The southern boundary began at the Dead Sea. The broad and desolate valley by which the depressed bed of that sea is protected to the south is call…

And your border shall turn from the south ... — Better, And your border will turn on (or, to) the south side…

And your border
That is, the south border, which is still describing:

Canaan was of small extent; as it is bounded here, it is only about 160 miles in length and about 50 in breadth; yet this was the country promised …