280 



EARTH FEATURES AND THEIR MEANING 



the lake (Fig. 308). Thereafter, the escaping water flows in a 

 braided stream across the late lake bottom and thence at the 

 bottom of the gorge through the moraine. 



FIG. 308. View of a drained lake bottom between the inorairip-covered ice front 

 in the foreground and an abandoned marginal moraine in the middle distance. The 

 water flows from the ice front in a braided stream and passes out through the mo- 

 raine in a narrow gorge. Variegated glacier, Alaska (after Lawrence Martin). 



The outwash plain or apron. The water which descends from 

 the glacier surface in the glacier wells or mills, eventually arrives 

 at the bottom, where it follows a sinuous course within a tunnel 

 melted out in the ice. Much of this water may issue at the ice 

 front beneath the coarse rock materials which are found there, 

 and so be discovered with the ear rather than by the eye. The 

 water within the tunnels not flowing with a free surface but being 

 confined as though it were in a pipe, may, however, reach the 

 glacier margin under a hydrostatic pressure sufficient to carry it 

 up rising grades. Inasmuch as it is heavily charged with rock 

 debris and is suddenly checked upon arriving at the front it de- 

 posits its burden about the ice margin so as to build up plains of 

 assorted sands and gravels, and over this surface it flows in ever 

 shifting serpentine channels of braided type (Fig. 308). Such 

 plains of glacier outwash are described as outwash plains or out- 

 wash aprons. 



Rising as it does under hydrostatic pressure the water issuing 

 at the glacier front may find its way upward in some of the ere- 



