MORPHOGENETIC CLASSIFICATION OF PLEISTOCENE GLACIATIONS IN THE ALASKA-CANADA BOUNDARY RANGE MAYNARD M. MILLER l)ep;\rtn~e~ltof Geology, Jlichig;rn State University, ;il~clthe Poi~~~tl;ctio~~ICJI Glacier Research, Seattle, LVashingto~l INTRODUCTION and dimension to tlie present-day Gree111:~udlce- cap.' By comparison, today's Cordil1er;ln glacier '1'11 11 fuiidametital relationship between Arctic terrain in tlifferent areas and the common genetic cover (fig. 1) is greatly reduced, embracing less tlenonlinator, climate, has been elaborated by l3~del than threr per cent of the formerly glaciated lantl- (1944; 1948). In a broader geomorphic sense, scape. Of this, the area of existing glaciers ill comparable 1:~ndscapesresulting from similar sets tlie Juneau Icefield and the Stikine Icefield of of climatic coilditions have been tertned morpho- the Alaska-Critisli Columbia Eoundary Ranges genetic regions (Thornbury, 1954). The form, encornpasses Iess than one-fifth of one per cent of tlimension, and geophysical character of glacia- the former total. There are certain features of tions likewise can be genetically related to climate. the present glacier system, however, which \yere 'I'lierefore, although a shorter time scale is in- characteristic of the coastal area in tlie periods of volved, it is appropriate to apply this concept to maximum glaciation. For example, it is still an the classification of ice sheets, both existent and ice-flooded landscape (figs. 2 and 3). Here some ancient. With respect to Cordilleran regions, a of the western glacial tongues descend to sea-level nlorphogenetic classification is especially helpful as they formerly (lid, although they are indeed far in delineating the repeated effects of Pleistocene inland froni tlie ice shelf which it is believed once glaciations, and the consequent accentuation of riillmed nluch of the Pacific shore along the outer features, ~vithoutpinpoitlting cycles or attempting edge of tlie Cordillera11 islands as far soutl~as the Olympic hlou~itaitis(fig. 1, ant1 Flint cf al., 1945, :L clironology. Such a classification paves the \yay for the f~lllestinterpretation of sequential map). lruntl forms \vhich, of course, is fundamental to In Alaska and northern British Columbia, the tlie establishtnent of either relative or absolute prime center of the continental "icecap" lay 200 clironologies. In this paper, the phases of Pleis- miles east of the present axis of the range and was tocene glaciation in tlie coastal Cordillera of the situated in the interior plateau region, probably Allaska-Canacla Boundary region are classified in near the northern end of the Cassiar ;\lountains. accordance with this concept. This pIaces it between the present watersheds of the upper Stikine, Taltu, Yukon, and Liard rivers CHARACTER OF THE CORDILLERAN (fig. 1). Fro111 this center, ice moved outward GLACIER COMPLEX in all directions. In its west\vard passage, it buried and overrode sections of the Coast Range In the culminating stage of the Pleistoceiie Epoch, the Cordilleran Glacier Complex reached in order to reach the sea. Johnston (1926: p. its most extensive develop~ne~itat the latitude of 137). from studies in the centr:ll plateau 11101111- tlie Taku Ilistrict of the Alaskan Panhandle. taitis, concluded that at one maximutn as m~~clias The ~~~I-~oleconfluent mass was some 500 miles 3.000 feet of Pleistocene ice huried even the wide on a line extending inland normal to tlie highest ridges, although he foutid no direct evi- present coast in the vicinity of Sitka, ,4laska. dence by erratics and striae ahove 6.500 feet. ,1. l~eri-~-~ody~-~ati~ically. the ice sheet was probably of In the Cassiar District, 250 miles east of IVran- the Polar type, completely filling the elevated sub- gell. 1,ortl (1943 : 1). 4) 01)servecl the maximtml continental platform between tlie Northern Rocky YIouiitains and the Coast Range and extending as lFlint (1957) e5tirnates its area at the Pleistocene maximum to have been 2,160,000 square kilometers. The far so11t1-1as the Columbia River in \I'asliiiigton pre5ent area of the Greenland Icecap is 1,726,400 square State. As such. it was similar in nature, form, kilometers. 248 RIA\7NAI<I) All. A21L1,EK LI'KOC..