The basal siltstones of the Edwardsville Formation of the Borden Group in southern Indiana contain almost tabular bodies (2 mi long by 60 ft thick) of crinoidal limestones showing abrupt lateral contacts with the contemporaneous and surrounding siltstones. Microscopic investigation of these bodies, which are thought to have developed in shallow depressions of the sea bottom, shows that they consist essentially of numerous alternations of coarse bioaccumulated crinoidal limestones and partially dolomitized calcilutites almost entirely devoid of crinoids. The coarse beds, deposited in place, were protected from any clastic influx by the screening effect of crinoidal growth, whereas the calcilutites, interpreted as accumulations of algal dust generated by phytoplankton corresponded to definite encroachments of the surrounding clastics. The temporary proliferation of the phytoplankton could result from the increased CO 2 concentration produced by the metabolism of the crinoids in combination with reduced circulation created by their growth. The algal precipitation of calcium carbonate would then make the environment lethal to the crinoids, and the related disappearance of the screening effect would allow an encroachment of the clastics, restricting the calcilutite area available for the next cycle of crinoidal growth. Repetition of this mechanism would lead to the gradual disappearance of the crinoids and to the burial of the carbonate body in the clastic sediments.