Vol. 65. ] OrACIA~ XROSlON I~ ~0RZ~r WAL~S. 281 1S. GLACIAL EI~OSlON in NORT~r WALES. By Prof. WILLIA~ M01iRIS DAWS, For. Corr. G.S. (Read March 24th, 1909.) [PLATE XIV--MAP.] CONTENTS. Page I. The Form of Snowdon ................................................281 II. Excursions around Snowdon ..........................................282 III. Welsh Terms, Local P]ace-~'ames, and Illustrations ............ 283 IV. Geological Structure of the Snowdon District ..................... 284 V. Indifference of Form to Structure ....................................285 Vl. General Erosion of the Snowdon District ...........................286 VII. l~amsay's ' Plain of Denudation '....................................... 288 Vlll. Date and Altitude of Ramsay's Plain .................................290 IX. Dissection of the Uplifted Plain .......................................291 X. Pre-Glacial Form of Snowdon .......................................292 Xl. Subdued ~ountains of Normal Erosion ..........................."293 Xll. Texture of Dissection ...................................................295 XIII. Interdependence of Parts ................................................295 XIV. Abnormal Forms of the Snowdon District ...........................296 XV. &bnormal Forms and ~ormal Processes ..............................300 XVI. Association of Abnormal Features with Glaciation ............... 303 XVlI. Various Methods of Discussing Glacial Erosion .................. 304 XVIII. Glaciers as Protective Agencies .......................................308 XIX. Special Features of the Theory of Glacial Protection ............ 311 XX. Glaciers as Destructive Agencies ....................................313 XXI. Special Features of the Theory of Glacial Erosion ............... 318 XXII. A Personal Section ...................................................... 320 XXIII. Confrontation of the Deduced Consequences of the Two Theories with the Facts of Observation in the Snowdon District .................................................................. 322 Valley-Heads : Cwms ............................................. 322 Valley-Floors : Lakes ............................................. 329 Valley-Floors: l~ock-Steps ....................................... 330 Valley-Sides ......................................................... 333 Hanging Lateral Valleys ......................................... 339 Glacial Overflows ................................................... 341 XXIV. tIistorieal Note ............................................................ 342 XXV. Closing Suggestions ...................................................... 343 XXVI. Bibliography ............................................................... 343 ' Few subjects in physical geology would possess greater intereat than a complete account of the denudations by which, after the disturbance of the strata, Wales assumed its present form.'--RA.~isxY, Mem. Geol. Surv. Great Britain, vol. iii (1866) p. 238 ; or, with verbal modifications, 2nd ed. (1881) p. 330. I. THE FoR~ oF S~owDo~. AN excursion around Snowdon, the highest of the Welsh mountains, in September, 1907, led me to the conclusion that a large-featured, round-shouldered, full-bodied mountain of pre-Glaeial time had been converted by erosion during the Glacial Period--and chiefly by glacial erosion--into the sharp-featured, hollow-chested, narrow- spurred mountain of to-day. Or, to phrase it in somewhat more 282 Vllov. w. ~tORltlS D,iVlS 0~ [Aug. 1909, technical style, that a body of ancient slates, felsites, and volcanic ashes, greatly deformed in Palmozoic time and greatly worn down in successive cycles of Palaeozoic, Mesozoic and Tertiary erosion, was reduced before the Glacial Period began to subdued mountain-form with dome-like central summit, large rounded spurs, and smooth waste-covered slopes, and with mature valleys drained by steady flowing streams which branched delicately headwards, with steepening slope, and which joined each other mouthwards in the accordant fashion that so systematically characterizes the drainage of all normally subdued mountain-masses. Also that this full-bodied mass was transformed during the Glacial Period, chiefly by the glacial excavation of valley-head cwms and by the glacial widening and deepening of the valleys themselves, irate a sharp central peak, which gives forth acutely serrated ridges between wide aml)hi- theatres: the serrated ridges changing into broad-spreading spurs as they are followed outwards ; the wide amphitheatres, backed by high rocky cliffs, opening by great rock-steps to irregularly deepened trough-like valleys, with oversteepened, undissected sides, sometimes smooth, sometimes of a peculiarly roughened slope; the smaller lateral