70 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



unable promptly to remove from their beds all the material which tlie 

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

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

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

 than they deepen them. 



Sui-face 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 many of these surface valleys are due to 

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

 change of fx-ont, by its alternately advancing and retiring, 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 compaiutively 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 mo.stly a sharp bottom with sloi)ing V shaped sides with often 

 a winding course, but valleys due to ice action of tlie glacier type 

 have a rounded bottom and more perpendicular sides, giving such 

 gorges a \J shape. They also end in a glacial amphitheatre 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 other- 

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

 eroding eifect of rains and fi'osts. Some of the sinks are from forty 

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

 aci'es to two thousand acres. The rim of sandstone surrounding 

 these depressions is generally nearly level, the out cropping rocks 



