Albert Barnes Commentary Psalms 78:15

"He clave rocks in the wilderness, And gave them drink abundantly as out of the depths." — Psalms 78:15 (ASV)

He clave the rocks in the wilderness — There were two occasions on which the rock was struck for water: one at Mount Horeb (Exodus 17:6), shortly after they came out of Egypt, and the other (Numbers 20:11), when they had nearly ceased their wanderings in the wilderness. Therefore, the plural term "rocks" is used here.

And gave them drink as out of the great depths — As if He had formed a lake or an ocean, furnishing an inexhaustible supply.