﻿Vol. 66, ,] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 183 



The Glacial accumulations of Ynyscynhaiarn are practically all 

 boulder-clays. These occupy a very considerable area in the 

 northern part of the district, and north of a line running north- 

 eastwards from Criccieth by Eisteddfa and Gesail Gyfarch to Foel- 

 yr-erw, cover and completely conceal the outcrop of the solid rock. 

 The hills of Pen Mynydd-ddu, Bryn Braich-y-saint, Moel Ednyfed, 

 Ymwlch, and Ystum-cegid are exceptions. To the south-east of this 

 line also the constant occurrence of striated pavements, roches 

 m out onnees, travelled rocks perched upon eminences, and drift- 

 pockets under the lee of upstanding crags, makes it certain that 

 the whole country has been intensely glaciated. 



Two types of Boulder Clay may be distinguished. The most 

 •dominant is the Northern Drift, characterized by boulders from 

 €wm Ystradllyn, Moel Hebog, and the Pennant Valley. This 

 eovers the whole of the Drift area above the 300-foot contour, and 

 as far south as Moel Ednyfed. The other type is the Western 

 Drift, containing travelled boulders from the Lleyn(West Carnarvon- 

 shire) and Anglesey, which locally seems to have come in-shore from 

 Cardigan Bay. This Western Drift is well seen in the shore-cliffs 

 both west and east of Criccieth Castle, and its eastward extension 

 forms the ground-rock of the Criccieth 'Xew' Esplanade and 

 the stony promontorj r of Merlljm, half a mile to the east of the 

 town. Travelled rocks of the type seen in situ on the Rivals are 

 found in the grounds of Bron Eifion, and as far up the hill as the 

 reservoir which supplies Bron Eifion with water; but the limit 

 between Northern and Western Drift is not an easy one to define. 



The boulders found scattered over the south-eastern driftless 

 area all belong to rock-types found in the Glaslyn Valley, and as 

 the striations noted also point to a glaciation from north-east to 

 south-west, we may take it that there the glacial sculpture was 

 accomplished by the Glaslyn glacier. Such a glacier with an ice- 

 supply from high up among the mountains of Snowdonia would be 

 more powerful than its neighbour from Cwm Ystradllyn ; and it is 

 possible that while the former was able to carry its debris right 

 out to sea, the latter, checked at Criccieth by the bar of western ice, 

 was compelled to stagnate behind that bar, and so deposited its 

 burden of boulder-clay on the land. The Glaslyn ice must have 

 ■advanced directly across the ' grain ' of the Tremadoc country, 

 and in its passage has sculptured each outstanding scarp from 

 Pen-yr-allt to Moel-y-gest into a roche moutonnee. The glacial 

 plucking and removal of all scree and loose rock on the steep 

 south-westward facing scarps seems also to have been accomplished 

 Tery completely. 



Of post-Glacial phenomena one of the most striking is the very 

 •complete removal of all fine-textured debris from the south-eastern 

 tract of the area here described. Except locally, where covered 

 by scree from a steep upstanding crag, hardly a trace of Drift 

 remains ; and, in many places, the rocks are still so bare that the 

 soil can hardly support the most meagre vegetation. Such scouring 



