The correlation of continental, brackish, and very shallow-water sediments by usual techniques sometimes presents serious difficulties. However, many stratigraphic problems can be solved by using spores and pollen, and some acid-insoluble animal microfossils. This method was used successfully on samples from two sections of the Mancos group from the Book Cliffs, Utah. Four groups of microfossils were extracted by treating the samples with phosphoric and hydrofluoric acids. These are: pollen and spores, hystrichospherids, dinoflagellates, and microforaminifera.

Pollen and spores are abundant in the upper two-thirds of the Mancos sequence. Plant species range throughout the interval, but relative populations of chosen genera and species change considerably with position. These population variations are interpreted as being a result of mild climatic changes, which most likely affected large areas simultaneously, and should, therefore, provide levels of correlation closely representing time lines. Thus, the upper two-thirds of the Mancos group can be divided into four pollen zones. Pollen is very scarce in the lower third of the section. Here, "top" occurrences of index species of microforaminifera and hystrichospherids provide a further zonal division.

Microforaminifera from the Mancos are closely correlative with the ordinary calcareous Foraminifera obtained from the same sections. Hystrichospherids seem to thrive at intermediate depths of the continental shelf. Their abundant occurrence in the lower part of the studied sections, and the corresponding scarcity of plant microfossils, suggest a deeper-water environment than in the upper part of the sequence, where pollen dominates over hystrichospherids.