Similar broad valleys at the upper courses of many rivers in British Columbia have been referred by Dr Dawson t to long-continued erosion in middle Tertiary time, and it is probable that the same conditions prevailed far to the north ward, producing the broad valleys of the upper Taku tributaries. The deep canyon-like valleys in the lower portion of the river basin represent a part of the erosion due to uplift in late Tertiary and Pleistocene time. The divide between the Taku and Yukon drainage basins is on the edge of an escarpment by which the surface drops from the high plateau 2,600 feet to the level of Ahklenvalley. The altitude of the pass is .5,100 feet, which corresponds very nearly with the average altitude of the interior plateau at this point. The valley is from twelve to twenty miles broad, and on its eastern side is the steep edge of a plateau corresponding to the one on the west and extending eastward to the base of the Cassiar range, forty or fifty miles beyond. Bounded by these approximately parallel plateau escarpments, the valley extends in an almost perfectly straight line for at least 250 miles in a northwest-southeast direction. The upper, that is, the southeastern, half of the valley is occupied by lakes. From one point on the escarpment, affording only a partial view of the valley, fifty-four were counted. Of these lakes, Ahklen* is the northernmost and by far the largest. This lake is ninety-five miles in length and from six to ten in breadth. Several small streams enter the upper end, but its main feeder comes in from, the northeast about midway between the head of the lake and its outlet. This stream, the Nisutlin, enters the head of an inlet about ten miles in length which extends at right angles to the direction of the lake. According to Mark Russell, who has prospected the stream, its current is very sluggish for seventy-five or one hundred miles above the head of the inlet.