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Outcrop 10:
Other Rocks in Rensselaer
ALONGSIDE GREYWACKE, other rocks in Rensselaer County are predominantly sedimentary rocks that formed as a result of weathering of the Taconic Mountains after colliding with the North American tectonic plate when New York State was underwater. These rocks primarily include shales, which is a brittle layered rock that forms from mud deposits. There is also some slate, which is shale that has been slightly metamorphosed.
Sandstone boulder on the Big Beaver Bog Trail
Photo: Dan Morse
In the Poestenkill Community Forest, there is a sandstone sample that was not formed on the Plateau. Sandstone is made mostly of sand sized quartz grains. This sandstone was formed somewhere else, and was brought to the Poestenkill Community Forest thousands of years ago by the glacier, and deposited when the glacier retreated. Look ahead for the sandstone boulder left by the glacier and now found in the roots of a tree. (Hint: the right edge of the boulder looks a bit like the profile of a bottlenose dolphin.) Note the worn cracks in the boulder where the original sand was laid down in layers.
Even though sedimentary rocks can have fossils in them, there aren’t any on the Rensselaer Plateau. Sometimes greywacke can have fossils, but it’s pretty rare!
Source: “Geologic Units in Rensselaer County, New York.” USGS Mineral Resources, USGS, mrdata.usgs.gov/geology/state/fips-unit.php?code=f36083.
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