The lower Eocene Simsboro Formation of the Wilcox Group, between the Trinity and Brazos Rivers in Northeast Central Texas, was deposited as a continental, fluvial complex grading southward into marine, beach deposits. On the basis of mineralogy, grain size, cross-stratification, bedding, and shape of the sedimentary bodies, this complex can be separated into six lithologic types: (I) Channel Deposits--medium-grained sand. Immature clay pellet-bearing, quartzose subgraywacke bordering on an orthoquartzite. This sand occurs in filled channels and exhibits pronounced trough or festoon cross-stratification. The rounded sand sized pellets of kaolin are probably detrital. The matrix consists of particles of kaolinite worn from the clay pellets and of clay pellets mashed by harder detrital grains. (II) Floodplain Deposits--illitic-kaolinitic, thinly laminated, silty clay. The deposits occur as thinly laminated (5 to 20 mm), silty clay in lenticular beds up to 20 feet thick. They are laterally associated with the trough cross-stratified channel deposits. Angular pebbles, cobbles, and boulders of this clay are locally incorporated within the channel deposits. (III) Bar Deposits--fine-grained sand. Immature clay pellet-bearing, quartzose subgraywacke bordering on an orthoquartzite. The finer grain size and tabular cross-stratification distinguishes these deposits from the lithologically similar channel deposits. (IV) Lake Basin Deposits--white, kaolinitic silty clay. These clays occur in a discontinuous 20 foot bed at the uppermost part of the formation beneath 0.5 to 13 foot carbonaceous clay bed (lithologic type VI). These deposits are laterally associated with very fine grained bar deposits. (V) Orthoquartzite Deposits--fine grained sand. Hard, siliceous, orthoquartzite. They occur in discontinuous, massive ledges, from 2 to 20 feet thick. These orthoquartzites were deposited in channels and bars, differing from the channel and bar deposits (lithologic types I and III) only in mineralogic composition. (VI) Swamp or Lagoonal Deposit--gray carbonaceous, kaolinitic, slightly silty clay. It occurs near the top of the Simsboro directly beneath a 0.5 to 3 foot lignite bed. It ranges from less than one foot at the Trinity River to 13 feet near the Brazos River. Mineralogic variations in the sediment load present in the numerous southward flowing streams are reflected in mineralogic variations among similar types of deposits. The trough and tabular cross-stratification in the channel and bar deposits is indicative of deposition in a lower flow regime. The source areas were characterized by uplift, moderate to high relief, and a humid climate. The rocks exposed in the source areas included primarily older sedimentary rocks and granites or gneisses, together with minor amounts of high grade and low grade metamorphic rocks, hydrothermal veins, and volcanic detritus. Most of the Simsboro detritus was probably originally derived from the Ouachita Mountains and the surrounding area with minor contributions from the southern Appalachians and other sources.