70 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



unable promptly to reiiiove from their beds all the material which the 

 steeper valleys at the head and the slopes on either side throw into 

 them in times of freshets and heavy rains. In their struggles with 

 this material they wander to and fro and widen their valleys more 

 than they deepen them. 



Surface valleys are valleys of erosion but cut out of surface 

 accumulations, such as drift or lake beds which have not yet become 

 solid rock. They are, in consequence, relatively wide and shallow. 

 This class would be any where of more recent origin than the other 

 classes. In drift regions man}^ of these surface valleys are due to 

 the uneven formation of the drift. The retreating ice sheet by its 

 change of front, by its altei'nately advancing and retix'ing, and the 

 streams which issued from underneath the ice caused an accumula- 

 tion of debris at various places along its margin. This debris 

 forming barriers between higher lands, enclosed basins which for 

 want of an outlet became lakes which ultimately drying up either 

 by evaporation or the erosion of a channel through the morainic 

 barrier left broad valleys comparatively shallow when their wide ex- 

 tent is considered. 



Another form of valley of erosion is that due to glacial action. 

 These are known by their being diflerently shaped than those due 

 altogether to the action of running water. Water cut valleys 

 have mostly a sharp bottom with sloping V shaped sides with often 

 a winding coui-se, but valleys due to ice action of the glacier type 

 have a rounded bottom and more per[)endicular sides, giving such 

 ©■orges a U shape. They also end in a glacial amjihitheatre which 

 is usually wanting in water cut channels. The original form is 

 always more or less modified by other causes. 



In the districts overlaid by the subcarboniferous Protean beds of 

 Kentucky, Tennessee and Northern Alabama, there are numerous 

 small basins or cavities known as sink holes. Some of them are 

 dry, but others form small pools or lakes and afford water in otlier- 

 wise dry districts. These sinkholes exhibit in a striking manner the 

 eroding effect of rains and frosts. Some of the sinks are from forty 

 to one hundred and ninety feet deep and cover an area of from five 

 acres to two thousand acres. The rim of sandstone surrounding 

 these depressions is generally nearly level, the out cropping locks 



