The Guil crosses various geological zones born from different periods of their flooding, first under the waters of a warm and shallow sea more than 200 million years ago, then under a 6,561 to 9,842 ft (2,000 to 3,000 m) deep ocean 160 million years ago. The river's route displays two types of rock: schistous rocks, impermeable to the east up to the glacial lock of Château-Queyras, then limestone sedimentary rocks carved out of cavities to the west.