Stratigraphic Column

The New York state (especially its Northern part) is known for its geologic features, that provide astonishing evidence for glaciation. Niagara falls, and High and Lower falls of the Genesee are examples of such topography. However, in many places thick layers of glacial deposits cover the bedrock, from time to time making it difficult for geologists to carefully study the long pre-glacial history of the North American continent. Nevertheless, some beautiful sections of Ordovician Queenston shale can be observed, and in some places the exposed areas reach up to 50 ft.

The rock layers in Rochester area are virtually undisturbed from their original horizontal position, dipping only slightly to the South. The Grenville basement, although exposed in the Adirondacks, is not visible in areas around Rochester. The bedrock in Western New York consists of lower and middle Paleozoic rocks, which are chiefly shales. Repeated transgressions and regressions (sea level rises and falls) are recorded in these rocks.

The oldest rocks visible in the stream banks, quarries and road cuts in Rochester, N.Y. are upper Ordovician. Approximately 1,000 ft of Queenston shale is the result of extensive erosion of the mountain belt produced by the Taconic orogeny, but only a fraction of it is exposed in outcrops. Eroded Cambrian and Ordovician sediments, along with pre-Cambrian igneous and metamorphic rocks were deposited in deltas. Queenston includes sandstone, siltstone, shale, and some limestone. It is generally reddish-brown, with some spots of green discolorations, and does not contain any fossils. The lowest visible Silurian unit is the Grimsby sandstone. It is very similar to Queenston, and the contact is difficult to discern. It is a little coarser-grained than Queenston, but was also produced by erosion of late phase of Taconics. The wave ripple marks, burrows and mudcracks identified in this part of Medina sediments are features of shallow sea environment. The upper Grimsby became more off-shore, as the shore line migrated to the East. Grimsby grades into white Kodak sandstone.

The Clinton group of the mid-Silurian is exposed in the gorge North of the Driving Park bridge. The Clinton group is heterogeneous, and consists of marine gray, green and purple shales, interbedded with fossil-rich limestones. It is interesting to note the famous Furnaceville hematite, possibly deposited by iron-precipitating bacteria.

The mid-Silurian Lockport dolomite is visible in old quarries and railways cuts. It is resistent to erosion, and therefore is a "ridge-maker" - it forms the upper part of Niagara falls and Upper falls of the Genesee.

The later part of Silurian deposits came from a nearby source, produced by a minor Salinic orogeny. Most of upper-Silurian deposits are limy shales. The Salina group also contains evaporites - anhydrite, gypsum, halite. The Vernon shale and Bertie limestone contain euypterid fossils, and were deposited in a hypersline environment. The latest Silurian is less saline, as indicated by brachiopod fossils (brachiopods live in normal marine environments).

Ordovician and Silurian stratigraphic section of Lower Falls shows Queenston shale, overlain by red Medina sandstone, followed by white Kodak sandstone, and, finally, by greenish-gray Maplewood shale.
(from Field Trip Guide Book, 66th annual meeting of New York Geological Association, October 7-9, 1994)

In early Devonian, not much deposition took place.

Mid-Devonian Onondaga limestone is highly fossiliferous, and was deposited in normal marine environment. Silica and chert make Onondaga resistent to erosion, and it forms a ridge parallel to Lockport. Onondaga limestone is followed by shales, which contain early abundant fossils of land plants and marine fishes (early sharks, too!)

The upper Devonian deposits are typically black, petroliferous shales. The fossils include brachiopods and conodonts, indicative of normal marine depositional setting. The caprock of the Upper falls is formed by late-Devonian sandstone. The fossils contained in the darker shale include carbonized wood and plant spores. The rock changes with increasing distance from the source area. Thus, the Catskill delta deposits started with sandstone and siltstones, which finally graded into limy shales. Minor irregularities of delta growth and minor local sea level changes are also recorded in rocks, which at times can make it difficult to map and correlate the outcrops.

The Mississippian rocks in New York State are very similar to the underlying upper-Devonian rocks, and are hard to distinguish. Following a long erosional period with little or no deposition, Olean conglomerate was deposited in Pennsylvanian. It is made of rounded white quartz grains, and is a result of chemically and physically mature thick gravel beach sedimentation.

During Mesozoic and early-Cenozoic, the sea line moved off the North American continent; the land was above the sea level and rising, which means that erosion rather than deposition occurred at that time. Pleistocene glaciation was the nest major deposition (and erosion) time. Rocks of many sizes and types were picked up by the glacier on its way to New York state, and the youngest rocks in this area are Quaternary glacial deposits, some of which are unconsolidated sands and silts.

Glacial History

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