Late-glacial deposits at Llyn Dwythwch and Nant Ffrancon, Caernarvonshire

• Published in: Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1962
• Subjects: Arctic; Tundra
• Online Access: http://dx.doi.org/10.1098/rstb.1962.0002; https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1962.0002

The stratigraphy of sediments in two lake basins in the Caernarvonshire mountains at 600 and 900 ft. O.D. is described and it is shown that the basal layers were deposited in the Late-glacial Period. The deposits of a third site occupying a kettle-hole in morainic gravels at 1223 ft. O.D. are described and here the Late-glacial Period is not represented. The relation of these sites to the youngest corrie moraines of the district indicates that the latter were formed during the post-Allerod climatic regression (Zone III). The vegetation history of the district was studied by means of pollen analysis of lacustrine deposits from the two first-mentioned sites (above). The vegetation of the Late-glacial Period at first formed tundra (Zone I) in which arctic-alpines, notably Betula , flourished together with species of oceanic and oceanic-northern distribution and calcicolous, eutrophic and moderately thermophilous plants. The spread of juniper scrub preceded the arrival of tree birches, which form ‘park-tundra’ in Zone II at Nant Ffrancon but failed to grow as high as Llyn Dwythwch. The birch ‘ parktundra’ is considered to have covered Britain south of the Forth-Clyde at low altitudes and to have occupied central and eastern Ireland at this time. The climatic deterioration of Zone III is clearly registered by the decline of juniper and tree birch and the local severity of conditions is demonstrated by the increased abundance of the chianophilous fern, Cryptogramma crispa, Lycopodium selago, Saussurea alpina and other montane herbs. The extensive occurrence of solifluxion, augmented by glacier streams, brought down silt and clay into the lakes. The ensuing amelioration in climate and the course of forest development through the Post-glacial Period is briefly traced and the persistence of certain elements of the present mountain flora from the Late-glacial Period demonstrated. A description is given of the spores of Cryptogramma crispa which together with Lycopodium annotinum and Saussurea alpina are new to British ...