C. F. Keil and Franz Delitzsch on Joshua 15:2-4

Jos 15:2-4 The southern boundary. This was also the southern boundary of the land of Israel generally, and coincided with the southern boundary of Canaan as described in Num 34:3-5. It went out “from the end of the salt sea, namely, from the tongue which turneth to the south,” i.e., from the southern point of the Dead Sea, which is now a salt marsh.