Indian Rock is a large glacial erratic boulder of granite gneiss, formed in the Proterozoic (Precambrian) era, 1.2 billion to 800 million years ago. It is estimated to weigh ~17,300 tons. The source area for the boulder was nearby in the Ramapo Mountains-Hudson Highlands; it is difficult to know for certain exactly where it was picked up by the glacier, but most likely not more than 5 to 10 kilometers from its current location. The boulder rests upon glacial outwash which in turn lies atop Triassic sedimentary red beds (sandstone and shale) of the Newark Basin (circa 145 million years old). The rock was carried to its current location by the internal flow of the continental ice sheet during the last glacial maximum, circa 21,000 years ago. The base of the continental glacier scoured the bedrock terrain across which it moved, thus plucking large and small blocks of rock from their position in the Ramapo Mountains and Hudson Highlands. Indian Rock got as far as Rockland County before being liberated by the ice and deposited along with gravels shifted by glacial meltwater.