Geography of Ohio 191 



Rivers cut rock, heat and cold crumble it, chemicals disintegrate 

 it, tool-laden winds scour it, lightning may rive it, and earth- 

 quakes disrupt it, but never is the effect that of glacial action. 



Contrast between glaciated and unglaciated regions. In countries 

 where glaciers have never scoured, we find the bed rock covered 

 by a mantle of residual soil, the thickness of which depends upon 

 a great many conditions. But where a glacier has moved over 

 the area, this residual material is largely, if not entirely, wanting. 

 Valleys in an unglaciated region are narrow or flat, according 

 to their age. When narrow, their cross-section is like the letter 

 V; when broadened by age, the sides of the V are more flattened. 

 Broad, unglaciated valleys have smooth sides, the slopes are con- 

 tinuous and uninterrupted; a view through such a valley shows 

 interlocking spurs, the inheritance of a natural tendency of streams 

 to swing. In an unglaciated area the surface reflects the texture 

 and attitude of the rocks; shoulders and escarpments mark the 

 harder horizons. From cliffs of a hard ledge, overlying a softer 

 horizon, continued weathering removes great blocks which come 

 to a temporary position of rest on the slope below. Accentu- 

 ated weathering along closely assembled joint planes carves the 

 ledges into columns and pillars, which sometimes are isolated as 

 great stacks and spires. The hills and mountains of unglaci- 

 ated countries have uniform outlines, except when folding has 

 tilted the layers ; then the harder beds break the otherwise even 

 slopes. 



Should such an area as the one just described be glaciated, either 

 the valleys would be buried, or their cross sections would be so 

 altered that they would resemble more nearly the letter U. If 

 the valleys were transverse to the direction of glacial motion, in 

 all probability they would be largely filled with drift; if parallel 

 to glacial motion, the^^ would be plowed out and their profiles 

 would be changed. The ice, feeding through these valley's, would 

 wear off the ends of the interlocking spurs. Instead of having 

 continuous slopes, near the axis of the valley the slope would be 

 oversteepened, while farther up, it might be made irregular by 

 deposits of drift. The cliffs and other evidences of differential 

 rock weathering would be partly or entirely obliterated through 

 glacial scouring and plucking. The remnants of weathering, such 

 as spires, stacks, and detached blocks on slopes, would be removed. 

 The hills would no longer have uniform outlines, but would be 



