Adam Clarke on Psalms 65:12

The pastures of the wilderness - Even the places which are not cultivated have their suffiency of moisture, so as to render them proper places of pasturage for cattle. The terms wilderness and desert, in the Sacred Writings, mean, in general, places not inhabited and uncultivated, though abounding with timber, bushes, and herbage.

The little hills rejoice - Literally, The hills gird themselves with exultation. The metaphor appears to be taken from the frisking of lambs, bounding of kids, and dancing of shepherds and shepherdesses, in the joy-inspiring summer season.