Trof, W. Keeping — Geology of Aherysticyth. 545 



around him looks much more regular than he would have expected 

 from his previous knowledge of the lower grounds. He may have 

 considered the whole country mountainous, and quite irregular ; but 

 now, from his elevated position, he is surprised to find how very 

 level and regular it looks. Now he sees all around a great plain, in 

 which the valleys appear as huge furrows ploughed out. The same 

 feature, viz. the regular height of the summits of most of the hills, 

 and their comparative evenness, may also sometimes be observed as 

 you look at a mountain system from a distance, when the tops of a 

 number of mountains are seen to be so many points in the same 

 straight line. Such a plane is known as a " Plane of Marine 

 Denudation " ; and it may be well seen from the top of Aran. 

 Mowddwy, Cader Idris, Snowdon, Plynlimmon, or nearer home 

 from the top of the hill above Cefn Hendre, Aberystwyth. The 

 height of this plane in Cardiganshire is from 500 feet to about 750 

 feet, gradually sloping upwards to the Plynlimmon mountains. 



This plane of marine denudation tells us of a period, far removed 

 from our age, where the sea gradually cut inland its level shores and 

 sea-bottom, levelling all down and leaving only the higher mountains, 

 as Plynlimmon, Cader Idris, etc., as isolated islands. Just in the 

 same way the sea is now cutting down another plane; it is slowly 

 eating away Constitution Hill, and every other hill along the coast, 

 and reducing all to one level — a modern plane of marine denudation. 



Lakes and their Modes of Origin. — Another feature which Mid 

 Wales has in common with most mountain districts is its numerous 

 lakes. A number of these may be visited from Aberystwyth, though 

 most of them are not easily accessible, for they are generally situated, 

 not in the valleys, but upon the summit plains of the hills. They 

 are depressions in the great plane of marine denudation. Such are 

 Llyn Eiddwyn, L. Gynon L. Fyrddyn, L. Gron, L. Egnant, L. y 

 Gorlan, L. Hir, and Llyn Teifi, etc. The five latter are the best 

 known under the name of Teifi Pools, and they will serve well as 

 illustrative examples of the most common type of lake in Mid Wales. 



Far up in the hills we find these irregular bodies of water with their 

 walls all formed of solid rock. They generally run more or less 

 along the strike of the rock-bed, their sides are not lofty (rarely so 

 much as 100 ft.), and in many cases at least their depth is not great. ^ ^ 



There are three principal ways by which lakes may be formed, 



(1) by the action of ice scooping out a hollow in the solid rock; 



(2) the formation of a dam across a valley, as by the moraine matter 

 piled up at the end of a glacier; and (3) by earth movements, 

 especially depressions. 



The Teifi Pools, together with the great majority of our Mid 

 Wales lakes, belong to the third class. Here we have not 



" A lofty precipice in front, 

 A silent tarn below," 



1 The depths of many of the lakes are not yet known to me. 



^ The visitor must take care to distinguish the natural lakes fi'om the artificial 

 reservoirs of water made by the lead-workers to serve for a steady supply of water 

 to their water-wheels. 



DECADE II. — VOL. V. — NO. XII. 35 



