Other experts say that lowering of the sea level by an ice age may have allowed the young river to scour down through the highlands near its source. Depending who you ask, the Hudson may have been ground deep by long ago glaciers.
Inwood Park extends to the Hudson River, which a geologist might describe as a fjord, because upriver it is deeper than at its mouth. The pothole is filled in now, to provide a stable foundation for the rebuilding. Such holes are carved by raging glacial meltwaters, which use huge cobbles as scouring powder. Near the south end of the island, in the excavations for rebuilding at Ground Zero, workers discovered a hidden glacial terrain, with a 40 foot deep pothole carved in bedrock. There are many small glacial potholes in Inwood, bored by ice and water in a process that produces pits oddly similar to the ones in city roadways.
Inwood Hill Park, at the north end of the island, displays sweeps of Inwood marble, metamorphic rock recrystalized from limestone built of sea creatures’ shells. Boulderers, urban climbers, practice their skills on many of Manhattan’s glacial erratics, especially on Rat Rock and Cat Rock, at the southern end of Central Park. The ice of the glaciers did not carve these rocks, the massive debris they carried did. In Central Park, many huge rocks stand out from the groomed landscape, some bearing the deep scoring of glacial striation, some shining with glacial polish. Manhattan can seem like one of the most manufactured environments in the world.
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