Al-Anbiya 21:31
Two parallel clauses.
وَجَعَلْنَا فِى ٱلْأَرْضِ رَوَٰسِىَ أَن تَمِيدَ بِهِمْ وَجَعَلْنَا فِيهَا فِجَاجًا سُبُلًا لَّعَلَّهُمْ يَهْتَدُونَ
And We placed on earth stabilizers, lest it sways with them, and We placed therein signposts and passages, that they may be guided
Al-Anbiya 21:31 · Translation: talal-itani
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Onto the earth "rawāsī" (firm, solid mountains) were set — "an tamīda bihim" (so it would not sway under them). Onto the same earth "fijājan subulan" (wide, open paths) were placed — "laʿallahum yahtadūn" (so they may find their way). Mass and passage, on the same ground, side by side.
Observation
In geology, mountains are structures bearing deep roots (isostatic equilibrium) that extend beneath the continental plates; these roots contribute to ground stability by balancing the weight of the crust against the mantle. Mountain ranges also shape natural passes and valley routes; it is among the established observations of historical geography that migration and trade routes have followed these openings.
Exegesis
Exegetes read the two ends of the verse together: on one side a firm mass that prevents sway, on the other an opening that makes wayfinding possible. Al-Ṭabarī conveys that "an tamīda bihim" points to the earth not rocking beneath people and leaving them unsteady; in the classical tafsīr tradition this "sway" is read as humans being unable to find a ground on which to live. Al-Bayḍāwī notes that "fijājan subulan" embraces the natural routes that pass between, over and around the mountains — that historical migration and trade roads most often follow these openings is the side of the verse's landscape language that touches human experience.
Sources
Tafsīr: al-Ṭabarī · Jāmiʿ al-Bayān; al-Bayḍāwī · Anwār al-Tanzīl.