THE WORK OF STREAMS 



93 



(7) The numerous falls of Switzerland were formed much as in 

 (3), but are largely due to the erosion of the main valleys by glaciers 

 so that the tributary streams enter their mains over falls. These 

 side valleys are called " hanging valleys." 



Exceptions — Falls not the Result of Erosion. — (1) A lava stream 

 (Fig. 70) may dam a valley and thus produce a fall. Many examples 

 of this sort might be 

 cited. (2) Limestone 

 (travertine) may be 

 deposited in streams 

 in such quantities as 

 to dam them, form- 

 ing falls (Fig. 71) and 

 even ponding back 

 the water to produce 

 lakes. Topolic Falls 

 in Dalmatia, east of 

 the Adriatic, afford 

 an illustration of the 

 construction of a 

 travertine dam. 

 These falls are 70 

 feet high and are 

 advancing down- 

 stream. (3) When 

 tributary streams 

 with steep gradients 

 carry a large quantity of coarse debris, they may deposit their loads 

 in the main stream in such amounts as to form temporary rapids. 

 Landslides also accomplish the same result. The Cascades of the 

 Columbia River were formed thus. (4) When a stream is forced 

 out of its valley by landslides (p. 73), glacial deposits (p. 155), or in 

 any other way, falls may result. 



Potholes. — When for any reason a strong, permanent eddy 

 is produced in a stream, as at falls or rapids, pebbles and stones are 

 given a rotary motion as they are carried through the eddy and wear 

 down the stream bed in this place, tending to produce circular holes. 

 These " potholes " (Fig. 72), " washtubs," " giant's caldrons," 

 or " kettles," as they are called, occur in hard granites as well as in 

 shales and limestone, and may be seen in the bed of almost any rapid 



Fig. 70. — Falls formed as a result of the damming of a 

 river channel by lava. (Modified after H. E. Gregory.) 



