Mohawk Valley Fossils

Lithologic formations in the Mohawk River Valley

This screen shows a time-corrected lithologic cross-section of the Mohawk River Valley. By "time-corrected" we mean that all points in a horizontal line along the page were deposited at the same time. By "lithologic" we mean that we are only concerned here with the rock-types.

The brick pattern indicates limestones, rocks consisting primarily of calcite, a mineral form of calcium carbonate. The mineral formed as shells of many different kinds of animals, plants and algae, was physically (and in some cases chemically) broken down and then deposited on the sea floor to become the rock we see today.

The horizontal-line pattern indicate shale. Shale is composed primarily of clay, and is the result of the erosion of other rocks. The clays making up the shales in the Mohawk River Valley were eroded from the mountains forming eastwards, during the Taconic orogeny.

Where the limestone pattern is interlayered with the shale pattern, so are the rocks. In real life, the interlayering (or, more correctly, interbedding) is on the scale of less than a foot. One would find six inches of limestone sandwiched between two 4 inch layers of shale.

Brief description of lithostratigraphy
There are four main stratigraphic units in this area of the Mohawk River Valley:

• The Lower Trenton Group

• The Denley Limestone

• The Dolgeville Formation

• The Utica Shale

• Bentonite beds

The Lower Trenton Group
The Lower Trenton Group consists of a number of formations. It primarily consists of limestones that were deposited in shallow water. The lower portions are light-colored. Locally, it contains vertically oriented burrows ("bird's eye" limestone), which are particularly diagnostic of intertidal conditions. Portions are dolomitic.

Denley Limestone
The Denley Limestone is a dark-colored, primarily black, fossiliferous micritic limestone. Beds are typically about 10 cm thick. Although typically very fossiliferous, the fauna is different from the underlying Lower Trenton Group.

Dolgeville Formation
The Dolgeville Formation consists of interbedded black limestones (similar to the Denley Limestone) with interbedded black shales (similar to the Utica Shale). The contacts with the Denley and Utica are gradational, except locally. In the lower Denley, the proportion of limestone to shale favors limestone; in the upper the ratio favors shale.
The contact along the New York State Thruway, between the Dolgeville and overlying Utica, is pronounced, and the upper beds of the Dolgeville there are plastically deformed into folds. These local strata are NOT conformable, but the result of a large slump of the overlying Utica shale into the basin to the east.

Utica Shale
The Utica Shale is a black, graptolitic shale with abundant pyrite. The shale is sub-bituminous, and fresh samples can be ignited. When a fresh sample is immersed in water, an oily sheen soon rises to the water's surface. The pyrite is evidence of anoxic conditions during deposition.

Bentonite beds
Throughout each formation can be found a number of diagenetically altered, volcanic ash beds, recognizable by their gray color and plastic (unlithified) nature. They typically form deep reentrants in the strata. They can often be recognized from a distance by their ability to form impermeable layers; plants find these layers a dependable supply of water and will often form distinct lines of vegetation at the bentonite beds, along an otherwise barren exposure of rock.

There are many bentonite layers in the strata. Efforts have been made to catalog all the different bentonites, but a thorough search can often reveal new beds. The different systems that various workers have devised for naming the beds are, therefore, usually not directly comparable.