In the Sierra 



have fallen into the channels, and which the 

 ordinary summer and winter currents were 

 unable to move, are suddenly swept forward 

 as by a mighty besom, hurled over the falls 

 into these pools, and piled up in a new dam 

 together with part of the old one, while some 

 of the smaller boulders are carried further 

 down stream and variously lodged according 

 to size and shape, all seeking rest where the 

 force of the current is less than the resist- 

 ance they are able to offer. But the greatest 

 changes made in these relations of fall, pool, 

 and dam are caused, not by the ordinary spring 

 floods, but by extraordinary ones that occur 

 at irregular intervals. The testimony of trees 

 growing on flood boulder deposits shows that 

 a century or more has passed since the last 

 master flood came to awaken everything 

 movable to go swirling and dancing on won- 

 derful journeys. These floods may occur dur- 

 ing the summer, when heavy thunder-show- 

 ers, called " cloud-bursts/ ' fall on wide, steeply 

 inclined stream basins furrowed by converg- 

 [ 63 ] 



