Vol. 65.] GLACIAL EROSION IS NORTH WALES. 315 



of erosion the further deepening of the water- or ice-channel 

 is very slow ; the erosive changes by the streams themselves then 

 take place chiefly at the stream-heads, while general weathering 

 reduces the inter-stream surfaces. 



As to contrasts in the behaviour of ice-streams and of water- 

 streams : — the ice-streams have an extremely small velocity as 

 compared with water-streams, and this involves very different pro- 

 portions between the cross-sections of the channel and the valley 

 in the two cases. Ridges that would abundantly suffice to separate 

 neighbouring water-streams might be overtopped by ice-streams ; 

 hence, when glaciers come to occupy a normally carved district, 

 peculiar features may be expected to result from the overflow of 

 ice-distributaries across low divides. The slow motion of ice- 

 streams involves a reduction of momentum and centrifugal force 

 to the rank of unimportant factors ; and this, taken with the large 

 =size of the ice-stream, may result in the straightening of the glacial 

 channel along a meandering valley which had been becoming 

 more serpentine under the action of the pre-Glacial water-stream. 

 A mature ice-stream is characteristically largest in its great upper 

 neve-reservoirs, and becomes smaller and smaller as more and more 

 of its volume is converted into a water-stream, which finally runs 

 -away in a small channel, of which the cross- section may not be 

 more than 1/100,000 of that of the ice-stream at its mid-length. 

 A mature water-stream, on the other hand, is subdivided into many 

 minute rivulets at its source, and the trunk stream characteristically 

 increases in volume towards its mouth. It may be pointed out 

 in passing that the best analogy of a mountain-glacier, which ends 

 in a warmer ' piedmont ' lowland, is a mountain-stream, which ends 

 in a drier ' piedmont ' desert ; but as we are here concerned chiefly 

 with the upper part of the Snowdon glacial system, this analogy 

 need not be pursued. 



On the basis of these resemblances and contrasts, it may be 

 inferred that when glaciers (assumed to be destructive agents) 

 -come to occupy the valleys of the pre-Glacial Snowdon district, 

 •as represented in fig. 6 (p. 307), they will proceed to scour out, 

 •enlarge and steepen the valley-heads, and gradually to convert 

 them into neve-reservoirs or cwms, and to deepen and widen the 

 valleys and gradually to convert them into channels suitable for 

 the large and sluggish ice-streams. Associated with the mature 

 enlargement of reservoirs and channels by deepening and widening, 

 there would be a correlated sapping of the superglacial slopes, 

 which would thereby be steepened : such steepening of valley- 

 heads and sides would be most pronounced in the earlier stages 

 of a glacial period, while the ice was still actively deepening its 

 course ; later, when the ice-deepening went on at a slower rate, 

 or almost ceased, the steepened slopes would be reduced to more 

 moderate declivity by superglacial weathering. 



These systematic changes of process and of resulting form through 

 ■the advance of a cycle of glacial erosion are as important in a 



