BRECKNOCKSHIRE AND EAST GLAMORGANSHIRE. 43 



The Old Bed Sandstone erratics have probably travelled from the 

 Beacons, 20 miles distant, and bearing 20 c W. of N. Some of the 

 boulders are composed of the yellowish-grey variety of Old Bed 

 Sandstone. The Mills tone- Grit boulders may have been derived 

 from the precipitous western side of Twynau Gwynion, or Cefn-cil- 

 sannus, in which case they have travelled in a direction correspond- 

 ing with that of the grooves on the adjoining rock-surface. The 

 highest point of the Millstone Grit at Twynau Gwynion, north of 

 Dowlais, 13J miles distant, is 1844 feet. This is the highest point 

 attained by the Millstone Grit in the area embraced in this paper, if 

 not the highest in the eastern part of the South- Welsh Coalfield. 

 There is an outlier, however, of Millstone Grit capping Pen-cerrig- 

 calch, two miles north of Crickhowell, in Brecknockshire, which is 

 over 2000 feet above the sea-level. Pen-cerrig-calch bears 18° E. 

 of N". from the Eglwysilan mountain, and is 20 miles distant. 

 The height above the sea-level of the group of erratics at Eglwy- 

 silan is between 950 and 1050 feet. This gives a fall from Twynau 

 Gwynion of 60 feet per mile, from Cefn-cil-sannus, 12 miles distant, 

 of 23 feet per mile, and from Pen-cerrig-calch of about 60 feet per mile. 



The lie of the longest axes of some of the principal erratics points 

 in the direction of Cefn Cadlan and Gwern-cefn-y-garreg. The 

 latter is 1444 feet high and 17 miles distant, giving a fall of 26 feet 

 per mile. It bears 40° W. of N". from the Eglwysilan mountain. 

 It seems improbable, from the smallness of the fall between the 

 position of these erratics and their parent rocks, that they have been 

 carried to their present resting-place on the surface of a glacier. 

 From the texture of the Millstone-Grit boulders, the author thinks 

 they are not derived from Pen-cerrig-calch. South of the Eglwysilan 

 mountain, these Brecknockshire erratics are of rare occurrence. A 

 small group of them is to be found in the " mixed " district at Caerau 

 three miles west of Cardiff. 



TJie Glamorganshire Erratic district extends north and south 

 from the top of the Bhondda valley, at the northern outcrop of the 

 Coal-measures, to "Welsh St. Donats, near Cowbridge. On the east 

 it touches the western boundary of the Brecknockshire erratic 

 district from Aberdare to Walnut-tree Bridge, and may be divided 

 from the " mixed " district by a line drawn from this last point to 

 St. Nicholas. Westward it probably extends at least as far as the 

 Neath valley. The Glamorganshire erratics in the area of the coal- 

 basin are composed of Cockshot and Carboniferous sandstone, and 

 are to be found chiefly in the Bhondda valleys and the Ely valley 

 north of Llantrisant. The Cockshot boulders are generally angular 

 and are to be found chiefly at heights of from 800 to 1000 feet above 

 the sea-level. They are probably chiefly derived from the mountains 

 on the south side of the Bhondda valley, between Pont-y-pridd and 

 Pcn-y-graig. West of Peterston, in the Ely valley, these erratics of 

 Cockshot and Carboniferous sandstone are associated with boulders 

 of Millstone Grit, Carboniferous Limestone, Old Bed conglomerate, 

 Lias, and Trias, which have all probably been derived from the areas 

 of their respective formations south of the coal-basin. 