\MI:K. PHIL. SUC. FIG.1. General hlap of Alaska and n'ort11 Pacific Coast. elevation of striae and erratics to be 7,250 feet.' 111(1rpliology, \vhic11 sl~oivstli:lt the alignmelit of -1s \zill be she\\ 11 later, these striae 111:1y be from a the Coast L[ountains chantlelled much interior ice lesser phase, with the evideilce for a greater glaci- froin ail extensive early glaciatiotl tlorth\vartl :ltioil havitlg heen erat1ic:~ted. Ne~rertheless,al- tlirough the .Atlit1 lo\~lanclinto the t1rain:ige sys- lo\zing for sufficient thickness of ice to produce tein of the Yukon Valley ant1 southwart1 through the striae, it may be assumed that in at least one the Nass Basin and the Skeen;~River channel to stage the glacier surface in the interior was at or Llixon Elltrance ;111d the open ocean at (_)ueeii probal~lyabove 8,000 feet. 0l)servations of the Charlotte Souild (fig. 1). sun~mitforms and related features along the crest- At least during the retlucetl ice-sheet phases of line of the Bouildary Iiange have let1 the author to the Pleistocene, the Juneau and Stilcine Icefields concltitlc that the correspotltling ice level of this actetl as "local" centers with tlieir o1l.1~"icecal)~" m;~xi~nu~iiwas of the s;tine order. 'I'he highest stantling higher thail the surface of the inore coil- axial peaks in the Talcu and Stikine Districts may, tiiiental glaciation to the east. That there was I~owever, not ha\e been overritlden by glaciers east-\vest transfer of ice cannot be denied; but from the east. Regardless of the extent of glacia- the movement was mainly over the lower 111oun- ticln, many of these axial ridges served as cleflec- taiils and cols between the high centers, with the tion agents. This is borne out by the regional iilost coilceiltrated flow in the over-tleepened -- trans-range valleys of the Taku, U7hiting, and "The summits of the higher Cassiar Mountains lie het~veen 7,000 and 8,100 feet, which approximates the Stikine rivers. height of those in the Kortliern Boundary Range in the Another iinl~ortantdrainage outlet was north of region of the Juneau Icefield. the Taku District through the broad saddle of the \o~.108, so. 3, loc,~] ('OKIlILLELl,4N l'LI':IS'I'O('ENE: GL,I\('IrYI'IOXS \iThite and Chilkoot Passes where a large tongue parallel to the structural axis of the range be- of continental ice penetrated the range to pass tween LIount Nesselrotle, 1Zou1it Selles, and southward into the Coastal Trough. The result llount Lester Jones east of TuIsequah (figs. 2 was inuch erosion in the main fiord system west and 4). These peaks rise to about 8,000 feet and of the present Cordilleran glaciation. This par- all have been subjected to extreii~efrost shattering tially explains the great length and depth of Lynn aiitl severe suhaeriaI cle~ludatio~iin late-Pleisto- Canal and Chatham Strait which, if considered as cene time. n single fiord, is over 200 nliles long, and 1,000 It is probable, therefore, that at these summit to 2,000 feet deep (Flint et al., 1945). Deep levels any direct sign of glaciation earlier than grooving, with a longitudinal trend on the valley the \I.~isconsinaii has been removed. If 11ot actu- ~valls,and broadly convex hogback ridges bor- ally inundated by the regional glaciation, these tlerillg the Taku River and Fiord prove that great peaks were certainly obscured by a deep nlantle of quantities of ice also scouretl this depression, local ice. 011 isolated, somewliat lower summits some of it fro111 the local center, but probably of 7 to 8,000 ieet elevation, unquestio~lablerem- much as spill-over froin the contiiiental glacier nants of a foriner extensive erosion surface pro- sheet which inundated the Taku Plateau. vide geomorphic evidence that this ice was sig- nifica~ltlythick. ~Ilthoughthe details are di6cult TIIE PRE-JYISCONSINAN INTERMONTANE to reconstruct, it is clear that the glacial cover in ICECAP GLACIATION this maxim~~mphase was confluent with the in- In the Taku District, at the maxirnum stage, terior "Icecap" of sub-continental dimension. the regional ice center was well east of the present Therefore, it is npprol~riateto refer to this great- water divide and probal~lyexisted as a11 elongated est high-stand as the Intermontane Icecap Glaci- zone or broad glacial ridge connecting the highest ation. It is considered to be pre-\\'isconsit~ai in bedrock ii~assifs. Its axial line 111:1y be assumed age and, as a regional reference, to correspond to 1l;rve extellded ten to twenty miles east of the to the Continental Ice-sheet Stage of ICerr (1936: 111-esent 1nternation:~lRountlarq ant1 to have run 1). 682). FIG.2. Oblique vie\\, northeast across soutlicrt~ portion of Juneau Icefield, Alaska-Canada Boundary Range. In foreground is the 4,000-foot nEvC of Taku Glacier. Mean elevation of closest iiunataks is 6,000 feet. Crestal 116~6in left distance at 6,200 ieet. Mount Nclles (8,000 it.) is cloud-topped summit in right distance. (Aerial photo by M. M. Miller, Sept. 8, 1962.) 250 11,IYSARD R4. hlILLER [I'KOC.4M1 H. I'HII.. SOC. FIG.3. View north up main branch Taku Glacier, Juneau Icefield. In middle foreground is the 1962 seasanal n6vE- line at 3,000 feet and the setni-pel-manent nPvt-line at 2,900 feet. This glacier is a prototype of a thickening icc mass in the current regime cycle of the Retracted Icefieltl glaciation.