Published in Trumansburg Free Press February 15, 2006
|Land & People
Rising and Tearing Down
by Bill Chaisson
At present the primary natural force shaping the local landscape is water. Tectonic forces are also at work in the Finger Lakes; the entire region is very slowly rising, but it is not a dramatic mountain-building event. Rather, it is a response to the relentless erosion of bedrock by water. The enormous stack of sediment that collected in this region discontinuously through the early and middle Paleozoic Era (~550–350 million years ago (mya)) has been eroding westward into the continental interior since the Alleghanian Orogeny (mountain building event) 280 mya and eastward into the Atlantic basin ever since it got big enough to collect sediment over 100 mya.
As the topmost, younger sedimentary rocks are weathered to sediment and eroded away, the mass pressing down on the elastic upper mantle decreases. The tectonic plate gradually floats higher, a phenomenon known as “isostatic rebound”. This process has managed to uncover not only the older, harder, sedimentary rocks, but also the gnarled metamorphic roots of Paleozoic mountain ranges. The present Appalachian Mountains are the product of this isostatic process; they have been left standing higher than the surrounding land, largely because they are more resistant to erosion, not because they have been actively thrust up.
The old metamorphic spine of the Paleozoic ranges runs through New England, Westchester County, northern New Jersey, southeastern Pennsylvania and so on southward. North and west of that spine are the folded sedimentary ranges, like the Endless Mountains of northeastern Pennsylvania. North and west of those are the merely tilted sedimentary strata of the plateaus, like the Allegheny Plateau into which the Finger Lakes are cut. On a drive from Trumansburg to Philadelphia you will leave the plateau and enter the folded mountains on I-81 just south of the New York-Pennsylvania border. You will encounter the sparkling, mica-rich metamorphic outcrops of the old mountain spine near the southern terminus of I-476 in the suburbs of the City of Brotherly Love.
Much of the erosive work over the past 200 million years has been done by water in its liquid form, but water in its solid state has done its share. Ever since the climate of upstate New York has been cold
|enough to seasonally dip below 32°F (you won’t be surprised to
learn that it has been like this for millions of years), water that
accumulates in crevices of bedrock each autumn has frozen and expanded
each winter. The ice pushes outward with a force of 4000 pounds per
square inch, fracturing the rock, exposing it to further erosion by
water each spring.
The really dramatic erosion, however, was accomplished by the Laurentide ice sheet. It advanced across upstate New York at least four times during the last two million years. Ice sheets are continental-scale glaciers; the last ice sheet extended from a center at Hudson Bay to an edge in northern Pennsylvania, a radius of 1300 miles. Ice sheets are parabolic in profile, which means they remain quite thick to very near their perimeter. At its maximum extent 18,000 years ago the ice sheet was 8,000 to 10,000 feet thick over Hudson Bay, but still over 5,000 feet thick above western New York.
Under enormous pressure of its own weight, glacial ice is transformed from a rigid to a plastic substance, into something like very stiff Silly Putty. When the base of an ice sheet encounters bedrock obstacles, the pressure may increase locally so that ice returns to a liquid state. The water liberated from the ice sheet trickles into crevices where, relieved of pressure, it re-freezes, fracturing the rock, which is then dragged forward with the moving ice sheet overhead. Often debris is incorporated into the plastic mass of the ice sheet and moves upward and forward in slowly surging lines of flow.
An ice sheet “retreats” when its melts back from its perimeter more quickly than it flows out from its center. Even while the ice sheet is shrinking in size, the ice continues to be squeezed outward in all directions away from the maximum thickness at the center of the mass. Therefore the landscape of the Finger Lakes was subjected to erosion continually during the tens of thousands of years that the ice sheet covered the region.
For millions of years the form of the local landscape was determined by the erosive forces of water and ice acting on a plateau that being gradually lifted by the tectonic force of isostatic rebound. The Finger Lakes landscape presumably looked more like the Cumberland Plateau of western Kentucky before the ice sheet smoothed and graded it. After the retreat of the ice, water resumed its erosive work. Then, about two hundreds years ago a bunch of guys with shovels showed up.
Next month: Reading the local landscape—features formed by water, ice … or shovels?
Hibernian Weather Channel Productions