UNCONFORMITY AT BASE OF ONONDAGA LIMESTONE 305 
be expected to retain it. The Onondaga limestone, however, 
shows some inclusions in its basal strata of fragments of the Helder- 
berg which were less readily removed by wave action than the 
subaerial clays which the advancing Onondaga sea must have 
swept away to other areas. While angular contacts, like the one 
shown in the figure, are not uncommon, the more usual character 
of the contact is a horizontal line which affords evidence neither 
for nor against unconformity with subaerial erosion. It should 
be pointed out, however, that adjacent disconformable beds, 
showing no discordance in dip, may have junction over a limited 
Fic. 1.—The disconformity between the Onondaga limestone and limestone of 
the Helderberg group at Manlius, N.Y. The hammer rests on the older formation. 
area in a flat plane which represents a considerable amount of 
subaerial erosion. Even limestones which have experienced the 
extensive erosion of the present cycle of subaerial degradation in 
New York may still retain a horizontal upper surface over a 
limited area. Some of the quarry sections near Manlius, N.Y., show 
that the contact of the Helderberg and the superposed residuary 
clay meet along a perfectly horizontal line, although erosion has re- 
moved from this particular area many hundreds of feet of rocks. 
While the Onondaga limestone rests on the Helderberg, or a 
thin band of sandstone which, like that at Splitrock, belongs to 
