276 Notes tipon the Diluvial and Alluvial Deposits of the Taffe Galley, 



of the softened, smoothed-off outline of the old red sandstone monntains, 

 of their tabular summits, and of the escarpments and terraces occurring 

 down their sides. Similar appearances occur in the sandstones of the 

 coal, only that the escarpments are steeper (usually bestowing upon the 

 mountain the name of Craig ; as Craig-yr-alt, Craig-yr-Esk, &c.) and 

 loftier. Appearances in some points similar to these, are observed on the 

 escarpments of the carboniferous limestone, as for instance, opposite to 

 Piercefield on the Wye. 



Tlie debris of which we are treating, usually fills up the platforms of the 

 sandstone mountains, lying there pretty thick, and giving nourishment to 

 ferns and timber trees ; or it may be, from the copious springs which gush 

 forth from such mountains, forming a swamp or bog. Between this, with 

 the occasional re-rppcarance of the rock, the rest of the mountain is com- 

 pletely covered up, the thickness of the covering depending upon its struc- 

 ture and steepness. On the upper platforms the debris is not usually 

 so thick as below, where it occasionally equals fifty or sixty feet. This 

 debris is intersected, and its stones washed out by the waters from above, 

 by which large masses are carried down into the main stream below, and 

 such channels are sometimes very deep clefts. 



The fragments in this debris are of variable size and quality, but usually 

 they bear a small proportion to the soil in which they lie imbedded : they 

 have no uniform dip, but lie in the greatest disorder. 



It is evident that this debris has been derived from the mountain upon 

 the breast of which it lies, both from the similarity of their composition, 

 and from the fact that had it been brought by a current from any distance, 

 the rocks would have been more rounded, and the soil have assumed more 

 of the appearance of sand. It would seem that this debris has been pro- 

 duced by the action of time and weather upon the faces of the mountains, 

 probably at a period when those faces were more abrupt, and therefore 

 more obnoxious to sucli agents, than they are at present. 



The lower part of this debris overlies the diluvial gravel. 



Upon the limestone mountains these phfcnomena are much less strikingly 

 apparent. These mountains are very deficient in water, and rarely give 

 rise to springs ; neitther are they so readily acted upon by weather. They 

 are neither so fissile, nor so readily pulverised as the sandstone rocks, and 

 consequently the debris, with which they are sometimes scattered, is com- 

 posed of very small fragments, with little or no intermixture of soil. 



There are observed scattered over the platforms of sandstone mountains 

 — such for instance as the table land between Pont-y-prydd and Pen-y- 

 coed, — a number of huge erratic blocks, derived evidently from the summits 

 of the mountain, and lying in all directions and positions upon the soil. 

 Some lie upon its surface, others are half buried, and of others only a small 

 part is seen. These blocks are rather weather-worn than water-worn j 

 and that they have lain iu their present position a long time, is evident. 



