KATAHDIN SYSTEM. 105 



beds of the present streams of that part of the State, and are more rounded 

 than those found in the midst of the rapids of this stream to the north of 

 the plains in question. The valley is not valley drift, but is an osar-plain. 

 The hills bordering the canyon on each side are from 400 to 1,000 feet 

 high. It is certain that a glacial river flowed into the north end of the 

 gorge, and the height of the lateral hills is such that it could not escape 

 except along the valley. The Seboois Valley broadens for several miles 

 north of its junction with the East Branch of the Penobscot. This wide 

 valley is bordered by plains of clay, sand, and gravel, and so also is the- 

 valley of the East Branch of the Penobscot from this point to Medway. 

 Whether any part of this is an osar-plain I can not now be certain. At the 

 time of my exploration in 1879 I had not diagnosed the level-topped 

 osar-plains, and regarded them as valley drift. The sedimentary plain of 

 these valleys is from near half a mile to a mile or more in breadth. My 

 notes refer to certain coarser gravels on one side of the plain, which perhaps 

 are a broad osar. A well-defined osar begins about 14 miles north of Med- 

 way and extends continuously southward to that place. While passing 

 along the river in a canoe I saw no osar ridges farther north from Medway 

 than this. This ridge is bordered on the east by the river and then by a 

 broad sedimentary plain extending for many miles southward. It is com- 

 posed of clay overlain by sand and gravel, all very nearly horizontallj^ 

 stratified. The ridge has steep lateral slopes on both east and west sides. 

 It is usually densely covered by vegetation and from the river does not 

 appear very different from the steep bluff of erosion in the alluvium on the 

 east bank of the river. None of the geologists who passed up this valley 

 appear to have noticed the ridge, but Thoreau must have seen it and recog- 

 nized its nature. He writes (Maine Woods, p. 294): "We stopped early 

 and dined on the east side of an expansion of the river [East Branch of the 

 Penobscot] just above what are probably called Whetstone Falls, about a 

 dozen miles below Hunt's. * * * There were singular long ridges here- 

 abouts, called horsebacks, covered with ferns." 



In a few places the osar expands into oval or elongated plains, not very 

 broad, biit rather flat on top, sometimes inclosing kettleholes. 



A comparison of the alluvial drift of the valleys of the East Branch of 

 the Penobscot and Seboois River above Medway with that of the valleys of 

 Pleasant River, the Piscataquis, the upper Kennebec, the Carrabassett, and 



