The platform carbonates of the Conococheague limestone (Late Cambrian) of the Great Valley, Maryland, carry clotted and digitate mounds from 0.5 to 10 m high, similar to Aitken's (1967) "thrombolites." These mounds are surrounded by cross-bedded oolitic-peloidal grainstones, intraformational conglomerates, and thin-bedded graded dolostones. Internally the mounds are composed of an upward directed network of cm-scale "fingers" of micrite-microspar separated by infills of skeletal packstone-wackestone with abundant remains of gastropods, brachiopods, trilobites, etc. Renalcids encrust the edges of the mud fingers and, in the interior, make an irregular, convex lamination which contains filament (?) molds. Stromatactis-like cavities cross-cut the Renalcid encrusted mud finge s and the interfinger infills. There observations suggest that the mounds were rigid frameworks constructed by encrusting colonies of algae and forams (?) and inhabited by a diverse marine fauna. The fauna, the association with current-worked sediments, and the position at the base of the shoaling-upward cycles indicate a wave- and current-scoured open subtidal bank environment for these mounds, which we interpret as early algal patch reefs.

In contrast, we find transported blocks of different boundstones to the east in massive off-platform breccias of the equivalent basinal facies of the Frederick Valley, Maryland. These boundstone blocks are composed of masses of Epiphyton-encrusted thin micrite plates (platy algae?) and are cut by cavities lined with early marine cements. These boundstones, we believe, were rigid framework algal reefs but were situated along the platform edge as an early Paleozoic marginal reef tract.

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