At a point which is not far east of the Kabul meridian an offshoot is directed southwards, which becomes the water-parting between the Kurram and the Logar at Shutargardan, and can be traced to a connexion with the great watershed of the frontier dividing the Indus basin from that of the Helmund.
The difference of the general level on the two sides of the water-parting is reflected in the length of their streams. On the west the drainage empties itself into the Atlantic after flowing only a very few miles, on the east it has to run 30 or 40 m.
Bending back westwards upon itself, the line of Afghan frontier now follows the water-parting of the Hindu Kush; and as the Hindu Kush absolutely overhangs the Oxus nearly opposite Ishkashim, it follows that, at this point, Afghanistan is about io m.
For the most part the country is flat, the only mountains being a low range which, rising in the west, runs south-east in an irregular line towards the Chilka lake and forms a water-parting between the district and the valley of the Mahanadi.
Thence the line either follows the latter to its confluence with the Madre de Dios, or the water-parting between that river and the Tambopata or Pando, to the valley of the Madre de Dios, from which point it runs due north to 12° 40' S.