Graded Bedding Ash
May 30, 2007
If you quickly drop a handful of unsorted sand (sand that includes a variety of sizes of grains) into a tall container of water, the particles will separate by sizes. In general, the larger, more dense and more rounded particles settle fastest. Of these, particle size is the easiest to observe. So, the largest particles settle quickly and progressively smaller particles settle on top of them.
In nature, this kind of pattern might form from a landslide into deep water. Underwater landslides are known as turbidity currents, which are common on relatively steep continental slopes. Graded bedding can also form in a different fluid; air. A violent volcanic explosion may blow a mixture of sizes of ash particles (volcanic rock) into the atmosphere. As they settle, the largest grains settle first to form graded bedding.
Graded bedding provides evidence of the original orientation of rocks. For example, if a geologist found a rock in which smaller particles on the bottom progressively changed upward to larger particles, you might infer that this rock layer has been turned upside-down by folding or some other geologic process.