Published in Trumansburg Free Press January 12, 2005
|Land & People
Where the Fossil Are
by Bill Chaisson
Upon finding out that I am a paleontologist many peopleís faces light up when they say "You must love this area." Then their brows knit slightly and they say somewhat less enthusiastically "There are fossils around here ... right?" The answer is "Yes ... and no." It depends what we mean by 'this area'. From the immediate Trumansburg area south to Ithaca the bedrock is largely free of fossils. North of the village, however, you find some of the most fossil-rich rocks in New York state.
When you look at the local rocks in a single gorge or roadside exposure they look as if they are in flat, level layers, but they are actually tilted ('dipping') slightly to the south. Imagine that you have glued several pieces of plywood one on top of the other in a block, and then tilted it toward the south. Then you held a band sander on top of the pile, tilted it slightly toward the north and then turned it on. After about half an hour you'd see the lower layers being exposed at the north end of the block and the upper layers persisting at the south end.
One of the basic rules of geology is that lower layers were deposited before the ones above them and so they are older. Just like the lower layers of plywood are exposed at the north end of the hypothetical block, the lower and older layers of sedimentary rock are exposed as you go north from Ithaca to Seneca Falls.
When you look at the rock in one of our local gorges you can see distinct layers ('strata') because between the deposition of one layer and the next something in the environment changed. Sometimes sediment accumulation stopped and sediment was eroded. Sometimes the size or the type of sediment that was accumulating changes. It indicates some sort of change in the physical environment at the time of accumulation and these changes often affect the life forms living in the shallow seas where the sediments were collecting. Consequently, different layers don't just look different, they may contain distinct 'assemblages' of fossilized life forms.
|Today Trumansburg is about 900
feet above present sea level. In the Late Devonian (380 million years ago)
when the sediments that make up the walls of Taughannock Gorge were accumulating,
the depositional basin was more 900 feet below Devonian sea level. For
this to occur you have to both lower the proverbial bridge (the land) and
raise the proverbial river (sea level). Sea level was relatively higher
in the Devonian because there was less glacial ice accumulated in the high
latitudes and therefore more water in the oceans. The ocean basins were,
on average, shallower because they were growing more quickly than they
are today. Ocean basins therefore had smaller volume and the shorelines
crept inward on the continents. Sediment accumulating on continents actually
weighs them down and they begin to sink into the plastic underlying layers
of the planet ('the astheno- sphere'). Collisions between continents also
depress the continents downward before they are lifted up into mountains.
Between Ovid and Seneca Falls, if you look at creek beds and in small quarries where the glacial debris has been removed by water or heavy equipment, you will find rocks of Middle Devonian age (400-385 million years ago) exposed. They alternate among limestones, siltstones and shales, and many layers are filled with the remains of shelled organisms (corals, scallops, snails and more esoteric fare). The seas were shallow, perhaps only 50 to 100 feet deep at times. Sunlight could penetrate to the seafloor and it was therefore covered with life.
But through the Middle Devonian the shelly lime accumulations become thinner and less frequent. The silts and clays became thicker, a sign that a mountain range was building to the east, shedding sediment into the shallow sea. By the Late Devonian the weight of the mountains formed a deep trough along their eastern side. The silts and muds of the Taughannock Gorge layers accumulated in this trough. It was too deep, too unstable and too dark for much life to live on the seafloor. Consequently there are very few fossils. Paleontologists stand there with the rest of the public, gaping at the beauty of the rocks themselves.
Hibernian Weather Channel Productions
Last revised: May 9, 2005