Some trails are more meandering in appearance and probably represent grazing traces as an invertebrate systematically combed an area of sediment for food. There are also resting traces produced by animals such as trilobites. Continued burrowing or bioturbation of the sediment will destroy primary sedimentary structures, and result in a massive, homogeneous, structureless rock. Burrows in Deep River Basin. Borings are usually circular.
Some snails produce borings or drill holes into other molluscs, such as clams.
Bivalves may bore into wood, rock, or other hard materials. Sponges also produce borings, often riddling shells with numerous tiny holes. Coprolites may contain fragments of undigested food, and thus provide valuable information about the feeding habits of fossil organisms. Coprolites smaller than 1 mm are called fecal pellets . Fecal pellets may be extraordinarily abundant in some environments, and are the dominant allochem in pelleted limestones.