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 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. Ice reentrant in lower left corner is satlie as in upper right hand corner of Fig. 6. (Aerial photo by M. M. Miller, Sept. 8, 1962.) GREATER, Ih'TI Frc. 5. Vertical view (flight altitude 20.000 feet). Deglaciatetl granitic surface east of Taku E'iol.tl, near Turner Lake (outlet visible in upper right). Fiord trend due south. This terrain affected by Rlountain Ice-sheet glaci- ation. Large arrows indicate southerly direction of overriding by Greater Mountain Ice-sheet. Dashetl lines follo\v longitudinal axes of a tributary valley glacier systeu~of the Intermediate and Lesser Mountain Ice- sheet glaciations. Contetnporaneous with, and subsequent to, the valley glaciation was a local glacier cotlditiot~, itidicateci by small arrows. Tarn lakes produced in final phase of deglaciation. (U. S. Navy photo, 1948.) scription implies that some of them may also be THE RETRACTED ICEFIELD AATD LOCAL niarginal drainage terraces. The composite de- GLACIER CONDITIONS velopment of some of the upper level cirques The next phase to be considered is typified in on the icefield Inay also relate to this phase. It the Taku District by the Alaskan Little Ice ,4ge. suffices to mention here that an Exteilded Ice- Although an icefield-type of glaciation (figs. 3 field coildition characterizetl the last significant ant1 6) it is much more retracted than those of Rt~ctuntiol~I~riforc the close of \\'isco~isinnn tilnr. tl~.Wiiconsiiian ;inti is ch;lr:~ctri-ize(l1)y o~llyotlc J71c;. 6. Vertical view (indicated flight altitude 20,000 feet) illustrating two contemporat~cousphases of glaciation. Norris Glacier on left, and Talcu Glacier on right . . . segments of the Retracted Icefield phase. On the inter- vening ridge small cirque glaciers represent Local Glacier conditions at elevations of 2,500 to 4,500 feet. The surrounding suriace of gr;ltiodiorite modified by the Greater, Interlnediatc and Lesscr Mountain Ice-sheets, the upper limits of \vhich are sholvn hp successive bcrm lcrels along tile flanks of thesc ridges. (U. S. Navy photo, Aug. 14, 1948.) inq)ortant disl)ursing center ill the Llount Ogilvie 'I'herilial LPaximum. 111 the morphogei~eticc1;~ssi- crestal nkvk (fig. 7, C). I, therefore, refer to fication, this is the T,ocxl Glacier Phase. This this modern coildition, represeilting esse~ltiall~the coildition is illustrated by small glacierets today, last two arid one-half n~illeilia, as a Retracted particularly in the coastal islands to the west, Tcefield Stage. well outside of present icefield positions in tlie (hefinal p11;1se is ~nentionetl. l'his correspoiltls 1:ountlary Range. Recognitioil of this plixse is to t11(, ?veil lnorc contr;~ctc~lcoii(1itio~i of tlic of in1po1-t;~ticcin genmorl)hologicd cc,nsitlerations A. MOUNTAIN ICE SHEET STAGE B. EXTENDED ICEFIELD STAGE I LEGEND C. RETRACTED ICEFIELD STAGE I e- '-A FORMER DISBURSING CENTERS PRESENT DISBURSING CENTERS BEDROCK 'WATER GENERAL REPRESENTATION OF SHIFT IN ICE DIVIDE AND CENTERS OF FLOW IN THE TAKU DISTRICT DURING THREE STAGES OF WISCONSINAN GLACIATION I;](;. 7. Gctirral reprcse~itatiot~of shift in ice divide and centers of dis1)ursement in the Taku District, Northern Boundary Range, during three stages of \Visconsinati Glaciation. I)eca~~seit focuses on selective glacial erosion dur- resu1t:lnt terrain features are usually allied with ing interst:~tlial tiine. Traces of its effect are those of each of the final icefield phases. It is fouild close to the present icefield and also in particularly related to the motlification of the iiol;ltetl areas of the tleglacintetl I~ighlantl. 