The geology of Ardèche reflects 500 million years of history. It consists of a crystalline base (granite, gneiss, etc.), sedimentary deposits (sandstone, limestone, marlstone, etc.) and volcanic rocks (basalt, Pozzolan, etc.).
Sedimentary rocks are composed of pieces of eroded rocks and broken down living organisms. Once buried, sediment layers are compacted and transformed into rocks.
Magmatic or igneous rocks are formed when magma cools slowly deep down (plutonic rock) or rapidly on the surface (volcanic rock)..