Popular Science Monthly/Volume 2/April 1873/River and Lake Terraces
|RIVER AND LAKE TERRACES.|
TRAVELLERS along the river-valleys of New England, and in other sections of our Northern States, will observe that the banks in many places rise by a series of terraces, which at a distance resemble the steps of an amphitheatre. Carved with singular uniformity upon the slopes, they are everywhere a striking and beautiful feature of these most picturesque and beautiful landscapes. In the valleys of the Connecticut, Merrimac, St. Lawrence, Kennebec, Hudson, and innumerable other streams, these levels have been utilized as sites for villages, country-seats, forest, and cultivation.
Northampton, Brattleboro, and Springfield, are built on terraces; and part of the charming village of North Conway, at the gate of the White Mountains, stands upon a similar level. Dartmouth College is upon an elevated terrace.
Terraces occur on both sides of the Niagara River, and on the east side four levels are described, the highest being 38 feet above the top of the American Fall. They occur also on the Hudson Highlands at Cornwall 180 feet, and at Cozzens 130 feet above tide-level. The Catskill Mountains are fringed with terraces almost to their summits; and on the east side of the Hudson, at Albany, eight distinct levels are passed on the line of the Boston and Albany Railway before reaching the summit station.
On Hoosac Mountain is a terrace 1,813 feet above the level of the sea, and near it an ancient beach 200 feet higher. They occur at Quebec, 500 feet; at Montreal, 400 feet; and, on the Genesee River, 1,410 feet above the ocean-level.
But terraces abound on lake-margins with the same distinctness as on the banks of rivers. Prof. Agassiz counted fifteen on the shore of Lake Superior, and the writer counted six, beautifully defined, at Portage Lake. Visitors at Watkins Glen may notice terraces sculptured on the amphitheatre of hills at the head of Seneca Lake, whose geological history is contemporary with that of the great gorge, the object of their visit. In Northern Utah lake-terraces are found, according to Hayden, nearly a mile above the ocean, and on islands in Barrow's Straits they occur at 1,000 feet elevation.
On some of the great Western prairies terraces extend like vast coast-lines bounding the plain.
Nor are they confined to North America. They have been noticed on the slopes of the Ural and Altai Mountains, around the Dead Sea, on the banks of the river Jordan, on the mountain-sides in the Great Sahara, and on the banks of the Nile above the first cataract.
The ocean, too, has its terraces. Darwin observed that, around Patagonia, the ocean had eaten deep into the rocky coast "a series of step-like plains." Roads are carried up the Cordillera on elevated terraces to a height of 9,000 feet.
These formations, so widely distributed and so uniform in their aspects, have an important geological significance. They are evidently among the latest results of the dynamic agents which have modified, and are still modifying, the surface of the globe. Those along the banks of rivers have been formed during the erosion of the valleys. Their history, therefore, begins with the development of the present
river-systems, and comprises what is known in geology as the "Terrace Epoch." They are most abundant and perfect in the drift latitudes—that is, where the continental floors are deeply covered by the waste and débris of the Glacial Period, which closely preceded that of the Terraces. If we examine the valley of a gently-flowing river, we may study all the processes by which it was formed, and step-like terraces distributed along its banks.
There is the channel along which the stream is flowing. By the side of it, at intervals, are verdure-covered meadows and deposits of shingle and sand, overflowed during periods of rain and freshet. These constitute what may be termed the river-flats or flood-plains. Something is added to it during each overflow. Meanwhile, the river-channel is deepening by the wearing action of the current and transportation of the materials of its bed. At length the waters are discharged along the channel, and no longer overflow the flood-plain, which becomes at once a terrace, the last formed and newest of the series, the oldest of which may be more than a thousand feet up the bank. Fig. 2 shows a section of a river-valley with terraces on one side only, a circumstance which frequently arises from sinuosities of the stream.
The newly-made terrace now really forms the bank or banks of the stream, and is itself slowly worn away and distributed elsewhere by the abrasion of annual freshets. Portions of it may thus disappear, but other portions remain.
It is obvious that terrace-formations occur in greatest perfection where the stream is not very rapid. Where it flows as a torrent, a flood-plain or delta may form only at its mouth. Sometimes, however, a swift stream is checked by the accumulations of débris or by rocky gorges, forming lake-like basins around which terrace-formations occur with great uniformity and beauty. The Connecticut River is 1,589 feet higher at its source than at its mouth; and, according to Prof. Hitchcock's excellent report on the Surface Geology of New England, twenty-two such basins, or levels, occur in its descent.
