Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 19

Persons living along the borders of the “Great Lakes” well understand what is meant by “The Ridge Road.” That road is a geological phenomenon. It is a record of high waters in the lakes. It possesses much significance, and opens an interesting vista through a chapter in the world’s history.

The Ridge Road runs parallel with the lake shore for many miles at a stretch. It is almost perfectly level and serves as a ready made road-bed for country roads. It consists of gravel and sands, and presents oblique lamination or cross-bedding in its internal structure. The materials have evidently been arranged by water. It has the general characters of a beach, and like a beach retains a level nearly uniform. Generally two or more of these ancient beaches run parallel with the lake, at different altitudes. The “ridge-road” south of Lake Ontario is 190 feet above the lake. The principal terrace bordering Lake Erie is 220 feet above the lake on the south side. On the north side, near Toronto, there are terraces from 30 to 500 feet above the lake. The Davenport ridge west of Toronto is 250 to 300 feet. At the west of Lake Ontario, near Dundas, the ridge is 318 feet high. Around Lake Huron are clayey deposits up to 500 feet. North of Lake Superior the upper terrace reported is 331 feet above the Lake.

There are other indications that the Great Lakes have formerly stood much higher than at present. Mackinac Island is a monument commemorating in stone the fact of the ancient high tides of the lakes. Get into a Mackinac boat and sail around the island. On all sides a wall of limestone rises perpendicularly from the water’s edge to a height of about 150 feet. Only on the south, for a narrow space, is the approach practicable. Here is the village; back of it, on the first rock-terrace, is the modern fort. The old Fort Holmes, captured by the British, is on the highest dome of the island, 350 feet above the lake. As we pass along the limestone wall which bounds the island, we see the waves breaking against the barrier. We notice the peculiar smooth concavities into which they wore the exposed surface. These are marks characteristic of wave action. Our eyes follow along the weathered buttresses to the summit. From bottom to top are the same records of warring waves. There was evidently a time when the lake acted at the height of 150 feet precisely as it acts to-day at the lake-level, before our eyes. We ascend to the main plateau of the island. On this rises a striking monument-like remnant of a formation which once covered the whole of this plateau to an altitude at least 134 feet greater. This is “Sugar Loaf.” But notice the fashion of its sides. Here, too, are the same smooth conchoidal depressions as the lake is still carving in the wall at the water’s edge. The waves have certainly been there. The time was when Lakes Huron and Michigan stood at least 200 feet higher than at present. Think of that condition of things. Picture the lakes filled up till the water covered Sugar Loaf. At present, Lake Superior stands twenty feet higher than Lakes Michigan and Huron. These are sixteen feet higher than Lake Erie; and the descent thence to Lake Ontario is 323 feet. This is according to Gannett. Now, we find lake-terraces up to three, four and five hundred feet above the present levels of the lakes. But let us assume the principal terrace south of Lake Erie as representing the highest flood of the lakes. This 220 feet above Lake Erie, 204 feet above Lakes Huron and Michigan, and 180 feet higher than Lake Superior. We have perfectly satisfactory proof that the water of the Great Lakes has stood at least as high as this. Now let us cast our eyes over the expanse. The sites of all the busy and populous lake cities are submerged. The greater part of the peninsula of Ontario is under water. The flood stretches westward of Detroit twenty-seven miles. There, near Ypsilanti is the ancient beach which marks the limit of the flood. Mackinac Island is submerged to the pinnacle of Sugar Loaf. Passing around to the head of Lake Michigan, we find the vast inland sea stretching southward and westward over a large part of the state of Illinois.

