The exposed strata of shales and sandstones exposed here were deposited during the Carboniferous Period about 325 million years ago. Here they dip gently NE but in the wider area they are gently deformed into open folds and cut by numerous faults. Coal seams and ironstones occur to the north and east. The quarry is formed in an igneous dolerite sill, which was intruded into the sediments about 290 million years ago (Early Permian).
The links to the pages below provide more detail about the geology of the Fossil Grove and the Carboniferous Period.
Geography of the Carboniferous
The British Isles in the Carboniferous
Animal life in the Carboniferous