Wiktionary
uncrossable
a. Impossible to cross.
Usage examples of "uncrossable".
He began to know where the rim of the Lakht was, just beyond that horizon line, that implacable, uncrossable ridge.
But there were also roads in the district of the Upper Parana, which I myself remember as a wilderness, uncrossed, uncrossable, where tigers roamed about and Indians shot at the rare traveller with poisoned arrows out of a blow-pipe, whilst they remained unseen in the recesses of the woods.
Now that the contact between them had actually been severed, he was coming fully to realize what it meant to be separated from his twin by an enormous and uncrossable gulf of time.
He began to know where the rim of the Lakht was, just beyond that horizon line, that implacable, uncrossable ridge.