גְּֽבוּל־שַׂ֭מְתָּ בַּל־יַֽעֲבֹר֑וּן בַּל־יְ֝שׁוּב֗וּן לְכַסּ֥וֹת הָאָֽרֶץ׃
Thou didst set a bound which they should not pass over, That they might not return to cover the earth.
Rashi on Psalms
You set a boundary for them, viz. the sand that is around its shore.