The Hammer Creek Formation is a mapped bedrock unit consisting primarily of conglomerate, coarse sandstone, and shale.
[1] The Hammer Creek Formation and other formations of the Newark Supergroup were deposited in the Newark Basin, just one of many Triassic rift basins existing on the east coast of North and South America, which formed as plate tectonics pulled apart Pangaea into the continents we see today.
The Hammer Creek is mapped from the southern borders of Dauphin and Lebanon Counties to the northeast to the Schuylkill River.
A laterally equivalent rock unit called the Brunswick Formation is mapped on the east side of the river and into New Jersey.
[2] Relative age dating of the Hammer Creek Formation places it in the Late Triassic period.