866 



SCIENCE 



[N. S. Vol. XXV. No. 648 



These are the Piedmont Plateau on the east, 

 the Appalachian Mountains, including the 

 Blue Ridge and joining the Appalachian 

 Valley northwest of the mountains, and, stiU 

 farther northwest, the Appalachian Plateau, 

 including the Cumberland and Allegheny 

 plateaus. 



Southeastward from the Blue Ridge the 

 Btreams flow directly to the Atlantic in most 

 of the area, northwestward from it they flow 

 into the Appalachian Valley, and south- 

 westward along the valley into Alabama. 

 Most of the drainage of the Appalachian 

 Plateau is dendritic and flows northwestward 

 into the Ohio River from the eastern margin 

 of the plateau. 



The characteristic topography of the Pied- 

 mont Plateau is a smooth, even-topped upland 

 into which the stream channels are rather 

 deeply dissected. The Appalachian Moun- 

 tains, from maximum heights of 6,600 and 

 6,Y00 feet in western North Carolina, become 

 gradually lower toward Alabama and Vir- 

 ginia. Around their southern end the Pied- 

 mont Plateau merges with the Appalachian 

 Valley. The same is true in less degree in 

 Virginia. The Mountains are rugged and 

 deeply dissected, especially where they rise 

 abruptly from the Appalachian Valley. The 

 great Appalachian Valley is a composite of 

 many small valleys, separated by sharp linear 

 ridges and mountains. The valleys follow the 

 beds of soft rock and the ridges the hard 

 sandstones. The height of the valley is 

 greatest (about 2,000 feet) in southern Vir- 

 ginia and descends in either direction to about 

 500 feet in Alabama and northern Virginia. 

 The Appalachian Plateau in Tennessee and 

 Alabama are typically flat, table-topped moun- 

 tains, more or less dissected by stream gorges 

 and narrow valleys. They are preserved from 

 erosion by beds of hard sandstone. In Ken- 

 tucky and farther north the sandstones are 

 less prominent and the region is extensively 

 dissected into a network of hills and knobs. 

 The summits of these, however, fall in general 

 into planes. 



Only the larger rock divisions were shown 

 in color on the model. These corresponded in 

 the main with the great time divisions — Cam- 



brian, Ordovician, etc. The Carboniferous 

 was shown in two divisions, the Pennsylvanian 

 and Mississippian, and the Cambrian was 

 divided into a lower siliceous and upper cal- 

 careous group. The Archean was divided into 

 gneisses and igneous rocks. These greater 

 divisions correspond closely with the great 

 geographic divisions, the topographic features, 

 in fact, being very largely determined by the 

 progress of erosion on the different forma- 

 tions, according to their solubility. Thus, the 

 Appalachian Plateau is formed mainly of 

 Pennsylvania sandstones and shales with bor- 

 dering zones ■ of the Mississippian limestone. 

 The Appalachian Valley is underlain in the 

 main by narrow bands of the Devonian, Silu- 

 rian, Ordovician and the calcareous division 

 of the Cambrian rocks, while along its south- 

 east border lie the siliceous Cambrian rocks. 

 These also form the northwestern part of the 

 mountains in a comparatively narrow band 

 extending throughout the Appalachian system, 

 with a few outliers farther southeast. 



The main mass of the mountains is com- 

 posed of gneisses through which have been 

 injected igneous rocks of various descriptions 

 — mainly granites. These rocks are, for the 

 most part, Archean, but include also some of 

 Algonkian age. They extend southeastward 

 over the Piedmont Plateau in broad areas. 

 Over the plateau there are also found large 

 masses of later igneous rocks of approximately 

 Carboniferous age. In this respect the Pied- 

 mont Plateau resembles the eastern part of 

 the Appalachian province in New England. 

 Other resemblances are seen in central Vir- 

 ginia and North Carolina, where sediments of 

 Silurian age are found. Knowledge of these 

 parts of the Appalachians is at present very 

 limited. Over the Piedmont also are isolated 

 basins of Triassic red sandstones and shales. 



The Appalachian structures also fall into 

 main groups similar to the geographic and 

 geologic features. The Appalachian Plateau 

 is underlain by rocks which are nearly flat. 

 In the valley all of the formations are steeply 

 folded, overturned, and, in places, faulted. 

 The local changes in the extent and type of 

 deformation express the differences in the 

 character of the rocks. Open folding at the 



