Clasts vary from well rounded to subangular and consist mostly of mafic and felsic volcanic rocks in roughly equal proportion. In Michigan basal beds are locally mineralized with chalcocite. Conglomerates are generally massive to crudely bedded and contain rounded clasts of mostly felsic volcanic rocks as much as 50 cm in diameter. Sandstones are mostly lithic arenites, commonly trough crossbedded with dominantly volcanic grains. Conglomerates are mostly near the base and finer grained rocks are higher in section.
Flows are pahoehoe or autobrecciated and commonly sparsely vesicular.
Numerous conglomerate and sandstone beds between flows are composed of volcanic fragments. Purple, brown, or reddish colors predominate. Flows range from 9 to 60 m thick, and most have autobrecciated and vesicular tops. Commonly strongly chloritized. Generally fine-grained and massive, but more felsic flows have plagioclase phenocrysts.