ORIGIN OF TOPOGRxVPHIC FORMS. 115 



rain of the arid area, to contribute to the planing down of its surface; but, 

 on the other hand, the volume ami consequently the energies of the stream 

 for corrasion are greatly increased by the copious contributions from the 

 mountain region. Therefore, in such cases corrasion by the streams reaches 

 a maximum, relative to erosion of the terrain. 



It is thus that canyons in the arid region are formed. They are found 

 wherever, from any cause, stream corrasion is decidedly more rapid than 

 surface erosion. 



Such canyons, when in homogeneous rocks, rarely contain vertical 

 cliffs. These are commonly formed in strata of differing hardness bv sap- 

 ping and undermining - , which will be explained later. 



In certain parts of the arid region, notably in the Great basin, the rain- 

 fall is so scanty that the drainage systems are very feeble. The little rain 

 that falls in the valleys is at once absorbed by the thirsty soil or the atmos- 

 phere, while the streams which flow down from the mountains, cutting, it 

 may be, deep canyons in their sides, dwindle away on reaching- the valley, 

 depositing, as they sink, their loads of detritus. With this detritus have 

 been floored to a vast depth most of the valleys of the Great basin. It has 

 been deposited there, instead of being carried oft* to the sea. The Great 

 basin, which is in reality a large number of basins more or less independent 

 of one another, is without outlet simply because of its small rainfall. Were 

 the rainfall to increase, it would soon contain many lakes, and as the water 

 rose these would overflow, the higher flowing into the lower and the lower 

 flowing into the sea. The streams connecting them and the sea, would soon 

 corrade channels, cutting them down to lower and still lower levels, and 

 progressively draining these lakes, and thus a drainage system would be 

 established. 



Sinks exist in other parts of the country, but are there due to different 

 causes. They are common in the Appalachian region. In these sinks the 

 water has an underground outlet through passages in the soluble limestone 

 with which the valleys are floored. They are common among the terminal 

 moraines of the continental glacier, in Minnesota, Wisconsin, Michigan, and 

 New England, where they are called kettles. Here glacial material has been 

 deposited so recently that time has not yet been afforded for the establish- 

 ment of drainage systems. 



