Читать книгу The Niagara River - Archer Butler Hulbert - Страница 34

What did the waters of the lakes do before the Niagara began its history? How long has it been at its present work? These are the questions interesting to every one; and by far more interesting to one who is making a study of the formative forces now contributing, and which have contributed to bring about the present characteristics of surface structure. A few important facts exist, and these now are beyond doubt, upon which rest the inferences concerning the age of the Falls. In ancient times the waters of Lake Erie did not find an outlet through Niagara River, so there was no channel ready made for the river when it began its present course. Even after the beginning of the river the upper lakes, Huron, Michigan, and Superior, did not discharge their waters through Niagara. Until comparatively recent times only the waters from Lake Erie discharged through this channel and therefore for many ages only a small fraction of the present volume could possibly have been at work on the Falls.

The striking features of the Gorge are modern, and have been very little affected by those agencies which are continually moulding the contours of land surfaces. The inclination of the river's bed has varied greatly with the ages, due to gradual uplifting or depressing of the earth's crust; consequently the current has varied greatly in velocity with these changes. A calculation of the work done by the river during each epoch of its history is indeed fraught with many difficulties. Much investigation, however, has been made along this line and with a rather satisfactory degree of success.

Niagara appears to have had a life peculiar to itself; but what is unique in its history, is the presentation of characteristics which in the case of other rivers have long since passed away. Rivers, and especially very large ones, appeal to us as "unchangeable as the hills themselves"; but the truth is, that the very hills and mountains are changing as a result of the forces exerted by water. Niagara, as viewed by the geologist, is unique, not on account of its having a different history than any other river, but for the reason that it had a more recent beginning. The calculation of the life of such a stream is interesting in itself, besides the other great questions settled by the solution of such a problem as the probable number of years that the river shall exist in its present form, the centuries which have elapsed since the ice retreated from this region, and the ascertaining of certain facts concerning the antiquity of man. In order to make a thorough study of these topics, one must take a view of the relief features of the Niagara region, and make a careful review of what conditions existed at the time that this district was covered by the great ice sheet, together with the changes effected during the retreat of the Great Glacier to the north.

Niagara River has its origin in the eastern end of Lake Erie, about three hundred feet higher than the surface of Lake Ontario. Passing from Erie to the last-mentioned lake the descent is not gradual, but one finds a gently rolling plain with almost no slope for nineteen miles until almost at the very shore of Lake Ontario, where almost unexpectedly one comes upon a high precipice from which a magnificent view of the lower lake may be gained, only a narrow strip of beach intervening. This cliff is called by geologists the Niagara escarpment.

When the river leaves Lake Erie its waters are interfered with by a low ledge of rock running across its channel. After passing this its waters meet no more troublesome obstructions until coming to the head of Goat Island. The river can scarcely be said to have a valley. One is reminded more of an arm of the lake extending out over this region. The country from Lake Erie to near the head of the Rapids above the Falls rests on a stratum of soft rock; from the Falls northward the underlying stratum is formed by a ledge of hard limestone, and beneath this a shale and two thin strata of sandstone. By the descent of the Rapids and the Falls, the waters are dropped two hundred feet, and thence through the Gorge they rush along at an appalling rate over the descent, through the Whirlpool and on to Queenston for a distance of seven miles. From this city to the lake there is little fall and so only a moderate current.

The deep, narrow gorge extending from the Falls to Lewiston is the especial subject of study to the geologist. This canyon is scarcely a quarter of a mile wide, varying little in the distance from cliff to cliff throughout most of its course. This chasm opens up before the student with almost appalling suddenness, while travelling over an otherwise regular plain. Its walls are so precipitous that few opportunities are offered for scaling them; and their height from the bottom of the river varies from two hundred to five hundred feet. An examination of both sides of the Gorge shows the same order in the layers of rock and shale on comparatively the same level, with the same thickness of each corresponding stratum. If a superstitious person had come unexpectedly upon this gigantic fissure ages ago, he might easily have imagined it to have been the work of some mighty mythological hero; but the modern scientist has reached a much better, as well as a much more satisfactory conclusion, namely, that this immense cleft has been sawed by the force of the water, from a structure whose features were continuous, as is manifest by the similarity of the exposed strata on the two sides of the stream. To be convinced of the fact that the Falls are gradually receding, it is only necessary to observe them closely for a few years. The breaking away of an immense mass of rock previously described is one of the recent events in the history of the river. This establishes the fact that the Gorge is growing longer from its northern end through the agency of the waterfall.