'I'he crestal flanks of ridges aiitl the floors of cirqt~es (figs. 5 and 6). Most of the evidence of local tlie ice divide to the present position which is glaciation during the post-Glacial Thermal Maxi- coincident with the vater divide. For the Taku nlum stage has been covered by renewed develop- District such is representetl in the stage evolu- nient of the present Retracted Icefield stage. tion sequence of figure 7. From this figure one \Vith respect to the morphogetietic phases in- call visuali~ethat in the progressive alitl retro- volved, discussion of the actual time relationships gressive stages, the disbursing centers were far- of each glacial stage is not within the scope of ther west than in the illaxinl~nistages. In the this ~aper. An attempt at chronology, therefore, waning hemi-cycle of each glacial t~~aximum,the has not been detailed, other than to facilitate the shift has, therefore, occtlrred from east to west. distinction in terms. It is reiterated that eacli of Thereafter, tluriiig further deglaciation, there has the glacial phases discussed has recurred re- been gradual shrinkage atid splitting up of the peatedly in the Pleistocene, and iti these dis- icefields into several local centers at separate tricts sever;iI of the lesser ones have occurred points along the crestline. 111 the waxing liemi- simultaneously (as illustrated in fig. 6). For cycle, the re17erse sequence 11:~s applied. these reasons, an u~~qualifiedreference to any one In table 1, the five recognized iiiorpliogenetic ~llorphogeneticphase does not imply n selective types of glaciatio~lare sum~n;~rizetl.Some evi- event or stage in Pleistocene chronology. Tl~isis dence of i~lteriiiediate phases has been tletectetl why the term "phase" is used for general refer- I~utthe detail ant1 extent of these will only be ence to a type glacial condition, while the term clarified through systematic stutly of selected "stage" is reservetl for reference to a specific stages and sub-stages via the glacial stratigraphy time interval in which a known condition of glaci- on the fringes of these districts. 'I'he Yiounti~i~l ation I~aspertained. Ice-sheet Glaciations of tlie \2'isconsinan, how- Fieltl evidence reveals that the passage froill ever, were of such intensity that they tlestroyetl one distinct stage to another it1 both the nortl~ern lliost of the clistiilct evide~lceof ~,revious ~najor and southern sectors of the Boundary Range has fluctuations in tlie immediate vicinity of the changed the east-west I,alance of glaciation. 'I'his Juneau Icefield and tlie Stilcine Icefield. As Iias heen through a series of successive shifts of a result, morphological studies in these areas TABLE 1 !I. Internzontnne "Icecap" Ghrialiurt The t~~axitnuniphase, attained only in pre-\\'isconsinn11 time; of sub- conti- (Icerr's Continental Ice-shcet nental proportiotl. Generally covering all sr~mnlitsand overridillg most of Stage; Davis and Mathens' them but with some radial flotv from local centers. Confluent with ~nain I'hase IV) continental ice-sheet to the east, which was deflected north and south, but with westward spill-over through low passes and major trans-range river gorges. B. Mountain Ice-sheet Glariatiun, Corresponds to \\'isconsilia11 maxima ; of regional proportion ; with localizetl \vith stagcs. (Icerr's Moun~t;till crestal centers and nunataks of higher peaks protruding through the surface. Ire-sheet Stage ; Davis and 11 confluent mass of tnoulitain and low-land piedtnotlt and vallej,-filling hIathc\rs' I'hase 111) glaciers, extending only slightly beyond the topographic borders of ihe range; distinct stages designated as the Greater, Il~terniediatcand Lesser Mountain Ice Sheets. C. B.vtended Icefield (;In(-iution .