Sand dunes at Monahans Sandhills State Park display a variety of dune forms that develop under a unique trimodal wind regime. Large expanses of unvegetated sand form akle dunes having reversing slip faces. Smaller dune forms in the park include wind-shadow, coppice, transverse, barchan, and parabolic dunes. Blowout dunes also occur in the heavily vegetated cover sands of the Pecos Plain surface. Seasonal wind regimes can be divided into three groups: summer, winter, and spring winds. Persistent summer winds from the south-southeast build long transverse dunes. These are modified by erratic gusty winds in the spring (transitional), and winter winds from the north and west during the passage of storm fronts. Dunes shift position as much as 65 ft (20 m) under the influence of strong transitional winds but migrate slowly back to their approximate original location so that net migration over the course of a year is negligible. The Monahans Sandhills have been occupied by people for more than 10,000 yr, as evidenced by several archeological discoveries. Native plants and animals have developed special adaptations in order to survive on the limited resources of this sandy, everchanging environment.