1888.] 
155 
[Wright 
deed, so far as we are aware, there is no evidence of even local 
glaciers in the Allegheny Mountains south of Harrisburg. But, 
it is easy to see, that an incidental result of the glacial period 
was a great increase of snow and ice in the headwaters of all 
these streams, so as to add greatly to the extent of the deposits 
in which floating ice is concerned. And this Columbia formation 
is, as we understand it, supposed by Mr. McGee to be the result 
of this incidental effect of the glacial period in increasing the 
accumulations of snow and ice along the headwaters of all the 
streams that rise in the Alleghenies. In this we are probably 
agreed. But Mr. McGee differs from the interpretation of the 
facts given by Professor Lewis and myself in that he postulates, 
largely, however, on the basis of facts outside of this region, two 
distinct glacial epochs, and attributes the Columbia formation to 
the first epoch, which he believes to be from three to ten times as 
remote as the period in which the Trenton gravels were deposited. 
If, therefore, Dr. Abbott’s implements are, as from the lowest 
estimate would seem to be the case, 1 from 10,000 to 15.000 years 
old, the implements discovered by Mr. Cresson in the Baltimore 
and Ohio cut at Claymont, which is certainly in Mr. McGee’s 
Columbia formation, would be from 30,000 to 150,000 years old. 
But as I review the evidence which has come to my knowledge 
since writing the paper in 1881 I do not yet see the necessity of 
making so complete a separation between the glacial epochs as 
Mr. McGee and others feel compelled to do. But, on the other 
hand, the unity of the epoch (with, however, a marked period of 
amelioration in climate accompanied by extensive recession of the 
ice, and followed by a subsequent readvance over a portion of the 
territory) seems more and more evident. All the facts which Mr. 
McGee adduces from the eastern side of the Alleghenies, comport, 
apparently, as readily with the idea of one glacial period as with 
that of two. I must add also, that the subsidence which Mr. Mc- 
Gee assumes in the valley of the Susquehanna above Harrisburg is 
much greater than Professor Lewis and myself had supposed nec- 
essary when we went over the field. Professor Lewis never, I be- 
lieve, placed the limit of the Philadelphia Brick Clay as more than 
two hundred feet above the streams either in the Delaware or the 
Susquehanna valley. But Mr. McGee speaks of deposits above 
1 See my paper on the “ Age of the Ohio Gravel-beds,” Proc. of Boston Soc. Nat. 
Hist., Vol. XXIU, p. 436, Deo. 21, 1887. 
