The shaking of earthquakes makes shallow soil layers slump down upon water-saturated sand beds below them, but because water does not compress, the material seeks an outlet wherever a fracture forms above it. Finer sediments don't have enough water and don't let the water out fast enough to form sand blows. Sand blows can be several meters across and tens of centimeters thick. They are considered to be strong signs of ancient earthquakes when they are found in buried sediment sections. This sand blow formed during the 1989 Loma Prieta earthquake.