THE WORK OF STREAMS 



127 



over the region (Fig. 116). Almost any topographic map of a desert 

 basin, however, shows that the slope of Piedmont plains is usually 

 considerable. 



In desert regions oases are often found on alluvial fans, since water 

 can be obtained here from wells or from the mountain streams. The 

 principal settlements of Utah are on the alluvial slopes at the foot of 

 the Wasatch Mountains, and many of the cities of Persia and Turke- 

 stan are situated on alluvial fans. 



(2) In Humid Regions. — Alluvial cones and fans are also de- 

 posited in moist regions where a main stream is unable to remove the 

 rock and silt carried into it by its tributaries. In such cases, the 



Fig. 117. — Lake Brienz and Lake Thun were formerly one lake, but have been 

 separated by an alluvial fan upon which Interlaken is situated. 



cone or fan forces the main stream over to the opposite side of the 

 valley, compelling it to undercut its bank. This may cause the for- 

 mation of rapids, with a shallow lake above. The Liitschine River, in 

 the Lauterbrunnen valley in Switzerland, has built an alluvial fan 

 which has divided the lake into which the river flows into two parts, 

 Lake Brienz and Lake Thun (Fig. 117). Fans in humid regions may 

 be of considerable extent, and are well-developed in portions of the 

 Rhone valley in Switzerland and in the larger valleys of the French 

 Alps. Since they are well-drained and usually fertile, they are often 

 the sites of villages. 



Alluvial Terraces. — Terraces are not uncommon in river valleys 

 and are composed either of rock, when they are called rock terraces 

 (p. 128), or of stratified clay, sands, and gravels, when they are known 



CLELAND GEOL. — Q 



