.
In tracing ancient beaches and terraces, their forms and internal
structure need to be recorded, so that the fact of their being true
shore records may be made plain to others. The elevations of various
well-defined points throughout the extent of an ancient shore should be
carefully measured, for, as will be noticed below, although originally
horizontal, they have, in many instances, been elevated or depressed,
owing to broad general movements of the earth’s crust. The continuous
tracing of individual shore lines for as great a distance as possible
is highly desirable, especially in a wooded country, in order to be
positive as to which ridge or terrace measurements of elevation relate,
and also for the purpose of observing the nature of the changes that
occur when a shore line gives place to other records. For example: some
of the ancient beach ridges about the west end of Lake Erie have been
found to be continuations of moraines. In other instances shore ridges
have been reported to end indefinitely and to be replaced at the same
general horizon by glacial records of various character. The correct
interpretation of phenomena of this nature is especially important.
Accurate measurements of the vertical intervals between well defined
beaches at many localities would enable one to identify special
horizons, providing orographic movements were not in progress during
the time the series was forming. This method has recently been
successfully applied on the north shore of Lake Superior, where the
character of the country does not admit of the tracing of individual
terraces for considerable distances.
The deltas of tributary streams should also be revealed in the
topography of the basin of an ancient water body. Changes in the
character of lacustral sediments near where rivers emptied are also to
be looked for. Sand dunes are frequently an important accompaniment of
existing shores, and their association, perhaps, in a modified form,
with ancient beaches is to be expected.
5. _Fossils._ Thus far only a few fossils have been found in the
stratified clays and sands or in the ancient beaches of the Laurentian
basin. Such observations as have been made in this connection indicate
an absence of the remains of marine life and the presence, in a few
instances, of fresh-water shells in all of the basin west of the
eastern border of the basin of Lake Ontario. To the eastward of Lake
Ontario, however, in the St. Lawrence and Champlain valleys, marine
fossils are common in deposits supposed to be contemporaneous with the
stratified clays to the west.
A careful search in the clays and beaches left by the former water
bodies might be rewarded by important discoveries. In this examination
microscopical organisms should not be neglected. If after a detailed
examination no fossils are discovered, this negative evidence would
have its value, as it would indicate that the physical conditions were
not favorable to life, and an explanation for this fact might be found.
It is scarcely necessary to mention that care should be taken not to
mistake the shells occurring in modern swamp deposits associated with
the ancient beaches for true lacustral fossils.
About the borders of the present lakes and sometimes even below the
level of the lowest of the ancient beaches the remains of the mastodon,
elephant, giant beaver, elk, bison, deer, etc., have been found. The
recency of the existence of such of these animals as are extinct may
thus be established, as well as the former distribution of those still
living in other regions.
Evidence of the existence of man has been reported from one of the old
lake ridges in New York, and it is important that this interesting
discovery should be sustained by evidence from other localities. Stone
implements especially should be looked for in undisturbed lacustral
clays, and in the gravels of the ancient beaches.
The remains of forests have been stated to occur in the lacustral clays
adjacent to the south shore of Lake Erie. It is desirable to know
the extent of these deposits and how continuous they are; also the
character of the plant remains they contain, and whether they have been
disturbed from the position in which they grew. Some of the questions
that may be asked in this connection are: Was the basin drained and
forest covered before the vegetable remains were buried, or were the
plants floated to their present position, or did they grow on moraines
covering the stagnant border of the retreating glacier and become
involved and buried in morainal material as the ice melted?
6. _Life in the present lakes._ The fauna of the present lakes has
a bearing on their past history, for the reason that in the deeper
parts of lakes Superior and Michigan crustaceans and fishes have been
found which are believed to be identical with marine forms. These
may be considered as “living fossils,” and are thought by some to
indicate that the lakes in which they occur were formerly in direct
communication with the ocean. If the occurrence of living marine
species in the present lakes is found to be widely at variance with
the history of the basin as determined from physical evidence, an
inquiry should be made in reference to the manner in which the species
discovered might migrate.