The forms of Vermont's mountains, even to the highest summits, were to a great extent rounded by glaciation, but as the rocks vary much in texture and are often steeply inclined, stream erosion has cut valleys deep and narrow, often mere gorges.
Its surface is much broken by the remains of the ancient plateau which has been worn down by erosion, leaving escarpments and ranges of flat-topped mountains, called chapadas, capped in places by horizontal layers of sandstone.
The plan here too was roughly quadrangular with a central court, but owing to the erosion of the hillside a good deal of the eastern quarter has disappeared.
The reason is to be found in its geographical position, a cold ice-covered polar current 68' running south along the land, while not far outside there is an open warmer sea, a circumstance which, while producing a cold climate, must also give rise to much precipitation, the land being C', thus exposed to the alternate erosion of a rough atmosphere and large glaciers.
This rocky barrier acts as a regulator for the water received from Albert Edward Nyanza and, by checking the erosion of the river bed, tends to maintain the level of the lake.
For many years, the town has been threatened by the erosion of the river banks.
The granite forms the prevailing rock in valleys of erosion.
Straits have been formed (I) by fracture across isthmuses, and such may be by longitudinal fracture as in the Strait of Bab-el-Mandeb, or transverse fracture as in the Strait of Gibraltar or Cook Strait; (2) by erosion, e.g.
Being protected by the water from the rapid subaerial erosion which sharpens the features of the land, and subjected to the regular accumulation of deposits, the whole ocean floor has assumed some approach to uniformity.
One of the scarps or steps is the result of a great fault or displacement of the earth's crust, and is known as the Balcones fault scarp; others are due to erosion and weatherin g of alternate layers of hard and soft rocks lying almost horizontal.
In the south-west the results of this erosion are seen in an accentuated form in the region between the White river and the South Fork of the Cheyenne river, known as the Bad Lands or terres mauvaises.
These Bad Lands were once a fairly level plain, but intricate stream erosion produced the labyrinth of ravines and ridges for which the region is noted.
Produced by long-continued subaerial decay and erosion, in later Cretaceous times this lowland extended from the Atlantic Ocean well toward the interior of North America; since then the whole continent has been generally elevated, and by successive steps the Appalachian belt has been raised to form a wide but relatively low arch.
Or more to the famous Blue Grass Region, in which erosion has developed on limestone a gracefully undulating surface.
The horizontal strata of the plateau present equal ease or difficulty of erosion in any direction; the streams and the submature valleys of the plateau therefore ramify in every direction, thus presenting a pattern that has been called insequent, because it follows no apparent control.
The rivers are shallow and more or less broken by rapids in the notches; rapids occur also near the outer border of the crystalline belt, as if the rivers there had been lately incited to downward erosion by an uplift of the region, and had not yet had time to regrade their courses.
Itis determined The Great structurally by a belt of topographically weak limestones VaJie and shales (or slates) next inland from the crystalline ~ uplands; hence, whatever the direction of the rivers which drain the belt, it has been worn down by Tertiary erosion to a continuous lowland from the Gulf of St Lawrence to central Alabama.
The coastal plain, however, is the result, not of a single recent uplift, but of movements dating back to Tertiary time and continued with many oscillations to the present; nor is its surface smooth and unbroken, for erosion began upon the inner part of the plain long before the outer border was revealed.
The erosion of the region must have been far advanced, perhaps practically completed, in very ancient times, for the even surface of the peneplain is overlapped by fossiliferous marine strata of early geological date (Cambrian); and this shows that a depression of the region beneath an ancient sea took place after a long existence as dry land.
The extent of the submergence and the area over which the Palaeozoic strata were deposited are unknown; for in consequence of renewed elevation without deformation, erosion in later periods has stripped off an undetermined amount of the covering strata.
The occurrence of the lake basins in the lowland belts on either side of the Niagara cuesta is an abnormal feature, not to be explained by ordinary erosion, which can produce only valleys.
The basins have been variously ascribed to glacial erosion, to obstruction of normal outlet valleys by barriers of glacial drift, and to crustal warping in connection with or independent of the presence of the glacial sheet.
They are usually fine-textured limestones and shales, lying horizontal; the moderate or small relief that they were given by mature preglacial erosion is now buried under the drift, but is known by numerous borings for oil, gas and water.
Thick, which cover the underlying rock surface for thousands of square miles (except where postglacial stream erosion has locally laid it bare), and present an extraordinarily even surface.
The complexity of the glacial period and its subdivision into several glacial epochs, separated by interglacial epochs of considerable length (certainly longer than the postglacial epoch) has a structural consequence in the superposition of successive till sheets, alternating with non-glacial deposits, and also a physiographic consequence in the very different amount of normal postglacial erosion suffered by the different parts of the glacial deposits.
The helicopter ride was demonstrating the widespread erosion of the surrounding plains.
The peneplain is no longer in the cycle of erosion that witnessed its production; it appears to have suffered a regional elevation, for the riversthe upper Missouri and its branchesno longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level.
On the east it is strongly undercut by the retrogressive erosion of the headwaters of the Red, Brazos and Colorado rivers of Texas, and presents a ragged escarpment, 500 to 800 ft.