238 
GEOLOGY OF THE FOURTH DISTRICT. 
At Penn-Yan, in the outlet of Crooked lake, and in the ravines of the neighborhood, the 
shales and shaly sandstones of the group are well exposed. The Cashaqua shale here em¬ 
braces thin courses of sandstone, and contains numerous fossils. Along the shores of Crooked 
lake, and in the ravines entering it, and at Hammondsport, its southern extremity, the rocks 
of the middle and higher part of the group are well exposed. They are everywhere known 
by the thin flagstones, covered upon their under surfaces by the short rigid stems of Fucoides 
graphica (see woodcut, page 241). At the southern extremity of Canandaigua lake, and in 
the deep ravines which extend from the valley of the same in the vicinity of Naples, the whole 
group is fully exposed, and these places offer good points of examination. The small lakes 
to the west of the Canandaigua, viz. the Honeoye, Hemlock and Canadice, with the ravines 
extending from them on either side, are all good points of examination. 
The Caneseraga creek, and its branches in the vicinity of Dansville, offer good exposures 
of the rocks of this group. 
The small streams flowing in the Genesee valley between Dansville and Mount-Morris, on 
both sides, afford good opportunities of investigation. The Cashaqua creek (see illustration, 
page 226), is the best of these. 
The Genesee, as before noticed, in its passage from Portage to Mount-Morris, exposes the 
whole series of rocks in fine mural escarpments which rise from 50 to 350 feet high. The 
examination of this gorge throughout its whole length, will give a most perfect and connected 
view of all the subdivisions of this group. 
West of the Genesee, the valley of Allen’s creek, of the Tonawanda creek, and the lateral 
ravines of the same, expose these rocks in a very perfect manner. The branches of Cayuga 
and Seneca creeks in Erie county give an exposure of the group throughout. 
The shores of Lake Erie, from Eighteen-mile creek to the State line, are composed of rocks 
of this group, presenting themselves in cliffs of 20 to 100 feet high. The streams and ravines, 
particularly the Canadawa, the Chautauque and the Twenty-mile creek, also give good ex¬ 
posures of this group. The deep gorge of the Chautauque creek not only exposes the rocks 
of this group, but their connection with the Chemung group, as well as the rocks of the latter 
through several hundred feet in thickness. 
Finally, scarcely any stream or ravine which flows over this group can be examined, with¬ 
out finding a good exposure of some portion of the strata. 
Thickness. — The thickness of this group on the Genesee cannot be less than one thousand 
feet. The fall in the river from the head of the falls to the base of the group near Mount- 
Morris, is nearly six hundred feet.* The rocks rise about two hundred feet higher; and the. 
dip in this distance of ten miles in direct line cannot be much less than three hundred feet, 
allowing for undulations. 
The rocks of this group extend along the shore of Lake Erie for about thirty or forty miles, 
before coming to the mouth of Chautauque creek. In the banks of this creek there are about 
* See “ Topographical Sketch of the State of New-York,” in Transactions of the Albany Institute, p. 97. 
