HYPOTHÈSES GLACIAIRES

Valley of the Llugwy aboveLlynOgwen, the Drift slill lying intact on roaminilated and grooved surfaces of rock.

The above remarks will prépare the uniliated to appreciate the well-ascertained fact, thatprevious lo the Tertiary glacial epoch, most of the grander contours of hill and valley were in Britain (and elsewhere in Europe) nearly the same as now. Much of the land was Ihen slowly depressed beneath the sea. As it sank, its minor features were somewhat modified, for lerraces wereformed on old sea marginSy and icebergs drifting from the north, and pack-ice on the shores , ground and grated along the coasts and sea botloms, smoo-

Uiing and striating the rocky surfaces over which ihey passed, and depositing, in the course of many âges, clay, gravel, and numerous boulders over wide areas thad had once been land. The groves and striations on the ice-smoothed rocks (except where locally deflected) stiU bear witness to the gênerai southward course of the océan currenta that bore the ice from its birthpiace into milder climates \

AU through Britain and Ireland Drift rises well up on the tlanks of the mountains; and in Caernarvonshire, and'North Wales generally, the surfaces is, over large areas, more or less covered by Irue glacial drift, rising from unterneath the sea to a height of about 2300 feet (700 mètres) on some of the mountains. Near the shore, it bas often been re-arranged and waterworn by the sea at a later period , during émergence and various terrestrial oscillations of level, and it is in this upper division that most of thèse shells etc. , are found, which are usually considered characteristic of the Newer Pliocène deposits. But in the high grounds it is geuerally in its native siate, consisting of clay , angular of stones, gravel, and boulders, sometimes, as in Cwm Llafar, on the west Qank of Carnedd LIewelyn' arranged in lerraces marking pauses in the re-elevation of ihe country. Shells were found by lUr. Trimnier, on Moel Tryfan , 1300 feet (400 mètres ait.) above the sea , in sand and gravel, and others were found by inyself at about the same height, less tlian two miles west of the peak of Snowdon, on a long slope of drift well cbarged with erratic blocks, one of which , of great size, is know as Mam-bras , or the large stone*\

* Near Glan Ogwen or Bethesda.

*'*' I>. A. Illustration: Bloc monstre à l'ouest de Snowdon.

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