Century after century the same eternal process of obliteration and renovation has proceeded. Millions of tons of silt have been deposited in this great alluvial basin. Levels have changed and the waters have wandered irresponsibly into a network of channels westward. Then the howling, desiccating winds of the north-west have carried back sand-waves and silt, burying villages and filling the atmosphere for hundreds of miles southward with impalpable dust, crossing the Helmund deserts even to the frontier of India. There is no measurable scale for the force of the Seistan winds. They scoop up the sand and sweep clean the surface of the earth, polishing the rounded edges of the ragged walls of the Helmund valley ruins. It is a notable fact that no part of these ruins face the wind. All that is left of palaces and citadels stands "end on" to the north-west. For a few short months in the year the wind is modified, and then there instantly arises the plague of insects which render life a burden to every living thing. And yet Seistan has played a most important part in the history of Asia, and may play an important rôle again. Arab records are very full of Seistan. The earliest of them that give any serious geographical information are the records of Ibn Haukel, but there are certainly indications in his account which engender a suspicion that he never really visited the country. He mentions the capital Zarinje (of which the ruins cover an enormous area to the east of Nasratabad, the present capital) and writes of it as a very large town with five gates, one of which "leads to Bist." There were extensive fortifications, and a bazaar of which he reckons the annual revenue to be 1000 direms.