Albert Barnes Commentary 1 Samuel 17:3

"And the Philistines stood on the mountain on the one side, and Israel stood on the mountain on the other side: and there was a valley between them." — 1 Samuel 17:3 (ASV)

In the middle of the broad open valley (1 Samuel 17:2) is a deep trench with vertical sides (1 Samuel 17:3), a valley within a valley. The sides and bed of the trench are strewn with water-worn pebbles. (Conder.)