What hemmed in this broad expanse of water on the south? This interesting question has not been completely answered. We know that in southern Illinois are the remains of an ancient barrier which crossed the Mississippi, and was worn down for the passage of the great river. The barrier is a prolongation of the Ozark range, from Missouri. The gap cut through is at Grand Tower. Perhaps here was the barrier which held the waters back at the west, until the Mississippi gradually sawed the notch which drained the inland sea. At the east, however, we know no barrier adequate to hold the lakes at the level of the 220 feet terrace of Lake Erie. The high flood of the lakes must have been 182 feet higher than the escarpment or wall of rock back of Lewiston, through which the Niagara river has cut its gorge. Undoubtedly, this escarpment, which runs east nearly parallel with the shore of Lake Ontario, was formerly much higher than at present; but we have no evidence that it stood 190 feet higher than in our time.

The Lewiston escarpment is at present 38 feet above Lake Erie, and could have dammed the lake to that height, at any time before the Niagara gorge was begun. The water, setting back to the site of Chicago, would have buried it 22 feet deep. Even this would overflow the present southern barrier of Lake Michigan, and inundate the prairie region of Illinois. Thus, the existence of a terrace, but 38 feet above Lake Erie would indicate that the great lakes once flooded the greater part of the state of Illinois. Now, if we examine the nature of the prairie deposit, it presents every indication of formation in the bottom of a lake. Here is the stratified arrangement; here are the clay and marls, and here are even the shells of the molluscs which dwelt in the water. These facts must be borne in mind.

The high water of the Great Lakes has occurred since the Drift was deposited—since the latest semi-stratified Drift was laid down. The lake terraces rest on the Drift. All the other lacustrine deposits attending the high water, rest on the Drift. The entire broad region covered by the high water is overspread by a sheet of lacustrine clays and thin seams of sand. These deposits exhibit a regular horizontal stratification. Occasionally we find a bowlder imbedded in them. Excellent material for bricks and pottery is furnished by the clays; and in many situations, as at Milwaukee, the absence of iron prevents the production of a red color in burning. The famous “Milwaukee bricks” are of a pale lemon color, or even, in some cases, as white as chalk. This sort of clay occurs on both sides of Lake Michigan.

These lacustrine deposits rise from the shore with the general slope of the earth’s surface, to the upper level reached by them. The lowest beds come to the surface at the highest elevation. Often these are sandy; and, becoming saturated with rain at the surface, they convey a sheet of fresh water under the other deposits to the lower levels. These water-bearing sheets pass under the cities which have been built in modern times on the lacustrine border. In some cases, as at Toledo, and many points on the north shore of Lake Erie, artesian borings have been carried down to the water-bearing strata, and thus artesian wells have been obtained.

We have been considering lake-terraces and high water in the Great Lakes. But every observer has noticed terraces also along the borders of rivers. On the lower Ohio they occur up to 160 feet above low water; at Louisville, 128 feet above low water; near Cincinnati, 120 feet. On the Connecticut, they range from 150 to 240 feet above the modern flood level. On the Missouri we find them up to 335 feet; on the Athabasca and Saskatchewan, up to 370 feet. There is no need of citing further; for these facts show that the rivers in all the northern parts of the country have been enormously flooded, as well as the lakes. These terraces, also, rest on the top of the Drift deposits. The flooded waters, therefore, in general, existed after the events which left the Drift overspreading the northern states.

Now let us reason a moment from the facts which have been brought to our notice. In our Second and Third Talks we have been led to speculate on the possible agency which transported the bowlders from their northern home. We have been thinking of glaciers as a satisfactory explanation; and this view was adopted by the late Louis Agassiz, and most ably defended by him. Now, suppose there really was a vast glacier covering the country as widely as the Drift at present covers it. The ice must have melted; it is not here now. Suppose it melted rapidly; what enormous floods must have been occasioned! With what fury those floods rushed over the country to the lower levels! How they moved and mixed and half assorted the sands and pebbles! May not such a flood have produced the results which we see in the semi-stratified Drift? And then may not an excess of water have remained in all the streams long after the southern portion of the glacier had disappeared, and the semi-stratified Drift had been put in place? Would not such a state of weather as accomplished the melting of the ice have been somewhat like our March and April weather, characterized by abundant rains? Do we not find here good grounds for the building of a theory of transported bowlders, half assorted sands and flooded rivers?