High Crag is composed entirely of lapilli-tuff of the Helvellyn Tuff Formation. This formation is part of the Borrowdale Volcanic Group, formed during a period of intense volcanic activity on the edge of an ancient continent during the Ordovician Period, about 450 million years ago. The Helvellyn Tuff Formation was formed by an explosive volcanic eruption which produced a large-volume pyroclastic flow of very hot gas and rock. Individual lapilli or pieces of semi-molten lava within the flow were flattened by the weight of deposits above them.