THE GLACIAL AND POSTGLACIAL LAKES OF THE GREAT

LAKES REGION.1

By FRANK B. TAYLOR.

GENERAL STATEMENT.

The Great Laurentian Lakes, or the Great Lakes, as they are commonly styled, are a group of valleys which have been turned into lakes. Geologically speaking, the lakes themselves are new and youthful forms, although the valleys in which they lie are much older.

The basins of the Great Lakes were once valleys with free drainage and no lakes, like the Ohio Valley of to-day. The events which changed them into water-filled basins were apparently associated with the glacial period, and. are therefore of relatively recent date. It is the later part of the Great Lakes history, comprising the glacial and postglacial epochs, that has engaged the attention of students most, because the facts relating to that part are the newest and most numerous. But in any comprehensive view, the fact should not be overlooked that the Great Lakes, or rather the basins in which they lie, had a long and complicated history before the glacial period and also a complex interglacial history. Only the main outlines of the earlier epochs are known at the present time, and it will suffice to enumerate them here briefly.

PREGLACIAL HISTORY OF THE VALLEYS OF THE GREAT LAKES.

PHASES OF DEVELOPMENT.

The preglacial history of the Great Lakes is simply the geological history of the region. For convenience it may be divided into three epochs, each one dominantly, though not exclusively, characterized by a particular phase of development. The first was the epoch of sedimentation or Paleozoic strata building the constructional epoch;

1 Published by permission of Director of the U. S. Geological Survey. This paper is an advance publication of Chapter XII of a monograph on Pleistocene deposits and glacial lakes of Indiana and Michigan, by Frank Leverett and Frank B. Taylor, U. S. Geol. Surv. Mon., vol. —, in press. This was not originally intended for separate publication, and inasmuch as it is merely an outline of the lake history, full and specific credit is not given here for the work of each of the several geologists who have materially contributed to the elucidation of this complex history. Such due acknowledgment will be found included with the complete discussion of the phenomena in the monograph.

the second was the epoch of land elevation, causing increase of altitude and inaugurating erosion-the epoch of emergence, and the third was the epoch of erosion or valley making the destructional epoch. These three epochs are not sharply and completely marked off from each other, although they may appear to be so in some parts of the Great Lakes area. For example, in the northwestern part, uplifts producing emergence of land areas occurred while in much of the region of the lakes farther east, sedimentation was still going on uninterrupted. Whatever land was then raised above the sea was attacked by the forces of erosion. Thus, to some extent, sedimentation, elevation, and erosion were all going on at one and the same time. But the successive dominance of the three processes distinguishes fairly well the three phases of development

OUTLINE OF GEOLOGICAL HISTORY OF GREAT LAKES REGION.

It is well known that the basins of the Great Lakes lie chiefly in depressions that were formerly filled and completely occupied by Paleozoic strata. While these strata were being laid down the whole region, excepting, perhaps, part of the Archean area south of Lake Superior and some parts of the plateau north of the Great Lakes, was under the sea. The rocks that filled these basins have very different characters in different beds. There are conglomerates and sandstones, shales and limestones, and in some places igneous rocks. Each one of these classes of rocks has many varieties with more or less variation in hardness and chemical properties, and these qualities exercised an important influence upon the rate and manner of disintegration under the forces of erosion. The formation of the Great Lakes basins has thus been dependent to a large degree upon the character of the strata out of which they have been excavated-upon their relative hardness, thickness, and arrangement.

This was the constructional period in which nature was getting ready for the subsequent making of the lake basins. The basins themselves, however, did not begin to be made until another great event in geological history had taken place-not until a change occurred in the relative attitude of the land and sea. Beginning at the close of the Paleozoic era there came an epoch of great earth movements affecting all of the eastern part of North America, including the whole of the Great Lakes region. In consequence of this the land now occupied by the lakes was lifted out of the sea to an altitude estimated by some to be relatively 2,000 or 3,000 feet higher than its present altitude. This was the time of the uplifting and folding of the Appalachian Mountains. This process probably occupied some thousands of years, but in a geological sense it was a relatively short

There is evidence, also, that there were some earlier movements of less extent affecting the region of Lake Superior and the northern part of Lake Huron especially, which probably made land surfaces of limited extent before the great movement which elevated the whole region.