\ stage of district proportion; several low1 centers. Esse~~ti:~llya n~o~~r~tait~ (Kerr's "lntcr~seAlpi~~e" Si;~gc; glaciation, confilled entirely within the fiords and valleys of thc range. Cor- 1)avis and Mathews' I'hasc I I) rcspo~ldsto p11ls;ttion limits of the latest \\T~SCOIIS~I~:LI~. I ). Retracted Icejield Glac-iation .\ sub-stage of local proportion with 114vdnre;ts restricted to i~~trrtnctliateat~d (Kerr's "Alpine" Stage; Davis high elel-ations. Corresponds to present situation; within lin~itsof niasi- ;tnd Mathcws' Phase I) tnum Little Ice 4ge pulsations. 1;. Lor-a1 Glacier Condition F;ntl phase (or initial phase) just before colnplcte disappearance; tlisconncrtctl glaciers or sn~allicefield systems only at the highest level. Characterizecl interstage periods including the 'Thermal Maxim~~m.Also all ;rdjutlrt phase, typified by minor glac-ierets itnd high-level cirque glaciers above or outside of the main icefields during earh of the other stagcs noted. 256 RIL4YNtlI must al~iiostwholly be concerned n ith landforms, flights carried out ill 1901 wit11 the support of a the surfaces of wl~icl~have heen severely altered grant froill the L\inericail T'hilosopl~ical Society. in Wisconsiiian and Recent time. Supl'lemental flights in 1960 and 1962 ;111d related ICerr (1936) and Davis and Rlathews (1944) ground ol~servationswere made utlder the aegis have recog~lizedfour general stages in the central of the National Geograpl~icSociety and the Fou~z- and southern parts of the British Columbia Coast d;ltion for Glacier Research. Range. These seein to correspond wit11 the major phases observed by the writer in this north- ern sector of the Coast Range. A notation of REFEREXCES their terininology is given in parentheses in the AITKEN,J. D. 1959. "Atlin Map-Area, British Col~i~n- table. The writer's references, however, are spe- bia." Geological Slrrvry of Ctrr~ada, Memoir 307 cifically to the Alaska-Canada Boundary Range and (Ottawa) : 1-89. are therefore inade on a slightly different basis.4 HUDEL,J. 1944. "Die morphologischen Wirkungen des Eiszeitklimas irn gletschcrfrein Gebeit." Gcol. Although Kerr's use of the term "Mountain Ice- Rzl~~dsclzau34 : 482-519. sheet" is retained in the present usage for refer- - 1948. "Die Klima morphologischen Zonen der Polar- ence to the lesser phases, it seems advisable to use lander." Rrki(tldc 2 : 25-53. other than his "alpine" iiomenclature so that the CAIKNES,D. D. 1913. "Portions of Atlin District, con~lotations\?rill be consistent with the field facts British Colurnhia: with special reference to lode miti- ing." Cnlzndtr nrpartrrzctzt of Mirzrs, Grologrlic.nl as we now know thein. I11 the first place, me are S~tr7,r)lHraltrlz, lfemoir 37 (Otta\va) : 1-129. dealing with glacier systems wl~ichare Subtem- DAVIS,N, F. G., and W. H. MATHEIVS. 1944. "Four perate "icefields" of far greater extent than any Phases of Glaciation with Illustrations from South- ice m;lsses found today in the Alps. The ~vord \vestern British Colutnhia." Joilrnal of Grologg 52, "alpine" also connotes a chronological develop- No. 6, 403-413. iiieilt or stage sequence correspondii~gto the high- FLINT, R. F. 1957. Glacial at~dPlci,sfoccric Geology. (New York, John Wiley and Sons, IIIC.) lanil glaciation in I'urope which inay not be war- FLINT,R. F. ct al. 1945. "Glacial Map of North Amcr- railted in the present comparison. It is a teri~? ica." Br~llctirzof tllc Grologicnl Socirtj~of Alr~rricn, 1)robal)ly more intlicative of the Local Glacier Speci;ll Paper No. 60: 1-37 and map. contlition (luring the fin:ll disappearaiice of icc Jorrxs.~.o~,lV, A. 1026. "Tlle Pleistocene of Caril)oo it1 the \tarmest iilterglacial :111tl ititrag1aci;ll inter- and Cassiar Districts. British Columbia, Catlatla." Trir~~,sa~.tio~r.sof the IZoycrl Socirt~,of Cnrrtrtllr, scr. 3, ~iit~re vals. Furthermore, it is one which is ap- 20, Scc. 4: 137-147. ~n-ol~intefor a tlesci-iptio~i of local colitlitio~ls I