Upper Devonian carbonates of west-central Colorado contain abundant stromatolites. These finely laminated, crenulated, and commonly brecciated calcareous dolomites and dolomitic limestones comprise most of the Dyer member of the Chaffee formation along the west side of the Sawatch Range, in the McCoy area, and in the White River Plateau.

These fine-grained carbonates display striking structural and textural resemblance to laminated sediments now being produced by algae on western Andros Island, B.W.I., and in Florida Bay. The environment of present-day stromatolitic sedimentation is intertidal where only occasional flooding occurs during spring tides or periods of storm waves. Carbonate mud deposited by these waters is laminated by the trapping and binding functions of filamentous blue-green algae. Desiccation polygons may become dislodged during flooding to form intraclastic breccias. If, in this case, "the present is the key to the past," these Devonian sediments represent quiet-water carbonate deposition in the littoral environment.

The Dyer in the eastern and northeastern part of the study area is predominantly stromatolitic, but to the west the lower portion is a neritic carbonate accumulation. During lower Dyer time the intertidal environment existed on the east and northeast and an offshore environment existed on the west. During upper Dyer time the intertidal environment regressed westward and southwestward behind the waning Upper Devonian sea.

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