hershey.] The River Terraces of the Orleans Basin. 



437 



upper channel of the Pearch Mine, to be described next. The 

 important feature of it is the landslide debris resting on the inner 

 border of the alluvium. 



The Pearch Mine is situated on the south side of the river, 

 between Pearch and Chinech Creeks, and it has been opened 

 chiefly on a channel which belongs to one of the most extensive 

 remnants of this terrace. The timber has been cleared from 

 the surface, and much of it is comprised in the Pearch farm, 

 about twenty acres in extent. The outer edge of the terrace has 

 been deeply cut by mining, and probably twenty acres of it 

 removed. 



On the outer border there is a rolling, waterworn bed-rock 

 platform about 100 yards wide, and whose average height above 

 the river is about ninety feet. It is encumbered with the coarser 

 portions of the gravel, including boulders up to three or four 

 feet in diameter. None of the original gravel is left on this 

 broad rim, but it is safe to say that it was at least twenty feet 

 thick, including the fine layers at the top, so that the surface 

 of the original outer edge of the terrace was probably about 110 

 feet above the river. Mr. Pearch corroborates this estimate. 

 Back of this ninety-foot platform there is a channel ten to 

 fifteen feet deeper, the width of which is not known, as mining 

 has not yet exposed the back rim. The perpendicular bank 

 fifty to fifty-five feet high is composed almost entirely of mod- 

 erately coarse river gravel of a rusty color, except in a few 

 places, where, by reason of its remaining in its original unoxi- 

 dized condition, it has a deep blue color. There are some strata 

 of sand and fine gravel displaying false-bedding. The gravel 

 in places extends practically to the top of the bank, but its sur- 

 face is irregular and the depressions are filled by a dark red 

 clay nearly free from rock fragments. 



The portion of the terrace remnant remaining is about fifty 

 acres in extent. It is distinctly undulating and rises toward the 

 base of the hills at a rate along one line of fifteen inches in 

 twelve feet. This topography is due to a series of torrent fans 

 (consisting largely of slate debris) which have been built up 

 over the river gravel at the mouths of several ravines back of 

 the terrace. Hence, the inner edge of the terrace is an undu- 