THE MAKING OF THE GREAT LAKE VALLEYS BY STREAM EROSION.

The forces of subaerial and stream erosion attacked the surface of the land as fast as it was raised above the level of the sea, and the sea itself, with its waves and tides and currents, attacked the new land all around its shores. Rain and frost, wind and sunshine and the various agents of chemical decomposition attacked every part of the new land surface. Most effective of all was the water that gathered into flowing streams. All of these, great and small, did their share in tearing down and sculpturing the new land-in carving valleys, hills, and mountains out of the elevated mass. Each one worked with an efficiency dependent upon its volume, the rate of its descent, the character and quantity of sediment carried and upon other factors. The first shapes of the newly emerged land determined the first drainage systems, but as the work of erosion went on the effects produced were greatly influenced by the variously resistent characters of the rocks and their relative position and arrangement.

In the building of the strata out of which the lake basins have been excavated, it happened that the region now occupied by the greater part of the basins was for the most of the time not adjacent to the shores of the ancient seas so as to receive coarse sediments, but was offshore some distance from the land, so that the sediments received were mainly of fine texture, mud which afterwards became shale, and limey ooze which afterwards became limestone. Conglomerates and sandstones indicating shore conditions or shallow water near shore occur, but are not common in the lake basins. Limestone does not rank as a hard substance in the scale of mineral hardness, but compared to the shales it is sometimes a hard, resistant rock, especially where it occurs in massive form and in great thickness. In the building of the strata it happened that, stretching westward and northwestward from central New York to northern Michigan, there was a group of beds whose arrangement and relative hardness predisposed them to unequal erosion and the formation of valleys bounded by great escarpments. In New York the Lockport limestone of the Niagara group is a massive bed of the hardest quality, 150 to 250 feet thick, while below it are shales and sandstoneschiefly shales-much softer, but containing two relatively thin, hard layers of Clinton limestone. These lower beds are several hundred feet thick. Then, again, above the Lockport limestone, are the

85360°-SM 1912- -20

very soft, marly, salt-bearing beds of the Salina formation, generally 200 or 300 feet in thickness. The selective processes of erosion led the streams to attack the softer strata with greatest effect, while the harder limestone resisted and formed the great escarpment which now characterizes it, from New York to Wisconsin. Extensive valleys were eroded in the soft rocks below it and the limestone ledge was driven back as fast as it was undermined. Other valleys were also excavated in the soft shales above.

Thus, Lake Ontario, Georgian Bay, the northern channel of Lake Huron, and Green Bay were excavated out of the soft rock below the limestones of the Niagara group, while Lake Erie, the main body of Lake Huron and all of Lake Michigan were excavated out of the soft strata above the limestones. Lake Superior appears to be somewhat exceptional. It is thought to be largely an original rock basin, or perhaps a syncline out of which the soft rocks have been eroded. These softer rocks were probably mainly those that lie below the limestone of the Niagara group.

Thus, the shape and size and arrangement of the lake valleys were primarily dependent upon the geological structure-upon the relative position and thickness of the soft beds and the distribution of their exposed parts. Where the soft beds were exposed to effective stream erosion they were removed more rapidly than the harder rocks, and thus became the main valleys of the region.

In the present attitude of the land the Paleozoic strata dip distinctly but gently southward in the basin of Lake Ontario; south in the eastern part of Lake Erie, and southwest and west in its western part; toward the southwest in the main part of Lake Huron, but toward the south in the northwestern part of this basin; toward the south in the northeastern part of Lake Michigan, toward the east in the southern part, and toward the south in the peninsula east of Marquette, while farther west the older rocks bordering Lake Superior on the south dip steeply northward toward the axis of the basin and the dips are various in other parts.

That these valleys were going through the process of development by erosion during practically all the time from the close of the Paleozoic to the beginning of the glacial period seems not improbable. Indeed, the time must have been very long to have made such extensive valleys by so slow a process. It might be thought that some movement of elevation or tilting had turned these old valleys into lake basins long before the time of the Ice Age, but no certain evidence indicating such a change has been found. Up to, or nearly to, the beginning of the Ice Age the valleys appear to have had complete drainage by rivers and held no lakes.