Shale is the rock most often associated with landslides. Weathering transforms the shale into a clay-rich soil which normally has a very low shear strength – especially when wet. When these low-strength materials are wet and on a steep hillside, they can slowly or rapidly move down slope. Overloading or excavation by humans will often trigger failure.
Shale delta: A delta is a sediment deposit that forms when a stream enters a standing body of water. The water velocity of the stream suddenly decreases and the sediments being carried settle to the bottom. Deltas are where the largest volume of Earth’s mud is deposited. The image above is a satellite view of the Mississippi delta, showing its distributary channels and interdistributary deposits. The bright blue water surrounding the delta is laden with sediment.
