106 J.W. SPENCER — RECONSTRUCTION OF ANTILLEAN CONTINENT. 



This, however, is rarely the case, for if the clenuded strata were restored 

 the position of the valleys would occupy the crests of the undulations, 

 as shown in the accompanyino- actual sections, where the broad valleys 

 occupy the surfaces of both anticlinals and s^mclinals. Another char- 

 acteristic of old valleys is that the streams flowing through them are in- 

 significant compared with their magnitude in both depth and width. 

 The limestones have been carried aAvay in solution and the finer mechan- 



,Sand Mt 



Feet 



Figure 1.— Section across Lookout- Wills' Valley, Alabama, at the Col. 



The incision in the tableland of Carboniferous sandstone here is about 4 miles wide and 500 feet 

 deep. From this section, for many miles downward in both directions, the general slope of the 

 floor of the united valley is about ten feet per mile, so that the tableland presents bold escarp- 

 ments more than 1,000 feet above the lower reaches of the rivers. It is an anticlinal valley. 



ical muds have been washed by the rains into the larger streams and, 

 suspended in the waters, they have been transported out of the valleys 

 and deposited on flood-plains or in the sea. Occasionally the streams 

 undermine the banks and obtain extraordinary cargoes, but the principal 

 widening agents are the rains and rills that everywhere wash away the 

 surface and undermine the mountain sides, which action is intermittently 

 retarded by the temporary protection of the unremoved materials of the 



' -^ ^- ^-~-.. Little Sand Mt. 



WUcal^J^Ji""' Feet 

 Figure 2. — Section from Pigeon to Little Sand Mountain, Georgia. 



This section represents a valley of about a dozen miles in width, with the complex geologic 

 base shown in the figure. It illustrates well how the valleys in the southern Appalachians are 

 produced by atmospheric denudation and not by mountain folding. 



landslides. While the tablelands are high, the streams are constantly 

 deepening their channels, but when the bottoms of the ravines are re- 

 duced to the baselevel of erosion, then the streams almost cease to corrade 

 and become geologic carriers of the surface washings of the valley. 



In the recently elevated mountains of Cuba and Jamaica, as in the 

 older Appalachian chain, the valley-making forces have overcome the 

 physical structure, so that the valleys are more or less independent of it. 



