. Bulletin of the Geological Society of America. Geology. GEOMORPHY AS EXEMPLIFIED IN VALLEYS. 107 When the depressions in the rock surfaces grow wider and deeper and are kept open in their descent, one can only conclude that they represent the molding of old land surfaces by running water, no matter whether the valleys be now buried by drift or submerged beneath the sea. The geomorphic studies especially consider narrow canyons with steep walls, made by rapidly descending streams; broad valleys with commonly sloping sides, arising from rain-washes upon the hillsides when the drainage of the d
. Bulletin of the Geological Society of America. Geology. GEOMORPHY AS EXEMPLIFIED IN VALLEYS. 107 When the depressions in the rock surfaces grow wider and deeper and are kept open in their descent, one can only conclude that they represent the molding of old land surfaces by running water, no matter whether the valleys be now buried by drift or submerged beneath the sea. The geomorphic studies especially consider narrow canyons with steep walls, made by rapidly descending streams; broad valleys with commonly sloping sides, arising from rain-washes upon the hillsides when the drainage of the depression was reduced to the baselevel of erosion; the burial and the reexcavation of the valleys ; their terracing ; the tilting of the land surfaces and the change of direction of the drainage or the closing of the basins into lakes. From all of these phenomena we can learn something of the geologic history of the land, while the rock formations indicate the contemporaneous history of the sea. To repeat, when we. Figure 3.— Cross-section (v) representing a baselevel Valley in Trinidad Mountains, Cuba. This district was recently elevated, and a stream at c is engaged in cutting back a canyon 600 feet in depth. find systems of valleys beneath the sea, unless there are other local causes, we are led to conclude that they are the remains of land features now submerged, or, in other words, that they are evidence of former conti- nental elevation. Under this interpretation the writer has correlated the extension of the great rivers into their fjords cut through the conti- nental shelves along the coast, which have been made known by the numerous soundings * Deformation of Land Surfaces. The gentle but broad undulations in the earth's crust which change the| relation of the land and sea and raise up barriers across valleys, so as to form basins, or divert the drainage of the land without producing crumpling and folding of the strata or other great distortions, such as in mountain
Size: 2717px × 920px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
