Groups of bioherms about 20 ft thick are exposed in the Hopkinton Formation at two localities. Bioherm cores contain scattered coral colonies up to 6 ft high in a fine dolomitic matrix. Dipping flank beds consist mainly of crinoid debris, with common stromatoporoids near the core. Coral growth forms seem to indicate that sedimentation in the cores generally kept pace with upward growth of the colonies. Lateral expansion of colonies was possible when the sedimentation rate fell; partial (or complete) burial of colony growth surfaces occurred when the rate increased. Similar coral bioherms are unknown elsewhere in the Silurian of the Midwest–GreatLakes area.