Geography of Ohio 401 



the more mature dissection. From the standpoint of the river 

 cycle now in operation, there is no typical old age topography in 

 the state; on some uplands, between troughs that are still being 

 deepened, one may see the broad gentle slopes of old age, the 

 remnants of an older base-level period. 



Lake plairi area. In the north and northwest parts of the 

 state, the relief is youthful. This is the region of the old lake 

 plain and the adjacent prairie land. The streams, since the 

 retreat of the glacier, have been worldng to the base-levels formed 

 by the different stages of Lake Erie; there have been three impor- 

 tant stages preceding the present (p. 214). Nearly all these 

 rivers have gorges that extend back a few miles from the lake; the 

 size of each gorge varies with the texture of the rock in which it is 

 cut. At a few places along the lake the streams are carving their 

 channels entirely in glacial drift ; but the glacial drift, the shale, and 

 the soluble limestones, yield quite readily to erosion. 



Another feature of the youthful topography in the lake plain 

 region is the extensive swamp areas; a long time will elapse before 

 the rivers will have pushed their tributaries across these swamps. 

 The state, therefore, is constructing artificial channels which lead 

 to the nearest streams, thus hastening what nature unaided would 

 do in time. 



Plateau area. In the plateau area there is much more dissec- 

 tion. Here are the headwater branches of the streams that 

 belong to the Ohio river and to the Lake Erie basins. This con- 

 dition of more advanced relief is not constant throughout the area. 

 In some parts post-glacial rivers have done but little trenching, 

 and the relief is largely the work of older drainage; the surface 

 is rolling and the minor divides are veiy irregular. 



The Ohio river is the immediate base-level of all these south- 

 flowing streams, but at particular places along their courses hori- 

 zons of resistant rock form local base-levels, to which the up- 

 stream parts for varying distances become graded. The whole 

 area is quite thoroughly drained; the stage is mature, but less 

 mature than would be anticipated at such a distance from the 

 sources of so long a river as the Ohio. The absence of relief of at 

 least an early old-age stage, in southern Ohio, considering the 

 length of the Ohio river, is due to the origin of this river, as already 

 explained (p. 392). Down the river towards Cincinnati the relief 

 is more immature; the stage of dissection, however, does not 



