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.