20 channel across Chase Mountain, to Tomias Lake Valley1. The channel was lowered as the ice retreated, and by the time the ice-dam in the old outlet of Swannell Valley had disappeared, the river had, apparently, been established in its present anomalous course, turning abruptly from the relatively broad through valley to the narrow rocky channel north of Chase Mountain, and again at right angles down the Tomias Lake through valley. At some time an ice-dammed lake occupied the through valley section of Swannell Valley, leaving terraces and delta deposits perched on the mountain slopes as much as 300 feet above the valley floor. A subsidiary effect of the retreat of the last large valley glaciers was the diversion of the streams on the massif northeast of Blackpine Lake. The glaciers in the northwest trending valleys of these streams, which previously had drained to Swannell River, apparently retreated to small cirque glaciers while stagnant ice still covered the broad Swannell-Mesilinka pass and blocked the lower ends of the valleys. The small streams in these blocked valleys overrode the southwest walls of their respective valleys, and cut channels that have now developed into deep, narrow canyons, by which the streams turn at an acute angle and flow directly to Mesilinka River (See Map 1030A). OSILINKA RIVER The upper branches of Osilinka River occupy broad valleys that unite to form a large southeast-trending trough near the south border of the map-area. The trough continues beyond the map-area, as the valley of what is now Discovery Creek, to Omineca River Valley. A mile below the mouth of Steele Creek, however, Osilinka River abruptly leaves this trough and flows northward through Uslika Lake and a narrow, tortuous, V-shaped valley north of it, to the mouth of Tenakihi Creek. There the river enters another broad, southeast-trending trough, which it follows beyond the limits of the map-area. The two large southeast-trending troughs are further connected by the broad, glaciated valley of Wasi Lake. A fragmentary terrace, about 100 feet above present river level, is present on both sides of Osilinka River Valley between the crossing of the Aiken Lake winter road and Uslika Lake. The terrace is in part followed by the winter road, and near the 37 mile-post it is seen to be a mixture of silts and gravels, capped by about 8 feet of till. On the west flank of Conglomerate Mountain, stream gullies cutting through the terrace show it to be mainly crossbedded silt and sand, overlain by a thin layer of till. The terrace appears to be the remnants of the shoreline, marked largely by deltas of small tributary streams, of a lake that filled Uslika Lake Valley to a level about 100 feet higher than the present lake. It would appear that in pre-Glacial times the upper section of Osilinka River drained directly to Omineca River through the valley of what is now Discovery Creek. This valley seems to have had a tributary in what is 1 The valley occupied by Tomias Lake, and the part of the present Swannell River that is an extension of Ravenal Creek Valley, appears to have been originally occupied by Mesilinka River. It is probable that Mesilinka River was originally a major tributary of Ingenika River, and that in pre- Glacial times it was captured, at a point about 3 miles east of the map-area, by a tributary of Omineca River eroding northwest along the west side of Butler Range. At the elbow of capture, Mesilinka River changes its course by 120 degrees. Tomias Lake and Carina Lake (east of the map-area), joined by a small ‘misfit’ stream, lie in the abandoned valley.