Geography of Ohio 199 



These lake deposits have been exposed to weathering since 

 glacial times. This interval is variously estimated as from ten 

 to forty thousand years. However long it really is, w^eathering 

 has spared to an amazing degree the work of the glacial lakes. 

 Their old shorelines, with interesting details of structure, in addi- 

 tion to the cliffs cut in enduring rock, stand out conspicuously 

 to-day. Their deltas, creased, to be sure , by gullej^s and streams 

 during the post-glacial time, still bear their original outlines. 

 The locally thick mantle of lake clays has sometimes slumped into 

 corrugated ridges, but is always easily recognized. The islands 

 that often dotted the ice-front lakes are encircled by cliffs and 

 beaches; those near the shore were tied by bars, which cannot 

 escape one's attention even to-day. Their overflow channels 

 now are frequently only fossil river-valleys. No phase of glacial 

 history inspires greater zest in investigation. 



EFFECTS OF GLACIAL ACTION 



Usually there is no difficulty in distinguishing a glaciated from 

 a non-glaciated region. Normally, a country's surface is made 

 irregular by rivers and other weathering agents; but while that 

 country is buried beneath an ice sheet, these agencies cease to 

 act. The rasping and aggrading effects of a continental glacier 

 tend to decrease the irregularities of pre-glacial topography. 

 Eminences are smoothed out or rounded. Broken slopes are usu- 

 ally made more regular. Minor depressions are frequently filled 

 with debris. Thus, the usual effect of glaciation is to decrease 

 surface irregularity. At the same time, glaciers often increase the 

 relief. The extent of smoothing, and of giving added relief, 

 depends somewhat on the rock texture and structure of the region. 

 Easily eroded rocks suffer most from glacial action. When hard 

 rocks do not exist in the area itself, or in its immediate environ- 

 ment, the glacier is not supplied with effective tools for erosion. 



When relief is increased. Glacier ice cuts deep only when its 

 erosive powers are concentrated. Whether a given area under 

 glaciation will come out w^ith increased or diminished relief, 

 therefore, depends upon its topography preceding glaciation. If 

 it already bears a pattern into which the ice fits as it moves, the 

 relief of that pattern will be increased. Such is the case when the 

 country has valleys trending in the direction of the ice motion. 



