Bioturbation is the churning or reworking of sediment by living organisms. It may enhance or decrease porosity and permeability. For instance, burrows filled with sand grains in a mud or clay unit could be conduits for fluid flow if connected. Bioturbation can offer clues to the depositional environment; vertical burrows may indicate shallow water, specifically the tidal zone, because the organisms moved up and down in the substrate as the tide ebbed and flowed. Bioturbation is sometimes called TTB - tracks, trails, and burrows.