Trails may be straight or curved. Burrows may be used by organisms for dwellings, or may be produced as a subterranean organism moves through the soil or sediment in search of food. Burrows are commonly filled in by sediment of a different color or texture than the surrounding sediment, and in some cases, the burrows may have an internally laminated backfilling. Burrow fillings may become cemented and hard, weathering out of the rock in rope-like patterns. Borings are ususlly circular in cross-section.
Sponges also produce borings, commonly riddling shells with numerous small holes.
Rootmarks typically branch downward in a pattern resembling an upside-down tree. Root marks are sometimes gray or greenish, penetrating reddish-brown paleosols. This contrast in color can make them easy to see and identify. Stromatolites are the only type of biostratification structure we will study. In the geologic record, most stromatolites are found in Precambrian and lower Paleozoic limestones.