It is, however, sometimes difficult to reconcile Idrisi's account of the routes followed by his countrymen in this part of Asia with existing geographical features. Deserts and mountains must have been much the same as they are now, and the best, if not the only, way to unravel the geographical tangle is to take his itinerary and see where it leads us. Of Baghnein on the southern borders of Seistan, he says it is an "agreeable country, fertile and abundant in fruits." From there (_i.e_. the country, not the town) to Derthel one reckons one day's journey through the nomad tribes of Bechinks, Derthel being "situated on the banks of the Helmund and one of the chief towns of Dawar."
So we have to cross an open uncultivated region for 40 miles or so from Baghnein to reach Derthel, on the Helmund. Again, "one crosses the Helmund at Derthel to reach Sarwan--a town situated about one day's journey off," on which depends a territory which produces everything in abundance. "Sarwan is bigger than Fars, and more rich in fruit and all sorts of productions. Grapes are transported to Bost (or Bist), a town two days distant passing by Firozand, which possesses a big market, and is on the traveller's right as he travels to Benjawai, which is _vis-à-vis_ to Derthel." "Rudhan (?Rudbar) is a small town south of the Helmund."
The Helmund valley has been surveyed from Zamindawar to its final exit into the Seistan lagoons, and we know that at Girishk there is a very ancient ford, which now marks, and has always marked, the great highway from Kandahar to Herat. South of Girishk, at the junction of the Arghandab with the Helmund, we find extensive and ancient ruins at Kala Bist; and south of that again there are many ruins at intervals in the Helmund valley; but these latter are comparatively recent, dating from the time of the Kaiani Maliks of the eighteenth century.
Assuming that the Helmund fords have remained constant, and placing Derthel on one side of the river at Girishk and Benjawai on the other, we find on our modern maps that from the ford it is a possible day's journey to Kala Sarwan, higher up the Helmund, where "fruit and grapes are to be had in abundance," and from whence they might certainly have been sent to Bist, where grapes do not grow. Baghnein, separated from Derthel by a strip of nomad country, one day's journey wide, might thus be on either side the Helmund; but its contiguity to Ghur seems to favour a position to the west, rather than to the east, of the river, somewhere east of the plains of Bukwa about Washir.