Abstract: Harhor formation (Late Middle Eocene) outcrops in asmall syncline, situated to the north of the Haymana Anticline.The formation overlies the Çayraz formation (with abundant nummilites ,and assylinas) with a thin conglomerate ,and consists ofalternations of thick (50-200 cm.) sandstones ,and thin mudstone bands. Total visible thickness of the formation is 255 m. Theupper part, forming the centre of the syncline, is eroded. Thesecalcareous sandstones contain approximately 20 % muddy matrix ,and consist of volcanic rock fragments (more than 50 % byvolume), metamorphic schist, granite, limestone ,and chert fragments, as well as angular quartz, orthoclase ,and abundant calcicpiagioclase. Thus, the rock may be named as a litharenite, / evena volcanic - arenite. Heavy minerals consist of mainly glaucophane, garnet, apatite, zircon ,and tourmaline, in addition to abundantopaque iron minerals ,and a few others. The first three mineralsshow a good correlation in their «abundance» (Norman, 1969) within the rocks.Sediments of Harhor formation are probably derived from variouscomplex sources : basic volcanic, metamorphic, sedimentary ,andacid intrusive areas. In technically active geologic conditions, theywere first accumulated in relatively shallow marine waters (neritic environment) where from time to time, they formed submarineslumps, turning in turbidity currents. These currents resedimented the materials as turbidites in the deeper parts of the basins.The increase / decrease of abundance of various heavy mineralgroups at different stratigraphic levels may be explained by differential uplift in the source areas ,and widespread climatic changes which affected the rate of erosion.