The Rocks

by Fred Amos


Note to this Online Edition: This chapter, "The Rocks," represented the accepted stratigraphic nomenclature and interpretations when it was written in 1968. Since that time, detailed study has resulted in extensive modification to the story as told by the rocks. Therefore, this chapter should be considered a historical record of past interpretations rather than as a source of current information.

Two good references for more updated information include:
Geology of the Genesee Region of New York Since H.L. Fairchild, Centennial Colloquium Issue, Procedings of the Rochester Academy of Science, 1983

Geology of New York, A Simplified Account, Educational Leaflet No. 28, The N.Y. State Museum., The State Edu. Dept.., The Univ. of the State of N.Y., 1991


I. The Over-all Picture:

The bedrock in Western New York consists of a succession of lower and middle Paleozoic sediments-chiefly shales but including prominent beds of dolomite, cherty limestone and thick bedded sandstones which are weather resistant and tend to form cliffs. These formations are essentially horizontal, dipping only about 50 feet per mile to the south. Most of these rocks were formed by sediments deposited under shallow seas and represent repeated inundations of continental lowlands. The shallow seas were generally connected to the open Atlantic by channels to the northeast and occasionally to the south as well.

The sediments in Western New York were the debris from the erosion of older rocks southeast of this area, probably along what is now the Mid-Atlantic Coast of the United States. Igneous and metamorphic rocks, as well as sedimentary rocks, supplied fragments of quartz and feldspar and other common minerals. The clay minerals that formed shale in Western New York largely resulted from the chemical breakdown of feldspar minerals during weathering and erosion of the source area uplands.

The "basement" rock beneath the Paleozoic sediments was exposed in a core from a deep well (3,078 feet) drilled in Rochester before the turn of the century. It was reached in a search for gas in the Ordovician rocks. This local "basement" rock is a quartzite, and its relations to different kinds of basement rock elsewhere in Western New York are obscure; few similar records exist.

The sediments elsewhere in the area tend to thin out to the west, thicken to the south and east, but "feathering" out on the flank of the Adirondacks. These mountains consist largely of Precambrian igneous and metamorphosed sedimentary rocks. Covering all the bedrock in the area except along parts of the Southern Tier is a veneer of glacial deposits which varies greatly in thickness. Although most of these deposits are unconsolidated silts, sands, and gravels, some recently cemented conglomerates occur and may properly be called rock. Many of these deposits are treated in detail in the following sections.

II. The Rock Record Exposed

In stream banks in western New York and in road cuts and quarries, rocks of lower Paleozoic age can be seen. (Refer to the geologic time chart and corresponding rock column, fig. 1.) Sedimentary rocks in this area represent the Upper Ordovician, Silurian, Devonian, Mississippian and Pennsylvanian Periods. Rocks of Pleistocene age are present but sparse. Even in the Periods represented, the rock record is incomplete at many levels (indicated by spaces in the rock column in fig. 1 ), either because sediments were eroded away or were never deposited locally. Rocks of such ages and portions of the Periods not represented locally have been found and studied else where in New York, North America, or on other continents. There are still gaps of unknown duration in the record, however, between some Periods, and especially between Eras. They indicate intervals when continental uplift and regional mountain-building, (accompanied by erosion) were widespread on every continent.

III. Description of the Rocks by Geologic Period

The more prominent rock formations will be discussed in sequence from oldest to youngest as they were deposited. Other strata are not noted except in the geologic column, fig. 1.

Upper Ordovician. During this ace, all of New York State, except for the loftiest peaks in the Adirondacks, was submerged beneath marine waters. Then occurred a mountain-building up-lift of the entire eastern half of the state, called the Taconic orogeny. A result of this uplift was the erosion of thousands of feet of Cambrian and Ordovician sediments, as well as large masses of pre- Cambrian igneous and metamorphic, rocks. They were re-deposited in the form of a series of great coalescing deltas, beginning in the east, extending from New England to Virginia, and advancing gradually westward to the central Great Lakes area. In the vicinity of Rochester and Niagara Falls, these red rocks are called the Queenston Shales. Under Monroe County (Rochester and environs) these shales are 1,000 feet thick. The upper fifty feet are exposed at the base of the Genesee Gorge below Driving Park Bridge and Maplewood Park. Gorge Looking North from Bridge ------ Lower Falls Looking South from Bridge

