520 



SUBMARINE DRIFTS OF DIFFERENT AGES. 



flattened shingle and fine gravel, derived from the adjacent Cambrian and Silurian 

 rocks, At Caermarthen the river becomes already a navigable estuary, the channel no 

 doubt having been excavated by the river itself through mud and silt of its own depo- 

 sition. On the sides of this modern delta there are, however, hillocks which contain 

 indications of separate ancient operations. One of these mounds, called Goylan-goch, 

 on the left bank of the river, exposes a vertical section of 30 to 40 feet. The upper 

 half is made up of coarse gravel of a brown yellow colour, containing many bowlders 

 of Old Red Sandstone, millstone grit, &c. This mass is neatly separated from an un- 

 derlying accumulation of finely laminated black shingle and coarse grey Cambrian and 

 Silurian gravel, with here and there dividing laminse of fine, light- coloured sand. The 

 arrangement in this hillock, therefore, presents two periods of aqueous agency entirely 

 unconnected jj one anterior to, the other following the elevation of the adjoining coal- 

 field. Such results might be expected from the elevation of the Silurian rocks and lower 

 portion of the Old Red Sandstone, which, it has been shown, gave rise to the most 

 ancient state of the vale of the Towy, whilst the subsequent upheaval of the coal-field 

 produced the detritus which covers the Silurian drift. 



The soft and destructible nature of the mountains, and the absence of decided ridges 

 of elevation in the north-western parts of Caermarthenshire, account for the paucity 

 of large bowlders proceeding from that quarter, which under other circumstances might 

 have found their way into the vale of the Towy. 



Superficial detritus of Pembrokeshire. — The detritus which appears on the surface of 

 most parts of Pembrokeshire, is of a simple character, and, as in other parts of South 

 Wales, is of local origin. It consists of fragments of greenstone, porphyry, Cambrian 

 grits, &c, all of which can be traced to the various mountains forming the crest of the 

 county. In some parts, this detritus is exceedingly coarse, as in the tract west of Nolton 

 Haven, where the blocks of trap are numerous and vary in breadth from less than a foot 

 to several yards. In other tracts, as north of Haverfordwest, we meet with finely com- 

 minuted gravel ; but this is rare. On the whole, though much denuded in some di- 

 stricts, Pembrokeshire is exempt from any traces of those sediments, which in other parts 

 of South Wales may be referred to lacustrine and fluvio -lacustrine accumulation ; a fact 

 which harmonizes remarkably with its physical features. The region is truly one with- 

 out any freshwater streams of sufficient length and width to be worthy of the name of 

 rivers ; and there are no natural depressions of any extent in which surface accumulations 

 could be received. Such, indeed, must be the case in a tract, not only flanked on three 

 sides by the sea, but also fissured by the great chasm of Milford Haven and its tributary 

 creeks, through which the tide now flows by devious branches to the very heart of the 

 county. It is highly probable that a large portion of the coarse detritus, remnants of 

 which still partly encumber the surface of this tract, has been swept down and carried 

 out to sea through these deep channels. We are led to form this opinion, because the 

 lower and southern end of the county consists of undulating hills of slight elevation, 



