Geology – How did all those boulders get there?


The cliffs, granite ponds, and rock-strewn rolling slopes of the Lake Sagamore area were created by vast glaciers, pushing southwards from central Canada in sheets of boulder-studded ice over a mile thick. During the past 2 million years, four “ice ages,” each lasting hundreds of thousands of years, were briefly interrupted by warmer intervals. The current warm interval began about 11,000 years ago and is due to end a few thousand years from now despite global warming. The debris left by the melting glaciers includes the rocks, large and small, that litter the hillsides and were gathered by pre-colonial and colonial farmers and used for the stone fences and root cellars that still stand.

Putnam County has a long history of mining that is centered on a major fault line known as the Reading Prong that extends from eastern Pennsylvania through New Jersey. It becomes the long narrow depression of Peekskill Hollow on this side of the Hudson. The shape of the Hollow is due to glacial excavation of the central zone of weakened rock, shattered by repeated fault slippage that began as much as 150 million years ago and that continues to this day. In fact, this slippage causes nearly undetectable earthquakes every few months. Various minerals were deposited by the magmatic fluids seeping up through the broken crust along this zone. One of the first gold mines in the United States is located just north of Lake Mahopac. There are many other mines and prospects that yielded gold, silver, lead, tin, arsenic and bismuth in the 19th century, notably the Sunk Mine off Pudding Street.

Of greater economic and social interest were the open-pit iron mines along the Reading Prong in Orange and Putnam Counties. These mines fed the furnaces at Foundry Cove south of Cold Spring, supplying the metal for building the Cast Iron District in Manhattan and the “Monitor” gunboat of the Civil War. The largest of these pits is now Canopus Lake and neighboring Pelton Pond, while the most important (because of the quality of its magnetite ore) was the Tilly Foster mine between Carmel and Brewster. Dozens of water-filled pits and deep caverns, from Ludingtonville to Constitution Island, also date from this time. Today, the most important fact about the Reading Prong is that it is the main source of the radon gas that accumulates in unventilated basements throughout the region.

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