Lower Silurian. Unlike most localities in the state where rocks of this age are exposed, the boundary between the Ordovician and Silurian Periods in the Gorge at Rochester is not prominent. The lowest Silurian rock we see here is the basal Grimsby sandstone. It has the same red color and nearly the same texture as the underlying Queenston shale. They seem to merge and are both nearly horizontally bedded. Yet, in the opinion of geologists, the Grimsby is not the oldest Silurian rock in the state. In the Niagara Gorge, the older Whirlpool sandstone is present. It was apparently a wind-blown sand deposited in the shallow sea of far western New York. It is reported only as far east as Medina. The Grimsby consists of thick beds of sandstone and is mottled green and white in some layers. Local conglomerates, wave ripple marks, and mud cracks attest to the shoreline environment of deposition. It is also seen in the Niagara Gorge. Small brachiopods, ostracodes, and worm trails and burrows are seen in it in the western half of the area under discussion. This dark red sandstone quarried at Medina has been used in Rochester for curbs and sidewalks, and some early buildings. The upper Grimsby seems to pass gradually into a bed of white sandstone. This bed is called the Kodak sandstone in central New York, but "Thorold" to the west. (The latter may be older and not equivalent to the Kodak.) In the Genesee Gorge upstream from the Driving Park Bridge, the Kodak sandstone forms the lip of the falls in the Genesee.

Middle Silurian. A heterogeneous group of rocks called the "Clinton" is exposed in the steep river banks of the gorge north of the Driving Park Bridge. It is a classic Silurian geologic section; few outcrops of the same age in North America can rival it for variety in rock type and in richness of its fossil record.

Marine sediments were deposited in the area by a deepening sea whose shoreline gradually moved to the east during the Middle Silurian. Above the beach and the shallow off-shore Grimsby and Kodak sandstones there is the fine-grained green Maplewood shale. It contains a few marine fossils. Following this is an unusual rock; a sedimentary iron deposit. Though thin here, it is contemporary with deposits laid down in several areas of the United States. In Birmingham, Alabama, where it is thicker, it is the basis of a large steel industry. Here in the Rochester area, the formation is called the Furnaceville. It contains hematite, replacing the original calcareous fossil fragments. These iron-bearing sediments were deposited in shallow seas. It is believed that this was a result of the action of iron- precipitating bacteria, living in waters highly charged with iron minerals in solution. Above the Furnaceville, the Wallington (Reynales) limestone is prominent; some of its beds contain large numbers of thick-shelled brachiopods. The younger Sodus shale is greenish and at some levels contains vast quantities of a small pearly shelled brachiopod, very like a coquina. The Williamson shale contains some pyritic black beds filled with graptolites. The Irondequoit limestone is the oldest "normal marine" deposit in the local Silurian -- with corals, trilobites, brachiopods and bryozoa well represented.

The uppermost part of the middle Silurian in the Rochester area is dominated by prominent marine formations. The Rochester shale is a very thick formation (about 85' in the Genesee Gorge) that is spectacular as a bearer of a great variety of marine fossils in the lower layers. Over 200 species have been identified. The upper part is barren of fossils and this section is followed by a third section which contains a few fossils and has been called the Gates formation by some geologists.

The Lockport dolomite is a prominent ridge maker from Hamilton, Ontario, almost due eastward to Medina, just west of Rochester. The Niagara escarpment, as it is called, exists because of the resistance of the rock to the forces of erosion, and the dip of this stratum to the south. The Lockport dolomite forms the lip of Niagara Falls and the Upper Falls of the Genesee by the Rundell Library. The rock quarries near Lockport and Rochester are in the Lockport dolomite.

The Lockport dolomite offers many items of interest to students and collectors. In the upper portions. a variety of attractive minerals, some crystallized (described in another section) occur in vugs and crevices.

Corals of many kinds and other forms associated with early reef development are common but the dolomite replacement of the original limestone has coarsened the texture of the rock, and much of the fossil record is obscure.

