BROAD OSARS OR OSAR TBRRAOES. 441 



therefore have continued in a channel confined wholly or in part by ice, or 

 it flowed into a valley over which the ice had melted all the way to the sea. 

 In the last-named case the sedimentary plain is a frontal delta, and ought to 

 extend continuously down the valley to the level of the sea as it existed at 

 the time of deposition. If at any point the sedimentary plain in question 

 leaves the valley in which it was deposited and takes a course on the hill- 

 sides or passes over hills into another drainage basin, we have proof of the 

 continuity of the glacial river sediments over even the lower parts of the val- 

 ley where for a time they were found in form so much resembling valley drift. 

 The sediments here termed " osar terraces" cross hills and valleys just the 

 same as the osars, and these topograjDhical relations are inconsistent with the 

 hypothesis that they are valley drift, though in the valleys their situation 

 is such that they must since deposition have been subject to the action of 

 streams and often have been eroded by them, and often were overlain with 

 valley drift. If an osar-plain were confined wholly to a single valley we 

 should have no topographical test to distinguish it from valley drift. This 

 does not apply to the osar-plains of Maine. 



2. The material of the beds of ordinary streams in every part of the 

 State I have visited has been carefully examined with a view to determining 

 the amount of attrition to which the existing stream gravel has been sub- 

 jected. Everywhere the testimony of the gravel of the osar-plains, when 

 compared with that of the adjacent streams having the same slopes and size 

 of drainage basin, is substantially the same. The average shape of the 

 gravel of the osar-plains shows immensely more waterwear than the gravel 

 of the existing streams. The proofs of this are abundant and overwhelm- 

 ing. Only in the mountain regions where the slopes are 100 or more feet 

 per mile do the stones in the beds of streams show rounded forms at all 

 comparable to those of the glacial gravels. 



3. The quantity of the broad osar sand and gravel is usually much 

 greater than the valley drift of the adjoining regions. 



4. Many of the osar terraces do not extend across the whole of the val- 

 leys in which they are situated, and show no tendency to expand into a delta 

 at a broad part of their valleys, but sometimes end at one or both sides in 

 a well-defined bluff rising quite abruptly above the level of the adjacent 

 land. At such places we must grant they were bordered by ice walls, or 

 the alluvium would have spread obliquely outward across the valley. 