These facts show us the river working at a monstrous task. Its work is only partly done. Two questions come to us almost immediately: When this work is done what will it do? and, What did it do before its present work begun? The waters of Lake Erie could never have flowed to Lake Ontario without wearing away at the Gorge we now see. The birth of the river and the cutting of the canyon were simultaneous. Of this much we are assured.

A superficial study of a map of North America will show at once a great difference in the northern and the southern sections. From the region of the Great Lakes northward the district is one continuation of lakes, ponds, swamps, and rivers with many rapids. South of the Ohio there are few lakes, and the rivers flow on with almost unbroken courses. Here is a region much older than that to the north; and its waters have had ages more in which to mould down elevations and fill up depressions. The cause of this difference in the characteristics of the streams of the North and those of the South is to be explained by the great Ice Age. As far as we now know there may have been little difference in relief forms between the two sections before the encroachment of the ice. During the glacial epoch the whole northern part of the continent was covered with a thick ice sheet, which was continually renewed at the north, and as continually drifted slowly in a general southerly direction. As this heavy ice cap passed over the surface, it acted somewhat like a river in its erosive power, only working much greater changes. It not only picked up loose particles, but also scoured and wore away solid rocks along its bed. Thus the whole configuration of the country was changed.

At the southern terminal of the glacier, where it ended in the ocean, the ice broke away in large bergs, as in the northern seas to-day; but where the advancing ice met the warmer climate on land, it was melted and thus deposited at its terminal all the material it carried. The eroding power of this ice sheet, together with the deposit of its materials on melting, brought about a great change in the configuration of the country. Many old valleys were obliterated, while a number of new ones were carved. As the ice retreated northward with the change of climate, new lakes and rivers were formed. Many times the streams escaping from the lower level of lakes were forced to find an entirely new course, and so to carve a new channel of their own. The region of the Great Lakes and the Niagara River is no exception to this rule; and it is with the ending of the Ice Age that the history of the river begins.

A glance at a map shows a low range of hills or rather a gentle swell in the land surface forming the watershed between the lakes and the streams flowing to the south. At the time of the farthest southerly extension of the glacier it reached beyond this elevation; and its waters were discharged into the rivers flowing to the south. When the southern terminal had retreated to the north of this divide, but still blocked all outlet to the north or east, there was doubtless a number of lakes here discharging their waters across the present low watershed to the south. Some of these ancient valleys can still be traced for long distances of their course. These lakes passed through their varying history as those of to-day, their surface troubled by wind and storm and their waves leaving indelible carvings upon their shores.

One of these lakes occupied what is now the western end of Lake Erie, shortly after the ice front had passed to the north of the watershed mentioned. There are still very definite markings which show that its waters were discharged across the divide by a channel into the present Wabash River and thence into the Ohio. This channel can be traced throughout most of its course very easily. There are at least four distinct shore lines preserved to us, which show four successive levels of the lake as it reached lower outlets before the Niagara River was born. All of these old shore lines can be traced throughout most of their courses.

As the ice continued to retreat, next we notice the greatest change in elevation of the surface of the water. The ice front finally passed to the north of the present Mohawk River, thus allowing the waters to escape by that outlet, and, as a consequence, lowering the surface of the lakes by over five hundred feet. This drained a great extent of land and dropped the surface of Ontario far below the present level of the Niagara escarpment. Then for the first time the Niagara began to flow, and its Falls began their work. Immediately upon the formation of this new, lower lake it began the work of leaving its history carved upon the rocks, sands, and gravels which formed its shores. Its first ancient beach is more easily traced for almost its entire course than any of the other old levels. It does not even take the trained eye of the scientist to see its unmistakable history written in the sands. The earliest western travellers describe the Ridge Road running along this old, deserted beach as showing unmistakable signs of having been an ancient shore line of the lake.