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USGS Geology in the Parks

Visual Glossary

Flattened rock with foliation versus stretched rock with lineation.
Flattened rock with foliation versus stretched rock with lineation. Illustration by Anne Crowder.

Foliation

Foliation forms when pressure squeezes flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied in. Slate, schist, and gneiss (pronounced 'nice') are all foliated metamorphic rocks.

Folded gneiss. It's not hard to see that this metamorphic rock, called gneiss, has been foliated, then intensely folded. Photo by Edward P. Klimasauskas, USGS.
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