The Cambro-Ordovician story of the Mohawk Valley

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This outcrop near the village of Ingham Mills, NY, illustrates one of the most profound unconformities in the rock record of North America. An unconformity is defined as a gap in the rock record caused by a break in deposition and subsequent erosion. This particular unconformity, known as the Knox unconformity, can be traced as far south as Alabama. This surface represents a break in deposition between the lower Beakmantown Formation (Cambrian) and the upper Black River Group (Ordovician) of 35-40 million years.

"Karst" is defined as a type of topography that develops over carbonate rocks by dissolution of these rocks, creating caves, sinkholes and underground drainage. This labeled surface appears to be an example of "paleokarst". What we are seeing at this surface is an ancient sinkhole. The vertical lines in the lighter colored rock above are Skolithos trace fossils. A trace fossil is evidence of ancient life that does not actually preserve the organism itself. A fossil footprint is a good example - here we are seeing vertical worm borrows. This type of fossil is characteristic of a shallow, intertidal region. This is the Black River Group of Ordovician age. The width of this photo is roughly 5 meters (~15 feet).

Hoyt Formation, a limestone that is part of the Great American Carbonate Bank of the Cambrian Period. This photgraph shows well preserved fossil stromatolites exposed along the pavement. What we see preserved here is not the actual organism; stromatolites are blue-green algae that grow in mats in very shallow water. These mats create mud mounds, which, over time, can turn into something which has the appearance of a head of lettuce. In affect, this photograph shows a slice through this head of lettuce caused by erosion, exposing the inner structure of these stromatolite mounds. Molluscs are also preserved in this rock, further indicating that the water must have been very shallow during the time of deposition.