The Pearl of the Andes by Gustave Aimard

At about ten leagues from San Miguel de la Frontera, a miserable town peopled by some twenty or thirty Huiliche shepherds, on the road to Arauca, the land rises rapidly, and suddenly forms an imposing wall of granite, the summit of which is covered with virgin forests of firs and oaks, impenetrable to the sun. A passage of twenty yards at most, is opened by nature through this wall. Its length is more than a mile, forming a crowd of capricious, inextricable windings, which appear constantly to turn back upon themselves. On each side of this formidable defile, the ground, covered with trees and underwood, stage above stage, is capable, in case of need, of offering impregnable intrenchments to those who defend the passage.

This place is named El Canyon del Rio Seco, a name common in America, because not only has vegetation long since covered the face of this wall with an emerald carpet, but it is evident that in remote periods a channel by which the waters of the upper plateaus of the Andes, overflowing, either in consequence of an earthquake or some natural inundation, pour down to the plain -- had violently and naturally cut itself a passage to the sea.