ABSTRACTS 765 



from 900 to 1400 feet in height, the upper portions being generally 

 formed by a series of cliffs. The two plateaus are separated by Sequat- 

 chie Valley, which is about four miles in width. Its western side, the 

 escarpment of Cumberland Plateau, is notched by numerous deep, 

 rocky gorges, cut backward into the plateau by streams flowing from 

 its surface ; while the eastern side, the Walden escarpment, forms 

 an unbroken wall. The eastern half of the district is occupied by the 

 Tennessee Valley, the river itself having an altitude of between 600 

 and 700 feet, while rounded hills and irregular ridges rise several hun- 

 dred feet higher. Leaving the broad valley, which continues southward 

 into Alabama, the Tennessee River turns abruptly westward at Chat- 

 tanooga and enters a narrow gorge through Walden Ridge. This part 

 of its channel is very young in comparison with the valley toward the 

 north, and there is evidence that the river has occupied its present 

 course but a short time, having formerly fiowed southward directly to 

 the gulf. 



Geology. — The rocks appearing at the surface within the limits of 

 the map are entrely of sedimentary origin, and include representatives 

 of all the Palaeozoic groups. The Cambrian formations include the 

 Apison shale, Rome sandstone, and Conasauga shale, a series which 

 is calcareous at top and bottom and siliceous in the middle. The 

 Conasauga passes upward through the blue limestone into the Knox 

 dolomite — a great thickness of siliceous magnesium limestone, the 

 lower portion of which is probably Cambrian. Above the dolomite 

 are Chicamauga limestone and Rockwood shale, the latter becoming 

 brown sandstone in White Ash Mountain. The whole of the deposi- 

 tion which took place in this region during the Devonian is apparently 

 represented by a stratum of shale from ten to twenty-five feet in thick- 

 ness — the Chattanooga black shale, which probably rests unconform- 

 ably upon the Rockwood. Above the Chattanooga are the Fort Payne 

 chert and Bangor limestone, forming the lower Carboniferous, and the 

 Lookout and Walden sandstones, forming the Coal Measures. Nearly 

 all the formations exhibit an increase in thickness and in proportion of 

 sand and mud toward the east, showing that the land from which their 

 materials were derived lay to the east and southeast. 



The geologic structure is simple in the region occupied by the 

 plateaus, and complicated in the valleys. In the Cumberland Plateau 

 the strata are almost perfectly horizontal, while in Walden Ridge they 

 have a slight dip from the edges toward the center. Sequatchie Valley 



