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Monroe County
New York
Genealogy and History





The hard-rock geology or stratigraphy was thoroughly described by Dr. James Hall over half a century ago in "The Natural History of New York, Part IV., Geology of the Fourth District." The section of the strata beneath the city of Rochester is published in the proceedings of the Rochester Academy of Science, volumes I and II.1

Except in the southern part of the county the rocks belong to the Niagara period of the Upper Silurian age. The lowest rock is the Medina sandstone, which in the northwestern part of the county is at or near the surface, and is extensively quarried at Brockport. This red Medina forms the rock bottom of the southern part, at least, of Lake Ontario and the rock bluffs at all points along the south shore. Beneath Rochester the red Medina is over one thousand feet thick, but here and all over the county, except the northwestern portion and the lake border, it is buried under the shales and limestone of the Clinton group. The perfect section of the Clinton is finely shown in the walls of the Genesee canyon at the lower falls in Rochester. Here it rests on the gray top of the Medina, and in ascending order consists of about twenty-four feet of the Lower Green shale, fourteen feet of Lower limestone, containing a bed of hematite iron ore one foot thick, twenty-four feet of Upper Green and Purple shales, and eighteen feet of Upper limestone.

The Niagara group rests upon the Clinton and consists of eight feet of dark, gritty shales, exposed at the upper falls in Rochester, and the limestone upon which the city of Rochester is built, more than sixty feet in thickness.

The strata all have a slight inclination southward, which causes the Niagara rocks to disappear a few miles south of Rochester beneath the shales of the Saline formation, which in turn are buried, further south, under the Corniferous limestones of the Devonian age. The latter is found in Monroe county only in the south border of Rush and Mendon, producing the falls of the Honeoye.

The surface geology of the county has not been described in detail, and will only be touched upon here. During the millions of years following the deposition of the Devonian rocks the region was probably exposed to destructive atmospheric agencies, and a great thickness of rocks has doubtless been removed from this area.1

The long era of subaerial denudation was finally changed to subglacial during the Glacial period. The superficial decomposed rocks were crushed and removed by the ice sheet, the old drainage channels were largely filled with debris, and the final removal of the ice left a sheet of glacial drift over the whole territory. During at least the closing part of the Glacial period Western New York was depressed far below its present level, and following and laving the retreating ice front was a huge glacial lake which buried the most of Monroe county to a depth of 300 to 400 feet.2 As the ice retreated northward so as to uncover the Mohawk valley this became a new outlet of the glacial waters and the water surface fell to the level of the Ridge road, which is simply the beach of the glacial Lake Iroquois.3 The superficial geoloty of the county is thus a complex result of the action of glacial ice, stream drainage of the glacier and lake action at the ice front and subsequently.

The north part of the county is a comparatively smooth plain drained directly into Lake Ontario by many small streams which have cut deep into the Iroquois lake deposits and the subjacent ice drift. The southern half of the county has a hilly topography produced by the glacier rubbing the deep subglacial drift into elongated hills, parallel with the ice movement, and known as "drumlins" or "drumlinoids." In the east side of the county, in Perinton and Penfield, these drumlinoid ridges are very pronounced. They have a north and south trend and culminate south in the Turk's Hill drumlinoid mass, the highest land in the county. Through Henrietta and Rush, in the southern part of the county, the drumlinoids have a direction some ten to fifteen degrees west of south, while along the Genesee river and in the southwest part of the county these ridges have a trend more nearly southwest. In the northwest part of the county the drumlinoid character is discernible in the broad, smooth swells with a northeast by southwest trend.

A frontal moraine, marking a pause in the recession of the ice sheet, traverses the county from Brockport to Brighton. This is not strong, but is well-defined near Rochester as an irregular ridge cut by the main line of the New York Central railroad one mile northeast of Coldwater station. Along the Rapids road, in the southwest part of the city, the moraine becomes more broken, but between the river and Brighton it forms the most conspicuous hills of the region, the famous Pinnacle hills. These are mainly sand and gravel, with some masses of till, or unassorted glacial drift, and many large boulders, and with remarkable flow structure. To glacialists they have been well known and very puzzling. They have been described as an "esker" or a deposit made by an overburdened glacial river.1 But they are undoubtedly a part of the frontal moraine, of the nature known as "kame." They consist chiefly of the materials washed out of the glacier by the drainage, and accumulated at the front of the ice wall in the deep water of the glacial Lake Warren. Two other similar kame deposits are found in the county, but not directily connected with any morainic ridge. One is the group of remarkable sand and gravel hills inclosing the Mendon ponds, the other the sand hills and plains extending from the head of Irondequoit bay past Pittsford into the northwest corner of Ontario county.

Glacial gravels are found in hundreds of localities over the county, and the lake silts are abundant, chiefly in depressions. Irondequoit bay represents probably a preglacial river valley modified by ice erosion and then filled more or less by serving as a catch-basin during the Lake Iroquois episode. The sand hills at the head of the bay are remnants of the lake deposits, and the present conspicuous terraces, at an elevation of about 400 feet, on each side of the bay, represent a flood plain.

The lower part of the Genesee river channel, from the rapids above Rochester to the lake, is certainly post-glacial. It has here no valley proper, and near its mouth streams flow by its side directly into the lake. Above the city, or in the south part of the county, and as far as Mt. Morris, the river occupies more of a depression, of possibly pre-glacial origin.

The length of time since the ice retreat has been estimated by several writers at about ten thousand years. The Ridge road represents a pause in the lowering glacial waters some centuries subsequent to the melting away of the ice.

1"A Section of the Strata at Rochester, N. Y., as shown by a deep boring." By H. L. Fairchild, Proc. Roch. Acad. Science, Vol. 1, pp. 182-186.
"The Geological History of Rochester, N. Y." By H. L. Fairchild, Proc. Roch. Acad. Science, Vol. II, pp.215-223.

1Proc. Roch. Acad. Science, Vol. II, page 221.
2"Glacial Lakes in Western New York," by H. L. Fairchild, Bull. Geol. Sec. America. Vol VI, 1894.
3See numerous articles in geological journals by C. K. Gilbert, J. W. Spencer and Warren Upham.

1"Eskers near Rochester, N. Y." By Warren Upham, Proc. Roch. Acad. Science, Vol. II, pp. 181-200.

[Source: Landmarks of Monroe County, New York; William F. Peck, 1895.


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