Investigation of marine invertebrates that bore, rasp, scrape, or otherwise erode intertidal and subtidal outcrops of sedimentary rocks of the Pacific Coast suggests that they are very significant in attrition of submarine outcrops and in shaping the configuration of the seabed. Localities ranging from the intertidal zone to depths of 160 ft in submarine canyons show that physical and chemical processes eroding the rocks are relatively unimportant compared with intensive "bioerosion."

Rock samples collected are studied by X-ray radiography to determine the internal distribution of borers; thin-section petrography and induction furnace analysis are used to learn the exact lithology and carbonate content. All borers and other occupants of each sample are recovered and identified.

Important initial excavators are bivalve mollusks and polychaete annelids. Upon death their borings provide protected habitats for other boring taxa, giving rise to a sequence of excavations with time. Over 50% of the volume of some rocks is excavated, containing extensive internal passageways and galleries.

Some taxa are confined to certain depth zones or rock types; others are present from the intertidal zones to the deepest localities investigated (160 ft). Rocks most susceptible to attack are generally fine grained

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and soft, but durable calcareous lithologies are selected by chemical borers requiring a carbonate substrate. Places with relatively stable environmental conditions, such as the upper walls of submarine canyons, exhibit a diverse community of bioeroders. Their activities are responsible for a system of borings penetrating several decimeters into the rocks.

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