49:4 As the minds of men continually vacillate, because they do not sufficiently consider the infinite power of God, the Prophet, that he might remove all obstacles which might have rendered his prophecy doubtful, now declares that the Ammonites gloried in vain in their valleys. Some understand by valleys a fertile land, well watered. But the Prophet, as I think, refers rather to fortified places. He then says, that they in vain gloried in their deep valleys; as they were surrounded with mountains, so they thought that they could not be approached. He derides this vain confidence, Why, he says, dost thou glory in thy valleys, or, profundities? Flown down has thy valley. By saying, that the valley, or depth, had flown down, he alludes to its situation: for when any one considers a region situated among mountains, the land appears as flowing, like a river gliding between its banks. It is then a striking allusion to a deep place, when he says that the valley flowed down. It was the same as though he had said, “Thy depth has vanished,” or, “It shall not be to thee such a protection as thou thinkest.” But the meaning is, that though the Ammonites, confiding in their defences, disregarded all attacks of enemies, they would yet be exposed to plunder; for their mountains and valleys would avail them nothing, notwithstanding the opinion they entertained, that they were so fortified, that they could not be assailed.
He calls Ammon a rebellious, or a backsliding daughter, though he mentions no particulars. But Ezekiel and also Amos and Zephaniah, these three, clearly shew why God was so severe towards the Ammonites, (Ezekiel 25; Amos 1:13; Zeph. 2:9;) it was because they had uttered blasphemies against Him and his people, exulted over the miseries and calamities of the chosen people, and plundered them when they saw them overcome by their enemies. For these reasons, then, our Prophet now calls them a rebellious people: they had proudly exalted themselves against God, and exercised cruel tyranny as to the miserable Israelites, who were yet, as it has been stated, connected with them by blood.
Who trusts in her secrecies, or hidden places: rendered by some, “in her treasures.” But as אצר, atser, means to hide, the reference is, as I think, to strongholds; for the Prophet in the next words explains himself, Who can come to me? It appears, then, that the Ammonites thought themselves thus secure, because they were not exposed to their enemies, but protected by their mountains, as though they were in hiding places. This boasting sufficiently shews that they did not so much trust in their treasures as in their hidden places, because they dwelt in recesses. The meaning is, that though the Ammonites gloried that they were beyond the reach of danger, yet God would become the avenger of the cruelty which they had exercised towards their relations, the Israelites.
 Calvin, J., & Owen, J. (2010). Commentaries on the Prophet Jeremiah and the Lamentations (Vol. 5, pp. 60–62). Bellingham, WA: Logos Bible Software.