GLACIAL GRAVELS OF MAINE. 



is nearly horizontal to Kingman, and thence the slope rises moderately 

 steeply again to Springfield. From Sherman to Macwahoc, and again 

 from Prentiss southward, the gravel takes the form of a broad osar, or osar 

 terrace, of sand and rather fine gravel. At Macwahoc and Prentiss it 

 expands into complexes of reticulated ridges inclosing kettleholes and com- 

 posed of coarse gravel, cobbles, and bowlderets. For 3 or 4 miles near 

 Kingman the system takes the form of a narrow osar of rather fine sand, 

 and is somewhat interrupted. It is the narrowness and fineness of this 

 ridge near the bottom of the transverse valley that specially demands 

 explanation. The noncontinuity is in part, and possibly may be wholly, 

 due to postglacial erosion. On both theories there were broad osar chan- 

 nels north of Macwahoc and south of Prentiss. Both postulate a lake- 

 like expansion at Macwahoc, and another at Prentiss, in which or near its 

 margin was left a plexus of reticulated ridges. On the subglacial theory 

 the tunnel would be relatively small where it crossed the transverse valley, 

 and sedimentation scanty or in narrow ridges. This corresponds well with 

 the osar at Kingman, but for a long time I had difficulty in accounting 

 for the fineness of the sediment. Now Macwahoc and Prentiss are not far 

 from the same elevation, and only 3 miles or so from opposite ends of the 

 subglacial dam. During the retreat a broad channel was formed north of 

 the divide in Springfield, which extended itself as far north as the complex 

 in Prentiss. North of there the sediments M^ere scanty or absent until the 

 lake or broad channel was opened at Macwahoc. If the opening of this 

 lake was due to a clogged channel, the water may have overflowed later- 

 ally, so that the old channel was never thereafter used, except for local 

 drainage, and thus only sand would be deposited. But as the channel was 

 not permanently clogged, tlie coarse sediment from the north would mostly 

 stop in the lake at Macwahoc and only the finer pass on across the valley 

 to gradually fill the old tunnel or parts of it just preceding the time that the 

 ice retreated so far north that the tunnel was disused. On the superglacial 

 theory thfe order of events must have been substantially the same. The 

 opening of the lake at Macwahoc and deposition of the plexus of reticu- 

 lated ridges is essential to both theories. But the distance between the 

 complexes of Macwahoc and Prentiss is about 10 miles, and we must sup- 

 pose deposition in one began immediately after the other was ended, or 

 there would be an osar-plain or other body of retreatal gravels left over the 



