POST-GLACIAL DEPOSITS AT TARN WADLING, CUMBERLAND

Size: px

Start display at page:

Download "POST-GLACIAL DEPOSITS AT TARN WADLING, CUMBERLAND"

Shannon Briggs
5 years ago
Views:

1 POST-GLACIAL DEPOSITS AT TARN WADLING, CUMBERLAND BY D. WALKER University Sub-department of Quaternary Research, Cambridge, and Research School of Pacific Studies, The Australian National University, Canberra [Received 19 December 1963) SUMMARY The topographic setting and stratigraphy of shallow lake deposits at Tarn Wadling are described. A pollen diagram through the muds indicates their very recent age. GEOGRAPHY Tarn Wadling is a saucer-shaped depression, about 600 yd (0.5 km) in diameter, immediately east of the village of High Hesket in Cumberland and i mile (2 km) west of the River Eden (Nat. Grid. Ref ). The hollov^f lies in the sandy boulder clay of the Main Glaciation of the district (Hollingworth, 1931), the edge at about 410 ft (124 m) O.D. To the south-east the drift-smeared slopes of Blaze Fell rise steeply to 792 ft (240 m) O.D. but northward lies an undulating area of boulder clay at about 450 ft {136 m) O.D. Eastward, beyond the narrow rim of boulder clay, the ground falls to the River Eden at about 180 ft (55 m) O.D. Westwards, drumlins rising to between 450 ft (136 m) and 500 ft (152 m) O.D. lie between Tarn Wadling and the River Petteril. The site of Tarn Wadling was vacated by the ice by retreat stage H of Hollingworth (1931) but at that time melt-water probably poured through the region from west to east. With the opening of drainage down the Petteril valley (stage LM) the Tarn Wadling hollow would contain an isolated lake. During the Scottish Readvance Glaciation, the ice front of which lay 2 miles (3 km) north-west of the site, Tarn Wadling was probably temporarily incorporated in the large pro-glacial lake which stood at 440 ft (133 m) O.D. Tarn Wadling has been identified with the Tern Wathelyne of Arthurian legend (Armstrong et al., 1952) and was doubtless a lake over a very long period. It probably drained through the low col and trough at its south-western end, a course still marked by the parish boundary. Many attempts have been made to drain the basin but, although some of these might have been temporarily successful, the present dry state was achieved as recently as the Second World War. The bed of the tarn is now overgrown by Jmcus effusus and a well-marked break in slope around its edge probably indicates the former shore-line. The wooded Crane Moss at the north-western end is a small bog lying above the old shore-line. In a similar position at the southern end of the basin lies a narrow band of fen deposits. HISTORY The borings only reached the boulder clay, which presumably forms the base of the depression, around its edges. Compact red sand fills all but the upper few metres of the hollow. This is overlaid by grey-brown silty mud, a deposit of open water in which small 232

2 Post-glacial deposits 233 sedge fragments are included. The virtual overgrowth of this lake by reedswamp is evidenced by the superposed layer of dark brown, coarse detritus mud rich in grass and sedge remains. This built up to a considerable thickness on the western shore where a fen probably developed. This normal hydroseral process of moderately stable water level was arrested by a rise in the water, which resulted in the deposition of a characteristic dark 5.W, SECTION A N.E. SECTION C I I I I r I I I I I I 300 n r I I I I 500 m 80 m OnSE. SECTION B Pollen samples N.W. \ I I I I I I I I I [ I I I I I I I I I_L AOO m HH Coarse sand I Coarse detritus mud j Grey sllty mud Laminated mud Peat 1 Disturbed surface ' deposits Fig. I. Stratigraphy of the Post-glacial lake deposits at Tarn Wadling, Cumberland, as reconstructed from thirty-eight borings along the section lines shown. brown, laminated organic mud over all the older deposits. Subsequently, another fen grew up over the old beach fiat at the southern edge of the hollow but it is generally the disturbed and oxidized, laminated mud which forms the surface soil today. It is evident that marginal fens were only able to develop on the western and southern margins of the hollow in the past, probably because of the inhibition of plant growth by waves, caused by the prevailing winds, lapping against the farther shore. A pollen diagram was constructed from the analysis of samples taken at the point shown in Fig. i. As might have been anticipated in such a shallow deposit, the pollen of local plants so dominated the pollen rain as to make impossible any detailed correlation between this diagram (Fig. 2) and the general vegetation history of the Cumberland Lowland documented from other sites (Walker, 1964). The presence oi Alnus, however, certainly indicates a post-boreal age for the deposits, whilst the low Ulmus values and the high content of weeds such as Artemisia, Plantago and Rumex suggest a date later than 3000 B.C. by analogy with the findings at Scaleby Moss (Godwin, Walker and Willis, 1956). Indeed, the relatively high frequency of Pinus pollen leads to the supposition that

3 D. WALKER 234 all these muds may post-date the planting of pine in the region m recent historical times. The composition of the vegetation in and around the hollow is well documented in the pollen diagram. Birch trees were evidently very common and probably formed open woodlands with an undergrowth of Galluna on the poor sandy soils, particularly on the slopes of Blaze Fell. Hazel thickets probably occupied the better soils whilst the damp u. Fig. 2. Pollen diagram from the Post-glacial lake deposits at Tarn Wadling, Cumberland. The frequencies of individual pollen types are shown as percentages of total tree pollen, excluding Corylus and Salix. Note changes of horizontal scales. hollows harboured alder and willows. Elm and oak were rare in the immediate vicinity, and much of the land was generally bare of trees. The high values for grass and sedge pollen belong to the period when reedswamp was encroaching from the margins of the lake and the falls in their curves are thought to be the result of the rise in water level which culminated in the deposition of the laminated mud. The success of Isoetes lacustris, probably in newly flooded marginal strands, at this stage is notable. It is tempting to suppose that the rising frequencies of Quercus and Alnus at the top of the diagram denote natural or induced afforestation of previously cleared land but the confusion of these levels with stratigraphic changes makes such an interpretation dangerous. ACKNOWLEDGMENTS The author is grateful to Mr. E. Blezard and Dr. D. A. RatclifFe who first brought this site to his attention.

4 Post-glacial deposits 235 REFERENCES ARMSTRONG, A. M., MAWER, A., STENTON, E. M. & DICKINS, B. (1952). The Place-Names of Cumberland Part III: Introduction. English Place Name Society, Publ., 22, Cambridge. GODWIN, H., WALKER, D. & WILLIS, E. H. (1956). Radiocarbon dating and Post-glacial history; Scaleby Moss. Proc. roy. Soc, B147, 352. HOLLINGWORTH, S. E. (1931). The glaciation of Western Edenside. Quart. J. (teol. Soc. Lond., 87, 281. WALKER, D. (1964). The Late Quaternary history of the Cumberland lowland! (In press).

LATE-GLACIAL DEPOSITS AT LUNDS, YORKSHIRE

[ 343 ] LATE-GLACIAL DEPOSITS AT LUNDS, YORKSHIRE BY D. WALKER University Sub-department of Quaternary Research, Botany School, University of Cambridge {Received 3 October 954) (With 3 figures in the text)