The County River has carved its way through the local bedrock over millennia, leaving this natural arch, roughly one and a half metres thick and crossing a drop of two to three metres, as the only straightforward passage across its course for a considerable stretch of country.

The geographical significance of this crossing is easy to underestimate. Between Lough Melvin, roughly five and a half kilometres to the north-north-west, and Lough Macnean, around six kilometres to the south-east, this limestone span represents the sole easy natural crossing-point along that entire corridor. In a landscape where lakes and river valleys dictated movement, such a feature would have quietly shaped the paths taken by people, livestock, and armies for centuries. Limestone, being a soluble rock, is particularly prone to the kind of dissolution and undercutting that produces natural bridges and caverns, and the border country of Leitrim and Fermanagh sits atop exactly the sort of carboniferous limestone that makes such formations possible.