9 Hear. “You have set a bound which they shall not pass over, neither shall they turn again to cover the earth”. What then, because now the bitterest waves have received a measure, that we must be allowed to preach such things even with freedom; because they have had their due limit assigned, because they cannot pass over the bound that is set, nor shall they return to cover the earth; what is doing in the earth itself? What workings take place therein, now that the sea has left it bare?
Although at its beach slight waves do make their noise, although Pagans still murmur round; the sound of the shores I hear, a deluge I dread not. What then; what is doing in the earth? “Who sends out springs in the little valleys”. “You send out,” he says, “springs in the little valleys.” You know what little valleys are, lower places among the lands. For to hills and mountains, valleys and little valleys are opposed in contrary shape. Hills and mountains are swellings of the land: but valleys and little valleys, lownesses of the lands.
Do not despise low places, thence flow springs. “You send out springs in the little valleys.” Hear a mountain. The Apostle says, “I laboured more than they all.” A certain greatness is brought before us: yet immediately, that the waters may flow, he has made himself a valley: “Yet not I, but the grace of God with me.” It is no contradiction that they who are mountains be also valleys: for as they are called mountains because of their spiritual greatness, so also valleys because of the humility of their spirit. “Not I,” he says, “but the grace of God with me.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)