Inner Terai valleys historically were agriculturally productive but extremely malarial. Some parts were left forested by official decree during the Rana dynasty as a defensive perimeter called "Char Kose Jhadi", meaning 'four kos forest'; one kos equals about . A British observer noted, "Plainsmen and paharis generally die if they sleep in the Terai before November 1 or after June 1." British travelers to Kathmandu went as fast as possible from the border at Raxaul to reach the hills before nightfall.