64 F. T. THWAITES 
Turning to the fourth hypothesis, it is significant that the 
gravel seam shows most weathering of all the bedding planes in 
the quarry, thus indicating that it has been a trunk channel for 
ground water. It is difficult, however, to understand, first, why 
the waters have followed only this particular plane; second, how 
they carried such comparatively large stones considering the size 
of the opening, and third, how they could produce so widespread 
and comparatively regular a sheet of gravel without at all affecting 
Fic. 7.—Glacial gravel pocket in St. Peter sandstone. (Photograph by W. O. 
Hotchkiss.) 
the large joints which pass through it. Being 1o feet below the 
surface, the seam is beyond the reach of frost under present climatic 
conditions unless air circulated through it, so that we cannot appeal 
with certainty to enlargement by freezing. It is possible that 
weathering of an originally soft layer has been the cause of lateral 
enlargement of the bedding plane parting so as to permit a wide- 
spread deposit of sand and gravel. In the case of the gashes and 
joints in the sandstone the effects of erosion by the gravel-carrying 
waters is clear, while the source of the filling might readily be the 
gravel deposit which overlies a portion of the exposure. The 
