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History of the Mohawk Valley: Gateway to the West 1614-1925
Chapter 3: Mohawk Valley Rocks.

[This information is from Vol. I, pp. 92-99 of History of the Mohawk Valley: Gateway to the West 1614-1925, edited by Nelson Greene (Chicago: The S. J. Clarke Publishing Company, 1925). It is in the Reference collection of the Schenectady County Public Library at R 974.7 G81h. This online edition includes lists of portraits, maps and illustrations. Some images have been relocated to the area in the text where they are discussed. As noted by Paul Keesler in his article, "The Much Maligned Mr. Greene," some information in this book has been superseded by later research or was provided incorrectly by local sources.]

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A description of the rock formations which bear Mohawk Valley names — the Little Falls dolomite; Trenton limestone and shale; Canajoharie shale; Utica shale; Frankfort shale and sandstone; Oneida conglomerate; Clinton shale, sandstone, limestone and iron ore; Cobleskill limestone; Helderberg limestone; Oriskany sandstone.

In geological text-books of New York State, the Mohawk Valley furnishes about one-fourth of the rock designations in the geological charts. These rocks are among the most important in geology, both from a commercial and a scientific point of view. As the average reader could only arrive at a knowledge of our Mohawk group of rocks by much study of various authorities, they are here given a chapter by themselves. Thus the Valley reader can inform himself concerning the rocks which lie at his very door.

The Mohawk Valley rock group comprises the following formations, taken in their order from the lowest Valley formation upward. There is no mention of any of the other rock formations, except as they bound or overlie or underlie the rock which is under consideration. The rocks, as listed, do not always lie in conjunction, but this is their general order in the geological scale beginning with the lowest Mohawk Valley stratum.

[Map: Geologic Map of New York State Showing Surface Rock Systems.]

There may be some argument as to the listing of the Helderberg limestone above, as the Helderberg regarded as a mountain chain, lies mainly in Albany County but this mountain ridge also extends into Schoharie County, which is in the Mohawk Valley. It continues across the entire State to the Niagara River, forming the Cherry Valley Hills and the heights along the southern ridge of our Valley, known as the Susquehanna and Unadilla Hills or mountains. This sharp, marked ridge (of varying altitudes) is called the Helderberg Escarpment by geologists and its close association with our Valley gives it a place here.

The Mohawk Valley surface rocks, which have given their names to important rock formations in the text-books of geology, are described in the following paragraphs.

Little Falls Dolomite

Little Falls dolomitic limestone of the Cambric age is one of the oldest of the World's rocks. It is found in the southern Adirondacks, in the narrow limestone border between the Adirondack Precambric rocks and the Ordovicic shales of the central river section of the Mohawk Valley. Its most typical outcrop is on the face of the 200-foot Rollaway Cliff at Little Falls, where it is in full view of the thousands passing on the New York Central Railroad and Mohawk Turnpike. This notable exposure of dolomitic limestone at Little Falls has given the formation its name. It is hard, compact and light gray in color. Limestone is a rock composed of calcium carbonate. When crystalline it is marble — metamorphosed from limestone at a great heat below the earth's surface. Dolomite is a brittle rock, composed of calcium magnesium carbonate. Thus the Little Falls dolomite differs from limestone, in the presence of magnesium, not found in typical limestone. The sea which deposited the Little Falls dolomite swept the southern and eastern shores of the Adirondack region of Cambric times and the dolomite was probably deposited along these shores. Little Falls dolomite is the fourth rock of the Cambric series, lying above the Potsdam sandstone and limestone, Acadian limestone and Georgian slate and quartz. At Little Falls, however, it lies directly upon the Precambric syenite, one of the world's oldest rocks.

The famous "Little Falls diamonds" or white quartz crystals are found in cavities in the dolomite together with a substance called anthracite.

In passing our Little Falls dolomitic Rollaway, it is pertinent to here mention that most of the geological rock strata designations take their names from towns and places close to the main lines of travel, many of these terms being taken from towns which lie directly on or close to the New York to Buffalo Highway and its central section, the Old Mohawk Turnpike. This is particularly true when the specified rock occurs in an unusual form or as a noteworthy landscape feature — as in the case of the Little Falls Rollaway, one of the world's best known cliffs, because of its proximity to the New York Central Railroad.

The Trenton limestone, at beautiful Trenton Falls; the Little Falls dolomite, exposed on a sheer cliff, 200 feet above the eyes of the passing thousands; the Canajoharie shale, forming the sheer hundred-foot walls of a beautiful small canyon, at Canajoharie; all are marked instances of the appropriate selection of geological terms in accessible localities.

