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). These soils were deposited by tributaries of local waterways, particularly the Brazos and Trinity rivers. There is a considerable contrast in soil composition to the north around Cypress Creek; most of the surface there consists of tan-colored sand with small amounts of gray clay. The north and northwestern regions of Houston and Harris County feature a slightly steeper slope than other parts of the city, with occasional escarpments caused by faulting or erosion.