Published in Trumansburg Free Press May 11, 2005
|Land & People
by Bill Chaisson
It wasn't until my family moved upstate from Long Island to the Hudson River mill town of Beacon, that I encountered stone sidewalks. The stone sidewalks of Beacon were very much like those of Trumansburg. They were blue and the opposite of flat. As in Trumansburg, the roots of the trees planted between the sidewalks and the streets had lifted the stones slowly and inexorably over the years. Whereas concrete sidewalks tend to crack and disintegrate when lifted by tree roots, stone slabs simply go along for the ride. The sidewalks outlast the trees. Long after the trees have died and been cut down, the sidewalks remain in their tossed-around conformation.
Construction projects, like the repair of water or sewer pipes, will also knock a stone sidewalk out of true. The resetting of the sidewalk is the last hurried part of any such project and even if it get put back more or less flat, the stone will ride the post-project settling downward. Between the tree roots pushing up and the settling pulling down the walk eventually look like a rollercoaster without tracks. And people stop walking on them.
When no one walks on a stone sidewalk, the algae that naturally grows on a shaded, damp surface will not be worn away by foot traffic and the slabs will become slippery when wet. And people walk on them even less often.
The stone itself is often referred to as 'slate', which is a metamorphic rock. The sidewalks are actually siltstone, a sedimentary rock. Siltstones and shales can be converted into slates by the pressure and heat that is associated with collisions between tectonic plates. A prominent 'slate belt' stretches the entire length of the eastern United States and extends into southern Quebec. It was formed by collision of an island arc (analogous to Japan) slamming into North America 460 million years ago during the middle Ordovician Period. You can see slate outcrops on I-90 between the Hudson River and the Taconic Parkway.
Sheets of slate split along planes of 'cleavage' that
|are formed when flat minerals
of mica are pressed parallel by the pressure associated with the above
described collision. In contrast, the flat planes of a sedimentary
rock are formed as sediment accumulation surfaces at the bottom of a body
of water. The individual layers of sedimentary rock represent continuous
sediment accumulation. The visible boundaries between layers represent
interruptions in sediment accumulation. These planar boundaries constitute
the surface of our stone sidewalks.
Many of the slabs are near absolutely flat, but one will often see ripple marks and other sedimentary structures in the sidewalk surfaces. These are the work of currents on the bottom of the shallow sea in which these sediments were deposited. Pore water rich in minerals percolated through the sediment over millions of years and 'cemented' them into rock, preserving the ripples and other structures originally formed in the sediment.
Silt is the grain size that is smaller than sand and larger than clay. Silt may be told from sand by the fact that individual silt grains are too small to see. The classical way to distinguish between silt and clay in the field is to rub the sediment against your teeth. Your fingers are not sensitive enough to fill the grit of silt, but your teeth are.
There are few fossils in this sidewalk siltstone. In part this is a commercial decision. Fossils tend to be weaker than the surrounding stone and the presence of too many will cause the working surface to break up. Some of the original porosity of shells and wood remain in the fossils, allowing water to penetrate the exposed surface. When it freezes, it shatters the stone. So the fossils that you may find in paving stones are generally small and fragmentary. I have never seen a single fossil in a Trumansburg sidewalk, which leads me to believe that they are Late Devonian in age. The regional rocks of that age were deposited in a deep, unstable slope environment and few creatures were apparently present and fewer still preserved.
I don't know where the Trumansburg sidewalk stone was quarried. I strongly suspect that it was quarried locally. If you know, please donít hesitate to contact me with information. More on local stone quarries, active and inactive, another time.
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Last revised: May 11, 2005