FORMATIOiSr OF KAMES AND OSAES. 331 



of erosion bordered by rather steep walls (as north of Hogback Mountain, 

 Montville), but more often the erosion is diffused. We see that till is absent, 

 but we find no bank or margin which can be said to mark the limit reached 

 by erosion. A characteristic form is shown in PI. XXVI, A. The discon- 

 tinuous osars lie in regions so covered by marine clays that we do not know 

 what forms the erosion takes. 



2. It is evident that during the last days of the ice-sheet the englacial 

 morainal matter would appear on the sxu-face in consequence of the melting 

 of the ice above it. There would form on the ice a multitude of small 

 superficial streams and seeps which would carry off much of the finer part 

 of the exposed till and precii^itate it into the main chaimels. The larger of 

 these lateral tributaries formed the short tributaries of the osar rivers else- 

 where recorded. No matter whether the longer ridges were deposited by 

 subglacial or superglacial streams, in either case there must have been a 

 multitude of superficial tributaries that have left little or no gravel. ' Indeed, 

 their work was almost wholly erosive, not constructive. Their slopes were 

 probably steep. They simply carried away such of the till of the region 

 they drained as they could lift, and cast it into the main river channels, 

 where it either went to make up the osar-ridges or was carried into lakes or 

 the sea to form a part of the glacial deltas. The larger stones over which 

 these brooks flowed would be left in place and be but little polished. Being 

 in the upper part of the till the signs of water wash and wear would in 

 most cases long since have disappeared by weathering. The diffused ero- 

 sion of the englacial till could be accounted for by the action of a multi- 

 tude of these lateral streams. A diffused erosion of the subglacial till is 

 more difficult to explain, unless by the wandering of the streams. 



3. The flow of the ice no doubt often helped to bring osar matter 

 together. 



(a) At the end or front of moving ice. In this case the flow of the 

 ice constantly brings moraine matter to the front and throws it down for 

 the subglacial streams to act upon as they swiftly emerge from their 

 tunnels. In such a case the sediments deposited in front of the ice 

 would consist partly of worn matter transported from a distance by the 

 subglacial streams (or superficial, if such there should be), and partly of 

 matter which would otherwise form part of the amorphous terminal 

 morame and which happened to be dropped into the streams verj^ near 



