CORBASION BY GRAVITY STREAMS. 277 



(a) Rock bisins. — These will be expected mostly in 

 Alpine regions — localities of profound canons with steep 

 channel slopes and of pronounced canon constrictions. The 

 glacier depends for its energy on declivities and volume. 

 The rock basin evidences a dissipation of stream energy. 

 The declivity gives added energy while the rock basin is an 

 attempt to readjust matters in the opposite direction. So 

 long as the peculiar conditions last, which permit of 

 increased velocity, so must the rock basin heads recede and 

 the declivities be cut away. In homogeneous rock struc- 

 tures each will possess the cirque head, the one as a basin 

 sunken below the local base level, the other as a series of 

 small or mighty " steps " in the valley. Local variations 

 in rock structure, or in valley cross-sections on declivities 

 will give rise to basins with heads above base-level, and 

 the recession of such will give rise to " steps" with inter- 

 vening "treads," the "treads" tending to hold basins of 

 corrasion according to the degree of valley contraction 

 existing at those spots. 



All other things being equal, basins will have depths 

 proportioned to the strengths of the travelling glaciers. 

 The deeper and more confined the channels, and the more 

 pronounced the swarming of the Ice Sheet over the channel 

 rims, the smaller also the cross-sections are locally, so 

 much deeper and longer will be the basins formed. The 

 basin must be deepened and lengthened so long as the 

 vertical motion is not expended, or so long as the glacier 

 can transport its load either across any cross-section, or 

 along the basin base, as a whole. Therefore a glacier which 

 has decided velocity and depth, and which is confined within 

 high resistant walls (so as to prevent lateral escape) will 

 corrade its base to a much lower point than the associated 

 or neighbouring water streams. Fiord basin depths are 

 not related merely to the buoyancy of ice in water, but 



