Marginal Types
Across Kentucky the conditions were similar to those of Tennessee, except that the hard Coal Measure sandstones were less elevated and formed no plateau, and subsequent erosion, as the Ohio river is approached, has been more and more active, until in the immediate vicinity of the river the pleneplain is recognized with difficulty. The conditions north of the Ohio river are at present entirely unknown, and the only suggestion the present writers can offer is that probably the two peneplains gradually approach each other in that direction until they practically coincide. About the southern margin of the province the elevation between the two periods of baseleveling was so slight that the rocks have been practically exposed to baselevel conditions from nearly the beginning of Cretaceous to Neocene time, and as a result are deeply decayed and but poorly preserve the records of the past. In the Coosa valley the Tertiary peneplain is generally distinguishable, although subsequent erosion has cut deeply into its surface and, owing to the decay of the rocks, has reduced the least resistant members to a still lower baselevelthat at which the present streams of the region are flowing. Continuing eastward, the vertical interval between the Cretaceous and Tertiary baselevels decreases and in the vicinity of Atlanta they practically coincide, so that the recognition of the two peneplains is almost impossible. The streams have not cut below the old peneplains in their upper courses and the tributaries of the Chattahoochee and Tallapoosa rivers still flow upon the surface of the Cretaceous peneplain. On the southeastern margin of the province, throughout the piedmont plain, the Tertiary peneplain is well developed and only occasional monadnocks show the position of the Cretaceous plain. Although crystalline rocks are generally regarded as offering great resistance to erosion, they are, under baseleveling conditions, subject to very deep decay and probably at the close of the Cretaceous cycle were softened to a far greater depth than at the present time.