414 GLACIAL GRAVELS OF MAINE. 



and cause to the slopes of the h^iid. But gaps are not seldom found ha the 

 midst of a level plain, which we can not attribute to conditions of the land 

 surface. There is no change in the slopes, nor hills to produce crevasses, 

 nor narrowing of valleys. Such gaps must have been produced wholly 

 b}?- local conditions of the ice and glacial streams. Many of the osars have 

 been washed away by streams, but such breaks in the ridges are not con- 

 sidered as true interruptions of the system. Erosion gaps were made sub- 

 sequently to the formation of the ridge, an accident having nothing more 

 to do with the original form of deposition than if the gravel had been drawn 

 away to build a road. The osar in this report is considered "interrupted" 

 only when for some reason the gravel was not originally deposited continu- 

 ously. These gaps are so short, as comjjared with the long reaches of 

 gravel, that on the State maps they often can not be represented without 

 exaggeration. When mapped, the ridges are seen to have a linear arrange- 

 ment which the longest of the gaps do not obscure. If represented on a 

 detailed topographical map, the close connection of the ridges would be still 

 more clearly indicated. 



The ridges formed by a single glacial river, including its tributary and 

 delta branches, are marked as a single system. Osars marked as tributary 

 can be traced to a definite junction with each other or to points very near 

 each other, where they are separated by intervals no greater than are ordi- 

 narily found in the main ridges in the same region and on the same sort of 

 land surface. When osars approach each other as if they were tributaries, 

 but instead one (or both of them) expands into a delta and seems to end 

 before reaching the other, they are regarded as distinct systems (e. g., 

 Pleasant River and Lilly Bay systems). 



The osar systems are of various lengths up to 130 or 140 miles. 

 Briefly summarized, the more important facts are as follows: Their materials 

 are more or less rounded, polished, and assorted by flowing water. The 

 water flowed along the ridge. In most if not all cases it flowed southward, 

 as is proved by the direction of transportation, by the dip of the strata, by the 

 positions of the deltas, and by the fact that at the north ends of the sys- 

 tems the stones are usually much less waterworn than at the south ends. 

 The gravel usually rises above the land on each side. These phenomena, 

 as well as the meanderings of the systems, could be produced only by 

 rivers flowing between solid barriers that have now disappeared. Ice is the 