Upper Silurian. Consisting mostly of limy shales and a few salt and gypsum beds, the Cayugan series is very thick and covers a broad belt across western New York almost down to the latitude of the Finger Lakes. The Vernon shale and lower Bertie limestones contain most peculiar fossils-the eurypterids. The latter formations are unusual, dense, very fine-grained rocks which may have been precipitated in a hypersaline, land-locked sea. The latest Silurian limestones are more normal marine, as indicated by their brachiopod fauna.

Early Devonian. In the eastern part of the state, rocks and fossils of this age are little different from those just below. The Helderberg and Oriskany sandstones are found only on the eastern border of the area under discussion. The farther west one travels, however, the more noticeable the break becomes, since post-Silurian erosion removed the uppermost Silurian beds; and in the earliest Devonian age, no deposition took place. Middle Devonian rocks lie on the eroded, very uneven surface of Silurian rocks, like the Bertie limestone.

Middle Devonian. The oldest Devonian rock in the central Finger Lakes area is the Onondaga limestone. It is only moderately thick, but is highly fossiliferous. Corals abound; apparently a well-developed marine environment prevailed. Much of the rock has been replaced by silica; chert (a quartz mineral) nodules of sizeable masses and hardness occur in such quantities that they were a good raw material for Indian points and tools. The silica content helps make the Onondaga limestone resistant to erosion, so that its northern edge forms a noticeable ridge parallel to that of the Lockport dolomite, but about 12 to 15 miles farther south. The Thruway passes along just to the north of this ridge from Seneca Falls west to Batavia. There is a very large lime stone quarry in this formation near Le Roy. Much of this limestone has been used as road ballast.

The Hamilton group consists mostly of dark gray shales and thin silty limestones, usually quite fossiliferous all along the northern edge of the Finger Lakes area. Stream outcrops and railroad cuts reveal these formations. Notable are exposures at Kashong Glen south of Geneva, Menteth Glen and Deep Run south of Canandaigua, Jaycox Run and Wheeler's Gully north of Geneseo, Fall Brook-just south of Geneseo, Little Beard's Creek north of Leicester, East Bethany, Alden, and Eighteen Mile Creek (south of Buffalo on Lake Erie shore). These areas are a treasure trove for fossil collectors; the floras and faunas are known to geologists everywhere. The Hamilton is the most fossiliferous group of formations in western New York. In these rocks are the earliest reasonably abundant remains of land plants and marine fishes (early sharks). But the commonest fossils in the group are brachiopods (especially spirifers), crinoids (generally represented as stem fragments), corals, bryozoa, trilobites, mollusks and ostracodes. The group is thickest toward the southeast; over 1,000 feet south of Syracuse; 500 feet in the Finger Lakes and 200 feet south of Buffalo. On the eastern border of this area is the prominent Tully limestone, not recognized farther west. It is seen at Portland Point on Lake Cayuga and the upper falls of Kashong Glen at Bellona.

Upper Devonian. Beginning with a prominent bed of black shale, the Upper Devonian formations consist mostly of sandstones, shaly siltstones ("flagstone") with thick, limy, dark gray and thin black shales. It ranges from 1,000 feet thick in the east to about 200 feet in the west. The black Geneseo shale is first; it is petroliferous, bearing traces of oil. The Genundewa is a thin formation of dark, limy shales and coarse limestones. It contains mollusks (notably ammonoids), conodonts and plant fragments. The West River formation consists of dark gray and black shales. Fossils are few; generally small clams and brachiopods. It is in this formation that the peculiar concretions called septaria are found. They average about eight or ten inches in size, are dense, fine-grained and heavy. They apparently precipitated as impure limestone, about some nucleus (fossil or grain of sediment). They exhibit cracks and small vugs later filled with small masses of calcite, selenite, ankerite, and pyrite crystals. In the east, at Taughannock Falls on Lake Cayuga, the Geneseo shale at the base of the falls is overlain by the 200-foot Sherburne shale. Above this is a West River equivalent, the Ithaca shale.

