The Outer Terai ends at the base of the first range of foothills called the "Siwaliks" or "Churia". This range has a densely forested skirt of coarse alluvium called the "bhabhar". Below the bhabhar, finer, less permeable sediments force groundwater to the surface in a zone of springs and marshes. In Persian, "terai" refers to wet or marshy ground. Before the use of DDT this was dangerously malarial. Nepal's rulers used this for a defensive frontier called the "char kose jhadi" (four "kos" forest, one kos equaling about three kilometers or two miles).