Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/381

 former existence of a narrow sea-strait. Here, then, it appears to me we have undoubted proof that the lines have been formed by the sea, and not by lakes.

A few observations taken from notes made on the spot may render this statement plainer. The highest line is that in Glen Gloy. The valley by which the lake is assumed to have drained into Glen Roy is very narrow and encumbered with detritus from the hills on the sides. The summit-level is flat and marshy, and it appeared to me considerably below the level of the line. On the other hand, a line of stones, as if washed out of the detritus, appeared to show that the sea or loch had extended quite through the strait. I observed no indication of any stream of water larger than the present small rivulet having ever run here. But as the difference in height of this line from the upper line in Glen Roy is not great, and the erosive action of the river might thus have been limited, I shall not mention any other details.

The next pass is that from Glen Roy to the Spey. If a lake ever filled the valley of the Roy to the level of the upper line, a great river, fully equal to the Roy where it now joins the Spean, must have flowed through this pass. In time of floods, when swollen by the western rains and melting snows and glacier-ice, it ought to have left no uncertain mark of its passage over the watershed and down the valley of the Spey ; but I looked in vain for any indication of the former presence of such a mighty stream. It has cut no notch in the ridge or on the sloping declivity to Loch Spey ; it has formed no delta in this lonely tarn. The broad flat strath of the Spey shows only the narrow channel through which the present streamlet winds its way to the sea. No one who has ever studied the effects of running water in such situations could doubt that even in a few days or weeks such a river as the present Roy (and the old river at least could not have been smaller) would leave a groove in the soft alluvial hollow which centuries would fail to obliterate. The rapid running stream must have cut a deeper line on the low haugh than the mere wave-wash of a shut-in mountain-lake on the slope of the hill. Yet the one is distinctly visible for miles round and round; of the other there is not the faintest trace. It is a physical impossibility that the lake should have left such a deep and distinct line, and that the river should have flowed through the gorge and down the valley for the same time without leaving any mark of its presence.

On the other hand there are clear indications of the pass having once been a sea-strait. The bottom is broad and flat ; a notch is cut horizontally along the side of the hill where the water once stood, and a distinct line of stones left where the water has washed away the detritus. In other words, there is an old wave- washed beach. Then a curious series of little rounded bare knolls rise up in the old channel. From a lateral corry below Loch Spey great masses of detritus project into the main valley, but these are spread out and levelled down ; as if thrown into the sea, not as if heaped up in a river-valley. It is scarcely worth mentioning, though im-

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