 Houston is largely located within the Northern Humid Gulf Coast Prairies level IV ecoregion (34a), a subset of the Western Gulf coastal grasslands level III region as defined by the United States Environmental Protection Agency.This region is generally underlaid by Quaternary-aged sand, silts, and clays (clay, clay loam, or sandy clay loam) and covered by grasslands with occasional clusters of oak trees.The land surface in and around the city of Houston is composed of alternating layers of red, gray, sandy brown, and black organic clay; these strata generally dip to the southeast at a slope of 0.06% ( of vertical change for every of distance traveled).