Published in Trumansburg Free Press  September 1, 2004

Land & People
A BriefLandscape History of Western New York

by Bill Chaisson 

The bedrock geology of western New York is something like a very large cake that someone has left out in the rain ... for about 15 million years.  The layered rocks that make up the Allegheny Plateau are tilted less than 5 degrees to the south and a series of valleys that hold the Finger Lakes have been incised into the plateau.  The original erosive work was done by running water and more recent excavation was accomplished by continental ice sheets.  In short, it looks as if giant with rather indifferent ideas about hygiene carved out a few servings, tried to cover up the mess with frosting and then abandoned the whole business.

Nearly all of the bedrock in the region is composed of layered sediments that were deposited into shallow seas, which covered the continent through much of the Paleozoic Era (540-245 million years ago).  Each successive layer buried the one beneath it and the weight of the accumulation caused the plastic stratum below (the asthenosphere, if you must know) to give way, making room for even more sediment to be added.  Sediment deposition in this region probably ceased approximately 300 million years (Pennsylvanian Period).  Today this stack of rock is over 5000 feet thick.  Much of it is shale, which when it was mud (before being compressed into rock) was about ten times thicker.

All of this lay deeply buried below the surface through the Mesozoic Era (245-65 million years ago) during which the dinosaurs padded about the landscape.  During the first third of this period the North American continent was locked in embrace with the Eurasian and African land masses.  About 190 million years ago (Jurassic Period) the North Atlantic Ocean began to open.

An ocean in this context is little more than a hole in which to pour sediment.  As the ocean widened and deepened, North America slipped grain by grain into the basin.  The continent, relieved of this 'overburden', began to rebound out of the plastic stratum, exposing more and more sedimentary rock to the excavating power of running water.  It is this rebound that created the present-day Appalachian Mountains and the surrounding plateaus, of which

the Allegheny is the northernmost.

The region surrounding and including Hudson Bay is called the 'Canadian shield' and it is essentially the nucleus of North America, around which successive 'terranes' have been plastered.  The terranes accumulated sediments and are called 'platforms'.  Both the thick platform sediments and the thinner shield sediments were eroding into the Atlantic basin.  Because the overburden was thinner, the shield began to rise earlier than the platform.  Hence the platform rocks of the Allegheny Plateau tilt (or 'dip') toward the south, away from the shield.

Before the rise of the present Appalachians, rivers on the east coast flowed south and east across New York State, away from the rising Canadian shield and toward the opening Atlantic.  The Susquehanna and Delaware Rivers are relicts of this period, their paths now made tortuous by the ridges of Pennsylvania.  But the soft red shales at the southern edge of the shield were carried away more quickly than the more resistant rocks further south, creating a valley at the edge of the shield.  Rivers began to drain from the rising Allegheny Plateau north into this 'Ontarian River'.  These valleys now hold the Finger Lakes from Seneca east to Otisco Lake; note that they all converge toward the north.  The Finger Lakes from Keuka to Conesus originally drained into the Cohocton Creek, which still drains into the Susquehanna.  Keuka Lake retains the junction where two tributaries joined on their southward flow.

This was all rearranged over the last million years as at least five successive glaciations remolded the landscape.  All the north-south valleys of the Allegheny Plateau were greatly deepened as the ploughshare of an icesheet two miles thick scraped southward toward Pennsylvania.  During the melt back of the last glaciation the ice sheet stalled south of the present Finger Lakes, extruding sediment from its diminishing mass.  When the retreat resumed there was an enormous linear pile of dirt winding from the foothills of the Catskills to the Erie Lowlands.  It is called the Valley Heads Moraine and it completely blocked the drainage into the Cohocton/Susquehanna watershed from the western Finger Lakes.  All water draining off the northern slopes of the Valley Heads flows through the Finger Lakes and northward to the Great Lakes.

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Last revised:  May 9, 2005