COURSE OF DRIFT ALTERS WITH CHANGE OF STRIKE. 



519 



rivers Honddhu. These streams, therefore, like the Teme, the Onny, and the Lug, flow 

 in the same direction as the coarse drift ; but as the latter encumbers the whole talus 

 of the elevated mass, being lodged in upland combs on sides of hills, quite remote from 

 the river channels, it is clear, that it never could have been distributed by existing 

 fluviatile action. 



From the hydrographical basin of the Severn and its tributaries, we now turn to that 

 of Caermarthenshire and Pembrokeshire. The Towy, or chief river of this tract, rises in 

 the Cambrian or slaty rocks, and runs from N.N.W. to S.S.E., or transversely through 

 the strata until it reaches Llandovery, where it is deflected to the south-west. At 

 Llandeilo it is again bent still more towards the west until it reaches Caermarthen ; 

 thence it finally takes a direction to the S.S.E. and empties itself into the sea. From 

 Llandovery to Caermarthen, this river flows in a wide longitudinal denudation ; but be- 

 tween Caermarthen and the sea it passes through a transverse fissure in the junction 

 beds of the Silurian rocks and Old Red Sandstone. (See p. 350.) The westerly course 

 of this river, from Llandeilo to Caermarthen, has been caused by those convulsions 

 which threw the strata out of their original direction, and gave to them the westerly 

 strike of the Glamorganshire and Pembrokeshire coal-fields. Such ruptures, doubt- 

 lessly, caused those denudations by which the valley has been excavated, and in which 

 the Towy now holds its course. In the upper tracts watered by this river, it passes 

 through fissures transverse to the Cambrian strata, and, therefore, all the detritus has 

 been derived from them. Much of that which has been spread, in the form of low 

 coarse shingle, over the wide river plain below the town of Llandovery, may have been 

 deposited within the historical period, and will be alluded to hereafter. 



There is no part of South Wales in which the difference between ancient coarse 

 drift, which we presume to have been submarine, and that, which we know has been 

 formed under the atmosphere, is more clearly marked than in this part of Caermarthen- 

 shire. Let the observer quit the low modern river shingle of the vale of Towy, 

 between Llandovery and Caermarthen, and approach the elevated border of the coal-field. 

 Before he has passed the parallel ridges of Silurian rock which separate the vale of 

 Towy from the Old Red Sandstone, he finds large bowlders lodged at high levels, in 

 hollows between ridges of Silurian rock, and in positions where no streams can have 

 flowed, since the surface of the land assumed its existing outline. Here we see the 

 effects due to those great movements by which the Caermarthen Fans were thrown up, 

 and which, doubtless, produced currents adequate to the transport of blocks to a dL 

 stance of several miles in directions eccentric from the lip of the coal basin, and opposed 

 to the prevailing line of drainage. By following the edge of this basin to the sea, 

 between Kidwelly and Caermarthen, we find that as the hills diminish in height, the 

 transported materials diminish in quantity and size, the lower ridges having thrown off 

 little or no coarse detritus. In that part of the course of the Towy which lies beyond 

 the influence of these elevations, the river has never transported any debris except 



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