In downtown Richmond, Va., a highly diatomaceous deposit contains a diatom assemblage in which 87 taxa have been identified. The diatoms indicate that this sedimentary unit is correlative with Lithologic Unit 19 of the Choptank Formation of middle Miocene age. Also in Richmond, a sedimentary unit correlated with the Eastover Formation overlies the Choptank Formation and contains a sparse diatom assemblage in which 24 taxa have been identified. The diatoms in this upper deposit form an assemblage that is distinctive from that of the Choptank Formation and is of late Miocene age. Both diatom assemblages are dominated by taxa indicating deposition in a littoral or shallow neritic marine environment. Although reasons for differences in diatom diversity and abundance between the two formations are obscure, these differences appear to reflect the greater primary productivity of diatoms in middle Miocene than in later Tertiary time.