It is evident, as we have observed, that the highest terrace of a series is the one first formed and the oldest, but, when formed, was equally, with the last one, the flats, or flood-plain, of the river; whence it follows that the river was then much higher as regards the general level of the land than now. Its present deep valley was not excavated, but it by no means follows that the river was any higher as regards the level of the sea. A change of level has, indeed, taken place, but it has been of the land, not of the ocean. No truth in geology is better established than this perpetual oscillation of the crust of the globe, and from the unchanging ocean-level is measured the extent of the movement.
The process by which a river-valley is excavated, and terraces formed upon its banks, is directly connected with this elevation of the land. Indeed, it could occur only during a period of elevation, and may have commenced with the emergence of the land above the waters, for then would begin the flowing of streams and their concentration into larger ones, forming at last our magnificent system of rivers. During a period of subsidence, however, the rivers disappear, as their valleys are filled, and the land is overflowed by the invading ocean. Nor is proof wanting of submergence of a very large portion of this continent, especially that which is north of the fortieth parallel, directly following the Glacial and preceding the Terrace Epoch; and nowhere is that fact more apparent than in New England.
The occurrence of ancient beaches above the terraces on Hoosac Mountain, and among the White and Green Mountains 2,200 and 2,600 feet above the ocean, proves its former presence and the movement of its currents and waves.
At that period the site of the present rivers was the bottom of an ocean. It was during the progress of that period of continental depression and submergence that the glacial drift was modified and redistributed, forming enormous deposits, filling old basins and river-valleys, so that when the land emerged from the waters it was comparatively level, a few mountain-peaks rising above the plain.
It is at this point, as we have seen, that the present river-system with its terraced valleys begins, and the phenomena may occur in the following order:
1. Elevated beaches, indicating ancient sea-shores.
2. The highest river-terraces.
3. Continuous excavation of river-valleys, and formation of flats, or flood-plains.
4. Elevation of those plains above the overflow of the river, forming terraces.
The process of formation we have already described; nor does it appear that any dynamic agent was then in operation which is not in operation now. The work has been continuous.
The superposition of terraces early suggested the idea that their origin was due to a succession of sudden elevations of the land rather than to a continuous movement, and such, indeed, may have been the case in some instances. But their usual want of uniformity through long distances and of correspondence on opposite sides of the valley induces the conclusion that their immediate distribution is controlled by local circumstances, while the general cause has been a continuous and gradual elevation of the land, and the equally continuous action of running water. Currents of rivers are thrown from side to side by ice-borne bowlders and accumulations of débris, and pebbles will become so adjusted in the river's bed as to resist erosion, as shown in Fig. 3.
This is one of the causes of the sinuosities of rivers. The water, as Sir Charles Lyell observes, is thus frequently forced to cut new channels, by which means new terrace flats may be formed by redistribution of materials.
The transporting power of running water depends on its velocity. Hopkins, cited by Prof. Dana, says that its force varies as the sixth power of its velocity—that is, doubling the rate, increases the force sixty-four times. "If a stream running 10 miles an hour would just move a block of five tons weight, one of 20 miles would move one of 320 tons." When shallow streams are suddenly swollen to torrents, bowlders of considerable size are borne along with heavy roar—are broken by collision, and ground to pebbles and sand. Hence we find that heavy rock-masses are abundant near the sources of a river, finer materials along its valley, fine sand and silt at its mouth. The material thus comminuted is prepared for distribution over the terrace-flats and plains along the river's bed.
The following table is from Mather's "Report on the Geology of the State of New York:"
A stream having a velocity of—
|3||inches per second||wears away fine tough clay|
|6||""||removes fine sand.|
|24||""||removes pebbles one inch in diameter.|
|36||""||moves angular fragments having a diameter of two or three inches.|
The latter velocity is, however, very moderate, being little over two miles an hour. In estimating the transporting power of rivers, we must consider the important fact that rocks lose nearly one-third of their weight in water.
The formation of terraces on the borders of lakes has, equally with those along the banks of rivers, arisen from elevation of the land. By this means the drainage of Lake Superior became possible, and its terraces represent former levels of its waters. Nor less interesting is the fact that the great plains of Northern Africa, including the Sahara and the valley of Egypt, were emerging from the sea while the Nile was excavating its valley and carving into terraces the sands of the Nubian Desert more than 200 feet above the present bed of the river. The wave-eaten shores of Patagonia have been elevated above the ocean, and its terrace-plains, equally with those of lake and river valleys, constitute an epoch of geological history, and record the most profound of the earth's secular changes.