The Encyclopedia Americana (1920)/Valleys
VALLEYS, in general, depressions of some magnitude in the surface of the land. Two great classes of valleys may be recognized, — (1) structural, and (2) erosional. The former include depressions due to folding, such as synclines; basins due to subsidence of an area such as might occur in regions of subterranean drainage, or in volcanic regions; fault valleys which when narrow gorges, like that of the Rhine, are known by the term of “graben,” and others. Erosion valleys embrace by far the more common types, such as river valleys, which range from gorges with vertical sides and completely filled by the river, to depressions many miles in width, with flat bottoms and gently sloping sides. These are carved wholly by the rivers that occupy them. The larger valleys of this type are generally found along the border line between ancient more or less disturbed, and more modern coastal plain strata which lap up against the older ones. By continued erosion the edge of the newer strata is pushed further and further away from the older land, until a valley of great width is produced along the strike of the strata. Valleys of this type are well developed along the Atlantic Coast, next to the crystalline rocks of the Piedmont district. Some valleys of this type have suffered drowning, as appears to be the case in Long Island Sound. Other valleys of this type have become filled with water and transformed into lakes by the stopping up — by glacial drift or warping of the land of their outlets. Such is the case with the valleys now holding the water of some of our Great Lakes, notably Ontario. Valleys due to glacial erosion are not uncommon, although it is probable that glaciers usually do little more than deepen valleys originally formed by streams. A type not included in the two divisions mentioned is found in intra-morainal valleys within the glaciated region. Here the valley is the remnant of the plain around which hills of glacial drift are built. See Mountain; Flood Plains.