South of the northern highlands and west of the Indus River plain are the Safed Koh Range along the Afghanistan border and the Sulaiman Range and Kirthar Range, which define the western extent of the province of Sindh and reach almost to the southern coast. The lower reaches are far more arid than those in the north, and they branch into ranges that run generally to the southwest across the province Balochistan. North-south valleys in Balochistan and Sindh have restricted the migration of peoples along the Makran Coast on the Arabian Sea east toward the plains.