THE DATE OF THE GLACIAL PERIOD. 581 



decomposition. Thus it is natural that the more northern 

 moraines and glacial deposits, of various kinds, should appear 

 fresher than the southern. 



The peculiar facts brought to light concerning the oxidiza- 

 tion of the belt of oldest till, bordering the Wisconsin moraine, 

 in Pennsylvania, are worthy of close attention in this connec- 

 sion. As already noted, Professor Williams found by exteh- 

 tive field work that the moraine as marked by Lewis and 

 Wright across Pennsylvania was not the extreme boundary of 

 glacial action, but lay on an average twenty or twenty-five 

 miles back from that boundary. This attenuated border was 

 referred to by Lewis and Wright as "the fringe," but they 

 did not endeavor to ascertain its limit in that state. The 

 deposits over that area would, however, now be correlated 

 without doubt with those of Kansan age in the Mississippi 

 Valley. 



It is noteworthy, therefore, that the surface outcrops 

 over this attenuated belt (examined in thousands of places and 

 at all elevations ineastern Pennsylvania up to 650 feet above 

 tide, and under caps of glacial deposits only a few feet thick, 

 that vary from loose gravels to compact clays) are universally 

 fresh and undecomposed, showing that the already oxidized 

 deposit was laid upon a freshly glaciated surface, and that 

 time enough has not since elapsed to decompose or oxidize 

 the gneiss, limestone, and slate rocks to any appreciable 

 extent. A striking illustration of this has already been given 

 in connection with the mammoth coal-beds at Morea, Pa., 

 within one mile of the extreme limit of glaciation, and twenty- 

 five miles south of the moraine of Lewis and Wright (p. 154). 

 Here the surface of the rock is distinctly glaciated, and 

 covered with from six to ten feet of sandy till through which 

 water easily percolates. But the coal is rotted only to the 

 depth of three-fifths of one inch, while immediately south of 

 it, in the unglaciated region, it is rotted to the depth of many 

 feet. 



