422 
GEOLOGY OF THE FOURTH DISTRICT. 
MONROE COUNTY. 
The rocks of this county are the same as those in Wayne county. The Niagara group, 
however, becomes better developed, and its fossils more numerous. The Medina sandstone 
expands to a greater width, bordering the lake shore. The Clinton group perceptibly dimi¬ 
nishes in thickness, and its most prominent fossil, the Pentamerus oblongus, disappears 
before leaving this county. The Onondaga salt group seems even better developed than in 
Wayne county, and its numerous beds of gypsum are mere extensively exposed. The course 
of the Genesee river affords the best section of these rocks, and is quite sufficient to give one 
an idea of the position and nature of the strata without visiting any other place. 
Surface of the Country, Streams, <f-c. 
The general elevation of the northern part of this county is about three hundred or three 
hundred and fifty feet above Lake Ontario. All that portion of the county south of the canal 
maintains about this elevation, if we except the drift hills, which rise to the height of from 
fifty to one hundred feet. On the north of the canal the surface declines gradually, and 
almost imperceptibly, to the lake shore; on the east side of the Genesee being more uneven 
than on the west. In the northern part of the county, particularly north of the Ridge road, 
the surface is remarkably even.. We here find scarcely an undulation or depression disturb¬ 
ing the uniformity, except the channels of the present streams. 
East of the Genesee, particularly in the vicinity of the Irondequoit creek and bay, the 
country is very much broken into deep ravines and high ridges. The ridges are com¬ 
posed chiefly of fine sand, or sandy loam, with strata of pebbles or boulders near the 
bottom. Most of the boulders in these hills are from the rocks in the vicinity, those of the 
primitive rocks being comparatively few in number. In this and some other respects, the 
drift along the Irondequoit differs from that of any other part of the county. There have 
been vast accumulations of diluvial matter about the head of Irondequoit bay, and along its 
shores. It appears as if at one time this had been a great water-course, and that some inun¬ 
dation of sand,, gravel, etc. had filled it up, and changed the direction of the stream. The 
sand of these hills is stratified, and the strata often curved, or inclined at different angles. 
The banks of the bay are high, with deep ravines scooped out by the action of the smaller 
streams. 
The bed of the Irondequoit at Penfield, is much lower than the bed of the Genesee at Ro¬ 
chester and farther south. From examining the surface of the country, this seems the most 
natural course of the Genesee, and it may at one time have flowed in this channel. The 
Irondequoit is now a small stream, pursuing its course in a deep valley of denudation, show¬ 
ing that some more powerful agent was formerly active in this quarter. 
