They go up by the mountains; they go down by the valleys unto the place which thou hast founded for them. They go up by the mountains; they go down by the valleys - Taking the words as they stand here, springs seem to be what are intended. But it is difficult to conceive how the water could ascend, through the fissures of mountains, to their tops, and then come down their sides so as to form rivulets to water the valleys. Most probably all the springs in mountains and hills are formed from waters which fall on their tops in the form of rain, or from clouds that, passing over them, are arrested, and precipitate their contents, which, sinking down, are stopped by some solid strata, till, forcing their way at some aperture at their sides, they form springs and fountains. Possibly, however, vapours and exhalations are understood; these by evaporation ascend to the tops of mountains, where they are condensed and precipitated. Thus the vapours ascend, and then come down to the valleys, forming fountains and rivulets in those places which the providence of God has allotted them; that is, continuous valleys, with such a degree of inclination as determines their waters to run in that direction till they reach another river, or fall into the ocean.
Some have thought there is a reference to the breaking up on the fountains of the great deep, at the time of the flood; while the protrusion of the waters would raise the circumambient crust, so as to form mountains, the other parts, falling in to fill up the vacuum occasioned by the waters which were thrown up from the central abyss, would constitute valleys.
Ovid seems to paraphrase this verse: -
Jussit et extendi campos, subsidere valles,
Fronde tegi sylvas, lapidosos surgere montes.
Met. lib. i., ver. 43.
"He shades the woods, the valleys he restrains
With rocky mountains, and extends the plains."
Dryden.