Detailed field study of a thin sequence of Middle Pennsylvanian rocks in central Colorado reveals that faunas comprising abundant mature fusulinids lived in water deeper than 40 ft. Juveniles rarely became established in water as shallow as 15 ft, but unfavorable conditions associated with shallow water precluded development to maturity. Absolute depth figures are based on: (1) knowledge of the position of the shore of maximum transgression, (2) knowledge of the form of the surface on which marine beds were deposited, and (3) interpretation that distribution of faunal and floral groups in central Colorado was controlled by depth or by factors directly affected by depth.

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