The glacial and post-glacial history of the lower Teifi valley

Abstract

Gorges in the lower Teifi valley formed short cuts towards the end of the Glacial period; former meandering pre-glacial courses were abandoned and are now filled to considerable depths, mainly by boulder-clay. Each gorge must have been eroded by a river flow greatly exceeding that of the present river, even in the highest floods. During the retreat of the ice from south-west Wales and the Irish Sea, the margin of the ice lay near the mouth of the Teifi just below Cardigan and thence probably ran nearly eastward along the northern slopes of the valley. A glacial lake was then impounded in the valley and overflowed by various channels, beginning at a level of 650 ft, at first into Carmarthen Bay and later into the Bristol Channel through Milford Haven. When the ice retreated to the mouth of the Teifi most of the lake then remaining was drained and river flow was resumed. At this time erosion of most of the gorges began. Because the coastline of Cardigan Bay was submerged under ice, an enormous volume of melt-water draining off the ice surface entered the Teifi valley and caused rapid erosion of the gorges. Glacial deposits removed from the wide parts of the valley between the gorges must have materially assisted in their erosion. When the ice retreated so far up Cardigan Bay as to reveal the coastline the flow of melt-water diminished until it ultimately ceased, and the flow of the river became comparable with its present value. Erosion in the gorges has since been insignificant. The progress of erosion in one of the gorges at Henllan is shown by a series of river terraces at successively lower levels between Henllan and Newcastle Emlyn. Because erosion of the gorges virtually ceased while the Irish Sea ice still occupied the northern part of Cardigan Bay, it is evident that this happened long before the end of the Glacial period in southern Britain. The distribution of various deposits formed during the occupation of the valley by ice and the subsequent system of lakes are described. The profile of the river for a distance of about 30 miles from its mouth has been ascertained by approximate levelling. Seismic investigations have revealed the present depth of the pre-glacial rock floor at three points. Near the mouth this is approximately 100 ft below the floor of the present channel.