Algal stromatolites locally comprise as much as half of the Willow River Dolomite (Prairie du Chien Group) in the upper Mississippi Valley. They exhibit two basic forms: mounds which may range up to 4 feet in diameter and mats which may be extensive. Laminations in both range from one to several millimeters in thickness and exhibit a variety of textures and compositions in thin section.

The structure of the mounds is distinctive in that the laminations are irregular and non-parallel. Thin sections show them to consist of fine silt-size dolomite debris and recrystallized micrite, and significant quantities of coarse debris including biogenic sand-size material and large pieces of shell material. Small spherical pellets are rare. Most samples contain some

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quartz, as well as small intraclasts or oolites, trapped between laminations and in small cut-and-fill structures.

In the mats, laminae are essentially smooth and parallel. Mats are composed almost entirely of fine silt-size dolomite and commonly bear desiccation features which suggest a calm shallow environment. This is in contrast to the presence of intraclasts and algal clasts in the mounds which indicate a relatively high-energy environment. Together, both forms of stromatolites suggest a shallow, intertidal, locally hypersaline environment very much like that of the areas where algal stromatolites occur today.

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