Detailed study of Foraminifera from the subsurface in the Ocean City area indicates the presence of more than 3,000 ft of upper Miocene marine sediments equivalent to the Montesano Formation. The lithologic sequence consists of a lower clay shale unit up to 2,000 ft thick, a middle sand unit to 800 ft thick, and an upper silty shale unit about 1,000 ft thick. The lower clay shale unit does not appear to be present in the Montesano Formation outcrop area. In the Ocean City area the Montesano lies with apparent unconformity on older sediments and volcanic rocks ranging in age from late Eocene to middle Miocene. It is unconformably overlain by Plio-Pleistocene continental to shallow-marine sediments.

The absence of key foraminiferal species prevents recognition of the upper Miocene stages as defined in California. Comparison of the fauna with recent foraminiferal faunas indicates deposition in mostly middle to lower bathyal depths for the lower clay shale unit. The overlying sand contains a sparse shallow-marine fauna and physical sedimentary characteristics of a beach or bar deposit. The overlying silty shale unit contains a neritic fauna. Of particular significance is the abrupt transition from middle bathyal clay shale to shallow neritic sand without evidence of a break in sedimentation, thus indicating tectonic uplift rather than simple basin filling.

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