The narrowest and deepest gorge is hundreds of feet deep in the shale.
I pushed him on over a sloping peak of shale, and told him to hold his tongue.
If the layers were of sand, the rock is sandstone; if of clay, it is shale.
The inside of the cave is a shale that no one could dig into.
Green and red marl, shale, and shaly limestone with some veins of gypsum.