There are lots of depositional (and erosive) processes happening. They're all subject to local conditions. But the most common (in no particular order) are: acc

There are lots of depositional (and erosive) processes happening. They're all subject to local conditions. But the most common (in no particular order) are: accumulation of plant matter, flood deposits, human-generated organic and building debris, and earthworm casts. You can also see other things, like volcanic activity and wind-blown material, in fewer places.