Five facies were defined on the basis of a Q-mode cluster analysis of petrographic data derived from well core samples of five Guelph Formation pinnacle reefs. The five statistically significant facies are crinozoan facies, bryozoan facies, mixed skeletal facies, stromatoporoid-coral facies, and brachiopod facies. Additionally, empirical data were used to define an algal facies.

Pinnacle reefs initially formed as carbonate mud mounds when bryozoan and crinozoan "mud dwellers" baffled and trapped sediment. This mechanism allowed the mounds to grow upward into a zone near wave base. As mud-mound buildup reached wave base, an ecological regime suitable for the development of a stromatoporoid-coral reef was established.

Guelph Formation pinnacle reef growth ended after the deposition of an algal boundstone lithofacies. Crests of pinnacle reefs were at or close to sea level prior to a major sea regression marking the end of the Niagaran Series.

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