MA LA SPINA GLACIER. 24 1 



entrances to the tunnels are frequently high arches, and the 

 streams flowing into them carry along great quantities of gravel 

 and sand. About the southern and eastern borders of the 

 glacier, where the streams emerge, the arches of the tunnels are 

 low, owing principally to the accumulation of debris which ob- 

 structs their discharge. In some instances, as at the head of 

 Fountain stream, the accumulation of debris is~so great that the 

 water rises through a vertical shaft in order to reach the surface, 

 and rushes upward under great pressure. The streams flowing 

 from the glacier bring out large quantities of well rounded sand 

 and gravel, much of which is immediately deposited in alluvial 

 cones. This much of the work of subglacial streams is open to 

 view and enables one to infer what takes place within the tunnels 

 and to analyze to some extent the processes of subglacial depo- 

 sition. 



The streams issuing from the ice are overloaded, and, 

 besides, on emerging, frequently receive large quantities of 

 coarse debris from the adjacent moraine-covered ice cliffs. The 

 streams at once deposit the coarser portion of their loads, thus 

 building up their channels and obstructing the outlets of the 

 tunnels. The blocking of the tunnels must cause the subglacial 

 streams to lose force and deposit sand and gravel on the bottom 

 of their channels ; this causes the water to flow at higher levels, 

 and coming in contact with the roofs of the tunnels, enlarges 

 them upwards ; this in turn gives room for additional deposits 

 within the ice as the alluvial cones at the extremities of the 

 tunnels grow in height. In this way narrow ridges of gravel 

 and sand, having perhaps some stratification due to periodic 

 variations in the volume of the streams, may be formed within 

 the ice. When the glacier melts, the gravel ridges contained 

 within it will be exposed at the surface, and as the supporting 

 walls melt away, the gravel at the top of the ridge will tend to 

 slide down so as to give the deposit a pseudo-anticlinal structure. 

 Ridges of gravel deposited in tunnels beneath the moraine- 

 covered portion of the Malaspina glacier, would have bowlders 

 dropped upon them as the ice melts, but where the glacier is free 



