The Grands Causses, which encircle the Gorges du Tarn, are made up of limestone affected by karstic erosion for more than 20 million years. At the top of the canyons hence stand ruiniform reliefs with singular shapes. They consist of karstic recesses due to the erosion of the marls at their base which causes their collapse and forms a vast network of underground caves and siphons. Dolomitic limestones (magnesium) alternate their layers and form jagged rocks called lapiaz.