Intersecting the mountains are numerous ravines and passes.
The terrain was rough, covered with brush choked ravines and sharp granite bluffs.
Long from the Maranon to the Bolivian frontier, is naturally divided into two sections, the sub-tropical forests in the ravines and on the eastern slopes of the Andes, and the dense tropical forests in the Amazonian plain.
The picturesque effect of this sculpturing by water, wind and fire is greatly enhanced by the brilliant colours along the faces of the hills and ravines - grey, yellow, black and every shade of red and brown.
Cottonwoods flourish along the Little Missouri river, and in sheltered ravines grow stunted junipers and cedars, which seldom rise above the crest of some protecting bluff.