Geography of Ohio 225 



remaining absolutely static induces chemical alteration; lime- 

 stone is thus corroded below the level of wave action. But the 

 efficient weathering work of lakes is accomplished by waves which 

 depend upon winds. Winds through friction of the atmosphere 

 against the water's surface, roughen the surface into waves. 

 The impact of the waves upon the shore, driving tools against 

 it, cuts the shore. The effectiveness of wave work shows more 

 readily on a steep shore. When the marginal part of the lake- 

 basin has a gentle slope, wave work is slow in establishing a cliff; 

 if the slope is very long, the waves may never form a cliff, but 

 instead, will pile up material off shore, there constructing a 

 beach. After such a beach becomes well developed, wave work 

 may steepen its outer slope, or even cut it as a cliff. 



At all places along the border of a lake the waves are either 

 undercutting, or piling material up. At different times during 

 the year, depending upon the strength of winds, both processes 

 may be in operation at the same place. 



The shape of a lake has much to do with the efficiency of wave 

 work on its shores. The longer axis of Erie coincides approxi- 

 mately with the prevailing wind direction. As a result its shores 

 have been subject to vigorous wave work, both constructive and 

 destructive. In case the prevailing wind direction has been the 

 same through postglacial times, and there is no reason to think 

 otherwise, the ancestral lakes of Erie experienced similar effects. 

 The vigorousness of wave work to-day may be observed at most 

 any point along the lake. The lake cities have to make annual 

 appropriations either for retaining walls to save their front, for 

 lengthening piers, or for dredging to make it convenient for ves- 

 sels to land. 



The development of cliffs. Active wave work develops cliffs 

 either in unconsolidated material or in rock. The slope of the 

 cliff is determined by its material, being steepest in rock; but the 

 process of cliff development is the same in either case. Waves 

 impelled by the winds w^ear the cliff by impact alone ; when the 

 water carries stones, the cliff is cut more rapidly. At the point of 

 attack, the cliff is undercut ; when undercutting has proceeded far 

 enough, the overhanging material drops off. As a usual thing, 

 where cliff-cutting is in progress, the waves are strong enough to 

 develop a current along the shore, which removes the blocks 

 that have dropped into the water, as they are ground to bits by 



