Two types of bioherms occur in the Pitkin Formation (Chesterian) of northern Arkansas. They are composed of a complex series of ovoid thrombolite heads intergrown with various other algal and bryozoan biolithites and are distinguished from each other on the basis of geometry and faunal and floral content. Type I mounds have sharp lateral margins and interfinger with horizontally bedded, contemporaneous, flanking strata owing to having expanded and contracted in size during upward accretion. They are interpreted as having had synoptic relief of 3 m or less during growth. Type II mounds occur slightly higher stratigraphically, to the east of type I mounds, and formed in deeper water, farther out on the Ozark shelf. They are interpreted as having had synoptic relief of 6 m or more during growth. Their margins are similar to those in type I mounds, but owing to higher synoptic relief, they interfinger with sediments derived from erosion higher on the mound.