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In a paper published in the Quarterly Journal for 1898 I gave particulars of submerged rock-valleys in South Wales. Further instances of the same sort have since come to light, one of which is sufficiently remarkable to deserve description. It was discovered in building a bridge across the River Towy, at Drysllwyn, 9 miles above Carmarthen (to which place the tide now flows), and about 18 miles from the mouth of the river. The sources of the Towy are some 40 miles from Drysllwyn, on the borders of Cardiganshire and Radnorshire, at about 1700 feet above the sea; but tributary streams descend from the Black Mountain, 2000 to 2600 feet above sea-level, on the south of the Towy Valley, 12 to 16 miles above Drysllwyn.
At the bridge, the valley is narrowed by a detached hill of Lower Silurian rock rising about 180 feet above the river, which stands out into the valley and contracts the width to half a mile. The river now flows at the foot of the hill, which rises at an angle of 40° with the horizon, and is crowned by the ruins of Drysllwyn Castle.
Near the water’s edge on the north of the river, the rock was laid bare, for the foundation of the abutment of the bridge, over an area of 30 by 20 feet. It sloped down gradually to 23 feet below summer water-level at the western or down-stream side, and it was glaciated in large fullows, a foot and more across,
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