valleys hanging in strikingly discordant fashion over the floors of the larger valleys; and the streams, far from following graded courses in steady flow, frequently halting in lakes or hastening in rapids and cascades. II. ExccRSlO~-S AROt'~D Ssownox. My first excursion around Snowdon was made for the most part in company with the late Joseph Lomas, F.G.S., of Birkenhead, who met me in the early morning of SeptemtJor ]2th, 1907, when the steamship Saxonia reached the landing-stage at Liverpool. We started off at once, and by ferry, train and carriage, via Birkenhead, Chester, and Bettws-y~ reached the isolated hotel of Pen-y- Gwryd, near the eastern base ot Snowdon, in the afternoon. On the four days following we walked around the mountain by the valleys on the north-east, south-east, and south-west, ascended the opposed slopes on the east (the western spur of Moel 8iabod) and west (the rounded summit of Mynydd Mawr) in order to secure general views of the main mountai~l and its great cwms~ entered the cwms on the east (Cwm l~yli) and south (Cwm Llan) to gain a closer view of their structure and form, and had a run over Moel Trefan, a little farther west, to see it,s famous drift-cover bearing northern erratics and sea-shells. After Mr. Lomas's return home, I went up to the summit of Snowdon by the rack-railway from Llanberis, and had an admirable review from above Of much that we had seen from below. Following this introduction to North Wales, it was my good fortune, as a member of the excursion conducted by Dr. Marr and Prof. Garwood, to see something of the Lake District, and then during the Centenary celebration of the Geological Society in London and the sectional adjournment to Vol. 65. ] er.ACIAr. SROSIOS i~r }rOBTH WAZ~S. 283 Cambridge, to meet a number of geologists, British and foreign, of broad experience in many fields, but of varying opinions as to the efficiency of glacial erosion ill mountain-sculpture, and to discuss with them, greatly to my own edification, though perhaps to their fatigue, the different phases of this interesting problem. Following the celebration a few days were spent, partly with Prof. Penek, of Berhn, in Devon and Cornwall for a purpose which will appear below ; and then three field-days were given to the Snowdon district again, with headquarters at Snowdon Ranger, another isolated hotel, by Cwellyn Lake, in order to reconsider the whole problem on the ground, previous to the presentation of a general account of it to the Liverpool Geological Society the evening before sailing, and to writing an outline of this article during the homeward voyage to Boston, October 15th-25th, 1907. Such was the delay in completing the manuscript that, before it was finally ready for printing, the Snowdon district was again visited at the end of August, 1908, in company with several members of the Oxford Summer School of Geography; and a brief statement of the discussion here presented was made before Section C of the British Association at the Dublin meeting, early in September of the same veal III. W~Ls~t TaR)iS, LOCAL PLACE-.NAXtES,AXD ILLIZSTRXTIONS. Several Welsh geographical terms are used in this paper, because they appear on the local sheets of the Ordnance Survey. One of these is ewm (pron. cfim)--essentially the equivalent of cirque in the French Alps, of kar in the German Alps, of both in Scandinavia, of coire in the Scottish highlands (made over into corrie for use in the lowlands), and of combe, or sometimes cove, in the English Lake district--the name for the amphitheatral form of valley-heads. Other Welsh terms are crib, a sharp, steep-sided, serrated ridge, corresponding to the ar~te of the French Alps; moel, a smooth dome-like mountain, resembling the balds of North Carolina ; afon, stream or river, with its equi- valent in the Avons of England; llyn, lake (the Welsh ll having the guttural eh and the l sounds combined); and nant, vale or valley. For the convenience of the reader, the use of little-known place- names as guides to the location of physical features is avoided as far as possible. Nearly all the local names employed are given on the accompanying outline-map (P1. XIV), from which it will appear that the Snowdon mass, with a summit-altitude of 3560 feet (standing about 10 miles south-east of the Mena~ Straits, the narrow sea-arm by which Anglesey is isolated from North Wales), contains four large ewms :--on the east, Cwm Dvli (pron. Dull) ; on the south, Cwm Llan ; on the west, Cwm Clogwyn ; and on the north-west, a ewm not named on the Snowdon sheet of the Ordnance Survey, but which will here be called Cwm Dur-arddu, after a lake of that 284 PROF. w. MORRIS DAVIS O~ [Aug. I909, name which it contains ; also the smaller Cwm Glas, on the north- east. Between these