« ПредыдущаяПродолжить »
CHANGES OF LEVEL OF THE GREAT LAKES.
The following pages are devoted to the physical history of the lakes of the northern States. As avenues of commerce, as preserves of food fishes, as reservoirs of pure water, as resorts for the artist, the pleasure seeker, and the health seeker, their description is left to other peps. They are here treated only as physical features, the endeavor being to set forth their origin and the series of physical changes, past, present, and future, that constitute their history.
Rivers are the mortal enemies of lakes. The river that flows into a lake brings stones and sand and fine mud, and dropping these into the quiet water endeavors to fill the earth cup that holds it. The year's tribute of sediment may have as little apparent effect as the year's tribute of water, which quietly escapes to atmosphere and ocean ; but the river is long of life and steadfast of purpose, and if years and centuries prove too short, it resolutely persists through geologic ages. The river that flows away from a lake constantly deepens its channel of escape, and thus attacks the lake's rampart at its weakest point. If the rampart is of loose earth, this is rolled and floated away bit by bit, and the work goes on merrily ; if it is of firm rock, this is dissolved, and then the process is exceedingly slow. But time is long, and even by solution the rampart may be channeled to its base and the whole lake drained away.
Nevertheless, in spite of this warfare of extermination, waged in all lands and through all time, there continue to be lakes, and so there must be in nature lake-producing as well as lake-destroy. ing agencies. There are indeed many such, but a few only need be appealed to to explain the great majority of lakes, and the chief are upheaval and glaciation.
Some parts of the earth's surface are known to be rising and others to be sinking. Usually such changes are of imperceptible slowness, but occasionally there is a sudden movement of a few feet, involving rupture of rocks and an earthquake. Similar movements have abounded through past ages of the earth, and to them are due not only mountains and plateaus, but continents and ocean beds. This great natural process of uplift and downthrow tends to produce lake basins, and, as we have seen, its tendency is opposed by the great natural process of erosion and deposition by rivers. The two are so nearly balanced that the scale is thrown to one side or the other by the accident of climate. Where much rain falls the rivers are powerful and prevail, sawing gorges through ridges as fast as they rise, building up the floors of valleys as fast as they sink. Where little rain falls the streams are weak, and the displacement of the earth's crust shapes the land into lake basins. Where the least rain falls the basins are many, but the lakes are ephemeral, created by the storm and dissipated by the sunshine. Great Salt Lake, Utah and Humboldt Lakes, and a score of others in our arid belt lie in valleys shaped by crustal displacement.
A glacier is aptly called a river of ice. Like a river of water it has an upper surface sloping continuously from source to goal, and like a river of water it rests on an uneven bed of its own shaping. When an aqueous river is suddenly deprived of its supply of water, there remain along its channel a series of pools recording the inequalities of erosion. When a great glacier is melted away the inequalities of its erosion are recorded in a chain of lakes. Moreover, much of the material ground and torn by the glacier from its bed is carried forward in the ice and dropped in a long heap where the ice melts, constituting a moraine. If the final melting is gradual, a series of moraines partitions the valley, creating lake basins. While it is building a moraine, the ice front advances and retreats in response to small changes in climate, so that the dropping of detritus is irregular, and the surface of the moraine is made billowy, abounding in small lake basins. Thus from glacial erosion there arise rock-basin lakes, and from glacial deposition of detritus there arise morainedammed lakes and moraine lakes.
In that wonderful geologic winter known as the Age of Ice, the annual snowfall on the northern part of our continent was so great and the annual melting was so small that the snow accumulated year by year, and became cemented into a continuous, deep, and ever-growing sheet of ice. As the depth of the sheet increased the pressure of its own weight became finally insupportable, and there was relief by horizontal flow, the margin moving outward to a region of warmer climate, where it was melted. It was, in fact, a vast glacier, so vast that the figure of speech embodied in the title "river of ice” becomes here inapt. Instead of flowing from a mountain down a sloping valley, it flowed radially from a central plateau of ice, with little regard for the slopes of the land over which it passed. We do not yet know the center of dispersion, but the ice entered our land as an invader from Canada. The border States from Maine to Minnesota were overrun, and most of the land north of the Ohio and Missouri Rivers. Twice the van was pushed far into the domain of the sun, and twice it was compelled to retreat; but when the sun finally surveyed its reconquered territory, the land was no longer simply graven with a tracery of rivers; it sparkled with the sheen of innumerable lakes.
