GLACIAL GEOLOGY.
One of the last great episodes in geological history was the advent of great ice sheets from northern lands, invading and overwhelming Northeastern America and Northwestern Europe. Because of its recency (in the Pleistocene peri od), the record of this invasion is clear. It lowered the hills, deepened the valleys, scoured, grooved, and polished the rocks, and transported soil and boulders in its onward march, leaving them in complex deposits when it melted back. These deposits clogged the valleys, turning streams aside and causing them to carve new valleys, which are now gorges with rapids and falls, and by making dams across the streams many lakes were ponded back in the stream val leys. In its advance the ice sheet drove out both animal and plant life, and many interest ing effects on life were produced. A study of these records, and an interpretation of the events which they record, is the province of glacial ge ology.
The time of coming, the length of duration of the ice invasion, and the length of time since' its withdrawal, are not known in years. From 5000 to 30,000 years is the estimated time since the withdrawal of the ice—probably nearer the former than the latter. The duration of the ice
invasion was many times the length of the Post Glacial period, and was great enough for a large amount of work to be performed. The beginning and the end of the Glacial period are included in the Pleistocene, so that even the time of coming is a recent geological event, being post-Tertiary. There is increasing evidence that the Glacial period was complex, consisting of several ice ad vances, with intermediate periods of deglaciation, or interglacial epochs.
Much discussion has arisen on the question of the cause of the Glacial period, without, how ever, arriving at definite results. That the land in the glaciated regions at the beginning of the Glacial period was higher than now is demon strated; and it seems certain that, could the land be once more raised to that elevation, glaciation would again set in. The land is now rising again in the glaciated region, and the query may well be raised: Are we living in an interglacial period ? See GLACIAL PERIOD.