The lowest Upper Ordovician (Blackriveran) strata in the Lake Simcoe area represent the base of a major transgressive sequence of peritidal carbonates and siliciclastics. From detailed petrographic study of 12 cores that penetrate, in ascending order, the Shadow Lake, Gull River, and lower Bobcaygeon formations, seven distinctive lithofacies are recognized: (1) sandstone, siltstone, and shale; (2) dolostone and calcareous dolostone; (3) lime mudstone; (4) peloidal bioclastic wackestone and packstone; (5) peloidal bioclastic grainstone; (6) ooid grainstone; and (7) intraclastic bioclastic wackestone and packstone. Metre-scale cycles, like those reported from numerous other peritidal deposits, are apparently absent in this succession. In addition, these strata are commonly interrupted by a variety of hardgrounds. The carbonate strata can be grouped into four lithofacies associations that represent deposition in generally low energy, tropical paleoenvironments along a southward-facing, homoclinal ramp that was frequently influenced by storms. Rapid facies changes along depositional strike likely resulted from a combination of an irregular basement topography and coastline geometry. The coexistence of Heterozoan and Photozoan carbonates in the lower Bobcaygeon Formation probably reflects the increasing incorporation of cooler oceanic waters as the ramp gradually deepened during the Tippecanoe transgression rather than the onset of more temperate climatic conditions.