The Middle and Upper Mohawk Valley (in the counties of Montgomery, Herkimer and Oneida), although of small area compared with the State at large, furnish a large number of the geological terms not only of New York State, but of our national geological text-books.

Trenton Limestone and Shale

Dr. Miller says: "In the strata of Cambric age in New York, animal or plant remains are comparatively rare, while the Ordovicic rocks throughout fairly teem with fossils. If any formation deserves special mention, it is the Trenton limestone which is exceedingly rich in fossils. The type locality at Trenton Falls is justly famous as a collecting place for Ordovicic fossils. Among plants, none above very simple seaweed and algae are known to have existed. Among animals, hundreds of sea species have been described as occurring in the Ordovicic strata of New York." So great has been the change in the life forms of the sea that none of the Ordovicic species survives in our waters today.

The limestone at Trenton Falls is exposed, in its two miles of Gorge, often to a height of 200 feet. It is one of the most beautiful gorges in the country with equally beautiful falls. As the Trenton series was probably the greatest limestone era of geological time and as limestone enters so much into our life — in buildings and industries, a brief mention of it is pertinent here.

Limestone is a sedimentary rock composed chiefly of calcium carbonate. It may originate by precipitation of the mineral from solution in water of oceans or lakes, but this process is usually supplemented or replaced by an organic process, the material being secreted by aquatic animals or plants to form their tests or hard parts. These hard parts are not decomposed after death, but accumulate in layers which are afterward consolidated by the precipitation of calcium carbonate in the interstices. Such a deposit if incoherent is called shell marl or calcareous marl; marl being a combination of structureless calcium carbonate, clay and sand in varying proportions. A special variety of fine texture is called chalk. Some limestone consists partly of magnesium carbonate and is called magnesian or dolomitic limestone. All other substances contained in limestone, are regarded as impurities. When precipitated limestone, instead of crystallizing, gathers in successive, coherent stony layers, it is called "tufa." Mexican onyx is a "tufa". Crystalline, metamorphosed limestone becomes marble, changed at great depths under conditions of tremendous heat which makes the limestone flowing or plastic. Cherty limestone has silex segregated in concretionary masses, called chert or flint — the material of which arrowheads and other weapons were made by our Mohawks and Oneidas.

Pure limestone is white. A yellow color is sometimes given by iron oxide. The ordinary limestone color is gray, which is due to the presence of organic matter. Limestones constitute about one-fifth of all the earth's sedimentary rocks and the Trenton was the greatest of all limestone periods. The limestones are extensively used for building, road material, a source of lime and for the fluxing of ores.

The Trenton series form the surface rock over a large area in the United States and Canada. The strata is about 100 feet thick in New York State, increasing to 1,000 feet in Pennsylvania.

The Black River, birds'-eye and Lowville limestones belong to the Trenton group, the Lowville being a particularly fine stone for building.

Canajoharie Shale

The Canajoharie shale overlies the Trenton limestone with the Utica shale above it. With the Frankfort shale, these strata form the great central belt of rocks along the Mohawk on which the great majority of the Valley's population is located.

The entire terrain of shales of the Ordovicic period which is older than the Utica shales (but of Utica aspect) is now distinguished as Canajoharie shale. It lies generally in the lower Mohawk Valley (east of Little Falls) and in the Hudson Valley with outcrops of the greatest depth on the northern Vermont shore of Lake Champlain.

The Canajoharie Gorge is cut through this black shale and the falls drop over ledges of this rock. The typical exposure in this unusual and accessible Gorge has given the Canajoharie shale its name. It outcrops in the Rural Cemetery, on Van Schaick's Island at the mouth of the Mohawk. At Panton, Vermont, on the northeastern shore of Lake Champlain, the Canajoharie shale has a depth of 1,000 feet; the Canajoharie is correlated with the Magog shale, appearing on the boundary region between Quebec and New Hampshire.

In the Valley, the Lorraine gray shale and sandstone are termed the Schenectady beds and similar beds to the north and east are called Snake Hill beds from Snake Hill, south of Saratoga Lake. All these formations — Canajoharie shale, Schenectady beds and Snake Hill beds are of Trenton age.

Shale is a rock composed of clay, divided into plates that lie generally parallel to its bedding and that split easily in the direction of the grain. Shale is the geological term, although, technically slate is the only shale which can be readily split into "slates". Our old time school slates were in reality pieces of this shale which is such a marked feature of the Mohawk Valley.

