Small head-shaped structures (biscuits) and large ellipsoidal structures (domes) form in the 3-25 feet zone by agglutination and binding of detrital sediment onto the filaments of blue-green algae. Algal stromatolites accrete at the rate of 1 lamina (sediment-rich layer plus algal-rich layer) per day. The distribution, abundance and morphology are controlled by current velocity and rates of sediment movement over the bottom. Algal mats occur in areas of less than 60-80 gm/hr/ft bottom sediment movement. Biscuits and domes are restricted to the range 8-60 gm/hr/ft bottom sediment movement. Domes form in areas of rapid sedimentation and biscuits in areas protected from high rates of sediment movement. Streamlining is related directly to current velocity and sediment movement. Elongated forms have the axis parallel to the current flow.

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