Natural slope usually has a zestier of strong unloading and a zestier of weak unloading and corresponding zone of strong (or weak) weathering, and there is integrity fresh rock in its inside.

Beneath the Sahara are huge aquifers, basically a sea of fresh water, that's perhaps a million years old filtered through rock layers.

Shale traps gas more tightly than other rock. To extract it, the shale is blasted with huge volumes of fresh water at high pressure, a practice known as hydraulic fracturing, or "fracking".