Although the sands are not laid down in flat layers, the process may build a horizontal layer. Shale deposits tend to be layered somewhat more horizontally than sandstones. The river delta itself is a complicated, ever-changing arrangement of channels and bars, constantly moving along the coastline. At any one time the delta seems to be moving seaward as deposits are left, but later settling, compression, or elevation of sea level can cause it to move landward. Sand is important because its porosity enables oil to accumulate and its permeability enables the oil to move to a well.
Shale is important because it contains the products of former life on earth, and their hydrocarbons.
These escape to nearby sands, but often not to the earth's surface, because of covering impermeable shales. The acoustic properties of sands and shales often overlap, though there is a slight tendency for shales to have a lower velocity than sands. Mixtures of sands and shales are called clastic rocks. The word clast means break. Clastic rocks are made from broken bits of crystalline rock.