GEOMORPHY AS P:XEMPLTFIED IN VALLEYS. 



107 



When the depressions in the rock surfaces grow wider and deeper and 

 are ke|)t open in their descent, one can only conclude tliat they represent 

 the molding of old land surfaces hy running water, no matter whether 

 tlie valleys l)e now huried hy drift or sul)merged beneath the sea. The 

 geomorphic studies esi)ecially 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 i)henomena "we can learn 

 something of the geologic history of the land, while the rock formations 

 indicate the contemporaneous history of the sea. To rei)eat, when we 



Figure S.— Cross-sect ion (t;) representing n bnselevel Valley in Trinidad MoHut(ti}is, Cuba. 



This diistrict was recentlj^ elevated, and a stream at c is engaged in cutting back a canyon GOO 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 interi)retation 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 Itroad undulations in the earth's crust which cliange 

 thej relation of the land and sea and raise up barriers across valley's, so 

 as to form l)asins, or divert the drainage of the land without producing 

 crumpling and folding of the strata or other great distortions, such as 

 in mountain uplifts, have been by ^Ir Gill.)ert denominated epeirogenic 

 (continent-making) in contrast with orogenic (or mountain-making) 

 movements. Such undulations are well known in the gentle rising and 

 sinking of coasts. In the deserted beaches of the region of the Great 



•See U. S. Hydrogmphio charts nos. :jl-3fi, 21,21a, 1007, 1411, Coast Survey eliarts C,P, and numer- 

 ous hiirrior cliarts. Tht- Hydrographic eliarts have on them many soundings taken from the 

 British Admir.dty and uther surveys iidjacenl to tin; islands and coast of Florida nut given on the 

 Coa»t Survey cliaris. These British Admiralty charts have also been studied. 



