sentences:  Most of Cleopatra Hill, the rock formation upon which Jerome was built, is 1.75 billion (1,750 million) years old.Created by a massive caldera eruption in Precambrian—elsewhere more narrowly identified as Proterozoic—seas south of what later became northern Arizona, the Cleopatra tuff was then part of a small tectonic plate that was moving toward the proto-North American continent.After the eruption, cold sea water entered Earth's crust through cracks caused by the eruption.