To the north-west these hills form the watershed between the valleys of the Arghandab and the Tarnak, until they are lost in the mountain masses of the Hazarajat - a wild region inhabited by tribes of Tatar origin, which effectually shuts off Kandahar from communication with the north.
Other routes there are, open to trade, between Herat and northern India, either following the banks of the Hari Rud, or, more circuitously, through the valley of the Helmund to Kabul; or the line of hills between the Arghandab and the Tarnak may be crossed close to Kalat-i-Ghilzai; but of the two former it may be said that they are not ways open to the passage of Afghan armies owing to the hereditary hostility existing between the Aeimak and Hazara tribes and the Afghans generally, while the latter is not beyond striking distance from Kandahar.
To the north-west, and parallel to the long ridges of the Tarnak watershed, stretches the great road to Kabul, traversed by Nott in 1842, and by Stewart and subsequently by Roberts in 1880.
It is doubtful whether the ancient Arachotus is to be identified with the Arghandab or with its chief confluent the Tarnak, which joins it on the left about 30 m.
The Tarnak is much the shorter (length about 200 m.) and less copious.
The ruins at Ulan Rob At, supposed to represent the city Arachosia, are in its basin; and the lake known as Ab-i-Istada, the most probable representative of Lake Arachotus, is near the head of the Tarnak, though not communicating with it.
The Tarnak is dammed for irrigation at intervals, and in the hot season almost exhausted.
The high road from Kabul to Kandahar passes this way (another reason for supposing the Tarnak to be Arachotus), and the people live off the road to avoid the onerous duties of hospitality.
The long, straight, level-backed ridges which divide the Argandab, the Tarnak and Arghastan valleys, and flank the route from Kandahar to Ghazni, determining the direction of that route, are outliers of this system, which geographically includes the Khojak, or Kwaja Amran, range in Baluchistan.
In the valley of the Tarnak are the ruins of a great city (Ulan Robat) supposed to be the ancient Arachosia.