Exodus 15

22. wilderness of Shur--comprehending all the western part of Arabia-Petræa. The desert of Etham was a part of it, extending round the northern portion of the Red Sea, and a considerable distance along its eastern shore; whereas the "wilderness of Shur" (now Sudhr) was the designation of all the desert region of Arabia-Petræa that lay next to Palestine.

23. when they came to Marah, they could not drink of the waters--Following the general route of all travellers southward, between the sea and the tableland of the Tih ("valley of wandering"), Marah is almost universally believed to be what is now called Howarah, in Wady Amarah, about thirty miles from the place where the Israelites landed on the eastern shore of the Red Sea--a distance quite sufficient for their march of three days. There is no other perennial spring in the intermediate space. The water still retains its ancient character, and has a bad name among the Arabs, who seldom allow their camels to partake of it.

25. the Lord showed him a tree, which when he had cast into the waters, the waters were made sweet--Some travellers have pronounced this to be the Elvah of the Arabs--a shrub in form and flower resembling our hawthorn; others, the berries of the Ghurkhud--a bush found growing around all brackish fountains. But neither of these shrubs are known by the natives to possess such natural virtues. It is far more likely that God miraculously endowed some tree with the property of purifying the bitter water--a tree employed as the medium, but the sweetening was not dependent upon the nature or quality of the tree, but the power of God (compare John 9:6 ). And hence the "statute and ordinance" that followed, which would have been singularly inopportune if no miracle had been wrought.
and there he proved them--God now brought the Israelites into circumstances which would put their faith and obedience to the test (compare Genesis 22:1 ).

27. they came to Elim, where were twelve wells of water--supposed to be what is now called Wady-Ghurandel, the most extensive watercourse in the western desert--an oasis, adorned with a great variety of trees, among which the palm is still conspicuous, and fertilized by a copious stream. It is estimated to be a mile in breadth, but stretching out far to the northeast. After the weary travel through the desert, this must have appeared a most delightful encampment from its shade and verdure, as well as from its abundant supply of sweet water for the thirsty multitude. The palm is called "the tree of the desert," as its presence is always a sign of water. The palms in this spot are greatly increased in number, but the wells are diminished.