Albert Barnes Commentary Joshua 15:10

"and the border turned about from Baalah westward unto mount Seir, and passed along unto the side of mount Jearim on the north (the same is Chesalon), and went down to Beth-shemesh, and passed along by Timnah;" — Joshua 15:10 (ASV)

Mount Seir is not the well-known range of Edom. The name (“shaggy mountain”) is applicable to any rugged or well-wooded hill. Here it probably denotes the range which runs southwestward from Kirjath-jearim to the Wady Surar. Mount Jearim, that is, “woody mountain,” is through its other name, Chesalon, identified with the modern “Kesla”.

Beth-shemesh – that is, “house of the sun,” called “Ir-shemesh” or “city of the sun” (Joshua 19:41; compare to 1 Kings 4:9), was a place assigned to Dan and one of the cities that fell by lot to the Levites (Joshua 21:16). Beth-shemesh was the first place where the ark rested after its return from the hands of the Philistines (1 Samuel 6:12). It was the residence of one of Solomon’s purveyors (1 Kings 4:9) and was the spot where, at a later date, Amaziah was defeated and slain by Jehoash (2 Kings 14:11 and following). It is no doubt the modern “Ain Shems”.

Timnah, also called Timnath and Timnathah, likewise belonged to Dan and is to be distinguished from other places of the same name (Genesis 38:12; Joshua 24:30). Timnah (“portion”) was evidently, like Gilgal, Ramah, Kirjath, and several other towns, frequently used in Canaanite topography.