At a point roughly halfway along its length, the river Teiglin ran past the eaves of the Forest of Brethil, before it entered the southern parts of the forest and flowed on to meet Sirion. In this part of its course, the river cut a deep chasm with steep sides. The length of this ravine is unclear, but it ran for at least two miles² past the Crossings of Teiglin. It was also extraordinarily narrow; at one point, known as Cabed-en-Aras, it was said to be narrow enough for a deer to leap from one side to the other.

The Ravines were important to the story of Túrin Turambar, for it was here that he slew the Dragon Glaurung. Making the treacherous climb down one side of the ravine and up the other, he was able to conceal himself beneath the lip as the great Dragon made his crossing. With a thrust of his sword Gurthang, Túrin gave the Dragon a mortal wound. Before his death, however, Glaurung revealed to Túrin's wife Níniel that she was in fact his sister Niënor, and in her despair she leapt to her death from the brink of the Ravines of Teiglin.