Utica Shale

The Utica shale lies directly upon the Canajoharie shale in the middle and upper Mohawk Valley. Overlying the Utica shale is the Frankfort shale and limestone. These rocks were laid down at the close of the Ordovicic period, before the beginning of the great Taconic mountain revolution, of which the Berkshires, Green Mountains and White Mountains are the remains.

Typical outcrops of the Utica shale are along Starch Factory Creek, running through the New York State Masonic Home grounds, and beautiful Frederick T. Proctor Park, at the western limits of Utica. In drilling a well in Utica it was found that the shale extended to a depth of 700 feet.

Frankfort Shale and Limestone

The Frankfort shale and limestone is exposed in Frankfort Gulf south of Frankfort village. Both the Utica shale and the Frankfort shale outcrop in Central New York and on the western side of the Adirondacks.

Oneida Conglomerate

The Oneida conglomerate and Medina sandstone were the first Siluric sediments to form in Central and Western New York, the two being practically of the same age. These coarse deposits were washed into the shallow sea from the northern lands of the Adirondacks and Canada. The Oneida conglomerate extends from Central to Western New York. A conglomerate rock is one composed of heterogeneous materials, typically a rock composed of rounded and waterworn pebbles or pre-existent rocks, sometimes called pudding stone. The Oneida conglomerate is banded with the Helderberg limestone and Oriskany sandstone, the three forming the narrow belt of the Helderberg Escarpment, from the Catskills to the Niagara River.

Clinton Shale, Sandstone and Iron Ore

Next in order, in the Siluric period, came the deposits of the Clinton series which consist of layers of shale, sandstone and iron ore. They are so named from the town or village of Clinton in Oneida County on Oriskany Creek. As Hamilton College lies in the midst of these formations, they have been particularly studied, inasmuch as Hamilton gives courses in geology. The Clinton limestone and sandstone have been extensively quarried here and have been used in the erection of the fine buildings of Hamilton College. The Clinton iron ore has been mined in two mines, one at the northern limits of Clinton and the other at Franklin Springs at its southern limits. This ore is now used in the manufacture of metallic paint at Clinton. Clinton sandstone is a rock consisting chiefly of particles of quartz sand, cemented with silica. The Clinton series extends from Central to Western New York.

At Clinton the following strata are exposed: Frankfort shale; Oneida conglomerate; Clinton shale, sandstone and iron ore; Niagara shale and limestone; Lower Salina or Vernon red limestone, Upper Salina shale and limestone; Cobleskill, Rondout and Manlius waterlimes; Helderberg limestone.

Cobleskill Limestone

At the top of the Siluric beds are the Cobleskill, Rondout and Manlius limestone named from their outcrops at Cobleskill, Schoiarie County, Rondout at Kingston, Ulster County, and Manlius in Onondaga County. These are not thick but are strongly in evidence across the state.

Helderberg Limestone

The Helderberg limestone forms the lowest stratum of the Devonic period. It takes its name from its marked outcrop on the picturesque Helderbergs which have their highest elevation and steepest fronts in Albany County. These mountains run into northeastern Schoharie County and also form the highlands known to geologists as the "Helderberg Escarpment" which runs from the angle of these heights south of Albany, westward to the Niagara River. It is this hard limestone which caps the southern Mohawk-Susquehanna divide, its heights and its highlands and, with the Oriskany sandstone and Oneida conglomerate, forms a narrow belt, from the eastern face of the Catskills westward through Central New York to the Niagara River.

Oriskany Sandstone

The Oriskany sandstone of the lower Devonic period outcrops on Oriskany Creek. Devonic strata comprise the whole Catskill and southwestern plateau province, except for a few small patches of Carbonic rock and thus cover more than one-third of the area of the State. They are more widespread on the surface than the rocks of any other age. The combined thickness of the Devonic strata is over 4,000 feet, which is considerably more than for any other Paleozoic period in the State. The Oriskany sandstone extends from the East Central to Western New York and to the eastern base of the Catskill Mountains. As previously mentioned, the Oriskany sandstone and the Oneida conglomerate outcrop, with the Helderberg limestone, along the summit of the Helderberg Escarpment.

The Oriskany is the second from the bottom of the long series of Devonic rocks and is the last of the geological strata which take their names from localities in the Mohawk Valley. The Catskill sandstones, which cap the Devonic period, might be included, as the Catskill Mountains form part of the Mohawk Valley, inasmuch as they cover the southern half of Schoharie County. However, this makes a conclusion of the eleven rock formations which bear geological designations derived from their typical Valley outcrops, and these same rocks, with the Precambric formations of the Valley's Adirondack region, constitute practically the entire Mohawk Valley surface rocks.

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