Among the many valleys of Daghestan that are interesting to the geologist there are none more remarkable than the channel by which the river Sulak passes through the chain of Cretaceous and Jurassic mountains which borders Inner Daghestan. Just above the entrance of this defile the four rivers Koissu unite in one stream, which in a series of cataracts tears through a stupendous chasm some fifteen miles in length, cutting the huge ridge almost at right angles to its axis.