Wherever the ice moved it swept forward the soil and all other loose material, and with them scoured the firm rock beneath, producing a polished surface of peculiar character, with many scratches and furrows parallel to the direction of motion. In some regions it did little more than this, but elsewhere it was a powerful agent of erosion, scooping out great hollows from the solid rock. For some reason not clearly understood the erosion was greatest along a zone parallel to the margin and a few hundred miles back from it, and here were formed the basins not only of the Laurentian lakes from Ontario to Superior, but of Winnipeg, Athabasca, Great Slave, and Great Bear Lakes. Within this zone of greater erosion the points of greatest erosion were determined chiefly by the pre-glacial shape of the surface. Where the land was high the overriding ice sheet was relatively thin, its motion correspondingly slow, its pressure slight, and its erosion unimportant. Where the land was low the deeper ice stream flowed faster, pressed harder on its bed, and eroded rapidly. How deep the original valleys were cannot be told, for the details of the old topography have been ground away, but we may be sure that they were shallow as compared to the existing troughs. The depths of Lakes Michigan, Superior, and Ontario reach from three hundred to five hundred feet below the level of the ocean, and their origin cannot be referred to stream erosion alone without incredible assumptions as to continental elevation.
Between the Great Lakes and over the country south of them are spread moraines and other deposits of ice-transported débris. They vary greatly in composition, structure, and topographic form, but have this in common, that their material differs in kind from the solid rock immediately beneath it, having been brought from more northerly points. Collectively they are called the Drift, and they dominate the surface, often concealing the rock for scores of miles. The typical morainic drift has a hummocky surface, abounding in small lake basins called “ kettles;" other varieties undulate more gently, and barbor broader but shallow lakes; and elsewhere the surface is smooth and completely drained. Over large districts, especially north of the Great Lakes, the drift is scant and irregularly spread upon an uneven rock surface, and there lakes are especially abundant, Many of them lie in rock basins, but the most are partly contained by walls of drift.
The Great Lakes, with the possible exception of Erie, all occupy rock basins, that is to say, they lie in hollows having continuous rims of solid rock; but these rims are in places coped by accumulations of drift in such way as to increase the depths and areas of the lakes and control to some extent the direction of their outflow. It is probable that the surplus waters of Superior and Ontario escape over the lowest points of their rocky rims, but if the drift were removed at the south end of Michigan the lake would find a lower outlet and become tributary to the Mississippi. The removal of drift between Huron and Erie would probably render them confluent, as Huron and Michigan now are. The removal of drift between Erie and Ontario would greatly reduce the upper lake, or possibly drain it completely, and would make it tributary to the lower at Hamilton, Canada, instead of at Fort Niagara.
As soon as the ice was gone running water began a work of reclamation, washing the earth from the steeper slopes down into the lakelets, and cutting down their outlets until they became so shallow that vegetation could take up the work and fill them to the top with peat. Half of the moraine lakes have been thus converted into marshes, and through extensive districts in Ohio, Indiana, and Illinois only the marshes, and the deposits of peat and marl where marshes have been drained, remain to show how numerous were the lakes. The drift-dammed lakes are better preserved, partly because in their region there is less loose dibris with which to fill them, partly because their outflow is often across resistant rock. Of progress toward the destruction of the Great Lakes there will be occasion to speak in another connection.
But the story of the lakes is not completely told by explaining the origin of their basins; there is also a history of their development as water bodies. During the period of greatest ice expansion the hydrographic basin of the Great Lakes and the valley of the St. Lawrence were not merely filled but overpassed, so that the rivers generated on the glacier in summer fell from its southern edge beyond the rim of the Great-Lake basin, and flowed to the Missouri, the Mississippi, the Ohio, the Susquehanna, and the Delaware. As encroaching heat gradually reduced the limits of the ice, its retreating margin reached and passed the basin rim at various points, and there accumulated between the water parting and the ice wall a series of glacial lakes, fed by the melting ice and discharging southward across the passes of the great divide. The precise order of events has not been made out, but there was a time when the western part of the Superior basin contained a lake discharging to the Missis. sippi River by way of the St. Croix, there was a time when the southern part of the Michigan basin held a lake discharging to the Mississippi by way of the Des Plaines and the Illinois, and there was a time when a lake, occupying the western half of the Erie basin and covering the Maumee valley, overflowed at Fort Wayne to the Wabash River, and thus sent its water to the Ohio. At a later stage a single sheet of water covered the southern part of the Huron basin and all the Erie, and encroached slightly on the Ontario. Then the ice retreated from all the