A fold in a rock main content.

A fold in a rock

Part of Hall of Planet Earth.

A Fold in a Rock-Schist AMNH/R. Mickens

This rock illustrates how seemingly brittle material can flow, given enough time and heat. It formed over 440 million years ago, when a new mountain range grew on what would become the margin of North America. Originally a shale, the rock was buried to a depth of 8 kilometers, heated to 550 degrees Centigrade, and folded. It was transformed into a schist containing the minerals gray quartz, shiny mica, and red garnet.