544 P^'of. W. Keeping — Geology of Aberystwyth. 



Amongst these, too, Mid Wales has its own physical characteristics, 

 which distinguish it from other Cambrian districts. We miss 

 especially the high precipitous escarpments with their scree slo^Des 

 below, and the sharp peaks of the more northern Cambrian areas, 

 and the scenery is altogether less rugged than those. This is suffi- 

 ciently explained by the homogeneous character of the Mid Wales 

 rocks, the rare occurrence of thick deposits of hard grit and 

 quartzite, and the absence of igneous rocks. It is not till we reach 

 Plynlimmon, and again go still further east to the neighbourhood 

 of Khyader, that we find more massive grit-beds and conglomerates, 

 and here the bolder features of steep rugged hill-sides and preci- 

 pitous escarpments are found. 



We have already, when dealing with the Glacial period, referred 

 to that moulding of the surface features by the action of ice which 

 has so profoundly modified the character of our scenery, but we may 

 here dwell longer upon those more angular valleys which have been 

 produced by the action of streams since the ice disappeared. Such 

 valleys are, as a rule, best developed in the neighbourhood of water- 

 falls, for there the cutting power of water is at its maximum. At 

 the Devil's Bridge, Pont Erwyd, and below Llyn Rhyddnant, 

 especially, deep chasms have been cut out by the long-continued 

 action of the water as it rushes down and hurls the stones, large 

 and small, like cannon-balls and gun-shot against the sides and 

 bottom of its bed. The characteristic water-cut sides are well seen 

 in the " Devil's Punch Bowl " of the Devil's Bridge, and the same 

 appearances should be noticed all up the sides of the chasm. They 

 are well seen lower down on the left bank of the waterfall above 

 the iron foot-bridge ; thus presenting to us the most convincing 

 proof that the gorge has been cut out by the water itself, and is not 

 a gaping fissure produced by violent earth movements. 



The Caradoc waterfall, between Traws Coed and Strata Florida, 

 is remarkable in that the slope of the fall corresponds with a sudden 

 fold in the rock -beds themselves, — a coincidence which is most 

 unusual. 



Those cylindrical holes known as " Pot-holes " are well seen 

 along the Eheidol in many places, as at the Parson's Bridge, Devil's 

 Bridge, and higher up around Pont Erwyd. These are good illus- 

 trations of that mode of working by water with the aid of hard 

 stones which we have already noticed. At the bottom of the Pot- 

 holes some stones are sure to be found, and it will at once be evident 

 that as the current of water flows over the hole, a spiral motion is 

 set up in the water of the Pot-hole, so that the stones are carried 

 round and round the bottom of the hole, always grinding their way 

 deeper and deeper down. It is interesting to notice that in this way 

 water may carve out great hollows below its level of outflow. The 

 holes vary much in size ; some are no bigger than beer tumblers, in 

 others one might hide oneself. 



The Plane of Marine Denudation. — Whoever has gone to the top 

 of the higher mountains of England, Scotland, Ireland, Germanj', 

 Switzerland, etc., must have noticed that the great spread of country 



