Late Cambrian algal reefs and bioherms in the Llano uplift of central Texas contain 4 genera of fossil algae. In decreasing abundance, they are Girvanella, Renalcis, Nuia, and Epiphyton. Detailed taxonomic investigations of the algae and studies of their host rocks show that changes in the ancient environment can be determined from (1) variations in relative abundance of Nuia, Renalcis, and Girvanella; (2) variations in growth patterns of Girvanella; (3) variations in the macrostructure texture and composition of the algal limestones; and (4) combinations of 1, 2, and 3. The occurrence of only 1 or 2 of these variables is sufficient to provide information about depositional environments. This can be accomplished by thin-section study of fossil algae in cuttings-size fragm nts.

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