Its not a mine. They would sell the dirt until they hit a natural spring that made the lake.
Technically they could have been Mining sand but it's also very possible they were using the gravel to build the Garden State Parkway or expressway over passes here in SJ we call em gravel pits
Yeah it was a rock quarry or something. They hit a spring. New Jersey has a lot of these situations actually. Especially South Jersey.
There never was mining there.
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