The Middlesex formation is a black shale, and can be seen near the base of Mount Morris Dam. The Cashaqua is a light gray shale which make up most of the sides of the Gorge at Mount Morris am. Fossils are prominent in this shale; mostly they are mollusks. Large ammonoids occur in concretions. In the east, the equivalent Enfield shale is prominent around the south ends of Lakes Cayuga and Seneca. The Rhinestreet is another black shale, with some thin, fine-grained siltstones. From Keuka Lake west to Attica, the Hatch dark shales and siltstones can be found at a level that exposes them prominently downstream from Lower Falls in Letchworth Park. The Grimes Sandstone occurs just below the Lower Falls. The very thick Gardeau formation contains siltstones, gray and black shales. The Lower, Middle, and nearly all the Upper Falls at Letchworth are in this formation. Fossils are rare but typically marine. The lower part of the Nunda Sandstone forms the caprock of the Upper Falls and extends up the Gorge to Portageville. The Wiscoy shale occurs south of Portageville in the steep stream valleys. It is a thick formation of black and gray shales. In the west, the Hanover formation exhibits typically upper Devonianlithology of black, petroliferous, fissile shales and greenish to gray shales. As fossils, the darker shale contains carbonized wood and plant spores, and some conodonts.

At this point it will be well to remember that as the distance from the source area of a sediment changes, so changes the resultant rock. This is especially visible in the Upper Devonian rocks of New York. The Catskill Delta, with its thickest and coarsest sandstones and siltstones in Eastern New York and Pennsylvania, extended westward to Lake Erie, manifesting finer-grained sediments, black, oily, and finally limy shales. The delta grew irregularly; occasionally the shoreline shifted back eastward, though its general trend was to the west. The result is a thick, interfingering mass of sandstones and shales that have been very difficult to correlate and map. The faunas varied accordingly and do not offer an easy solution to the stratigraphic problems.

The Canadaway Group is better represented in western Pennsylvania, as are the uppermost Devonian, Mississippian and Pennsylvanian formations shown on the geologic time chart. These poorly fossiliferous Upper Devonian formations are largely siltstones and shales with some zones of concretions and dark pyritic layers. Toward the eastern part of the Southern Tier, these interfinger and merge with coaser sandstones of reddish brown color. Many of the beds in the finer-grained gray and black shales to the west contain crude oil and gas in commercial quantities.

Lower Mississippian. These rocks are only sparsely represented on the highest hilltops near Jamestown and in Allegheny State Park. They closely resemble the Upper Devonian rocks and are difficult to differentiate.

Lower Pennsylvanian. After an interval of erosion lasting tens of millions of years, a coarse conglomerate was deposited along the Southern Tier. The Olean conglomerate consists of round, milky white quartz pebbles in a siliceous, sometimes iron-stained matrix. It is present south of Olean and is quite impressive, though not very thick, at Rock City.

The Olean conglomerate lies unconformably on the older rock formation which, at the noted locality in the hills a few miles south of Olean, are the shales and sandstones of the Chautauquan Series. The conglomerate represents a thick gravel beach of a sea that encroached on former dry land which had been subjected to the forces of erosion for a long time.

The Local Rock Column (fig. 1) shows 4 other unconformities as indicated by wavy lines. These represent periods during which the land in the region was raised above sea level and erosion took place over a great length of time. The surface of rock formations below the unconformities show the effects of erosion and are quite clearly different from the rock formations above the unconformity with one exception; the contact between the Queenston shale and the Grimsby sandstone in the Genesee Gorge. To the east of Rochester, and particularly in southeastern New York, there are fine examples of the unconformity between the Ordovician formations and the overlying formations.

Younger Rocks. During the Mesozoic and early Cenozoic times, most of eastern North America was above sea level and was rising; erosion, not deposition, was the prevailing process. Only in the Pleistocene, the youngest geologic epoch, were more sediments deposited in western New York. The continental glaciers, which apparently advanced and retreated over most of this area more than once, brought silts, sands, and rocks of many types and sizes from the northern Adirondacks and Laurentians in Canada.

Igneous and metamorphic rocks are common in our topsoils and beach sands. Multi-colored and attractively banded gneisses predominate. Some of these gravels were cemented into conglomerates as the melt waters from retreating glacier fronts precipitated limy and ferric minerals. Springs issuing waters charged with these minerals today are forming tufa, a light colored, fine-grained but spongy-looking deposit similar to "cave rock." Springs near Mumford and at Tufa Glen produce notable amounts.


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