Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: a Comparison of Methods

Home , Laramie River, Quaternary glaciation, San Juan Mountains

QUATERNARY RESEARCH l&289-310 (1982)

Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: A Comparison of Methods

THOMAS C. MEIERDING Department of Geography and Center For Climatic Research, University of Delaware, Newark, Delaware 19711 Received July 6, 1982 Six methods for approximating late Pleistocene (Pinedale) equilibrium-line altitudes (ELAs) are compared for rapidity of data collection and error (RMSE) from first-order trend surfaces, using the Colorado Front Range. Trend surfaces computed from rapidly applied techniques, such as glaciation threshold, median altitude of small reconstructed glaciers, and altitude of lowest cirque floors have relatively high RMSEs @I- 186 m) because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability. Surfaces computed for accumulation-area ratios (AARs) and toe-to-headwall altitude ratios (THARs) of large reconstructed glaciers show that an AAR of 0.65 and a THAR of 0.40 have the lowest RMSEs (about 80 m) and provide the same mean ELA estimate (about 3160 m) as that of the more subjectively derived maximum altitudes of Pinedale lateral moraines (RMSE = 149 m). Second-order trend surfaces demonstrate low ELAs in the latitudinal center of the Front Range, perhaps due to higher winter accumulation there. The mountains do not presently reach the ELA for large glaciers, and small Front Range cirque glaciers are not comparable to small glaciers existing during Pinedale time. Therefore, Pleistocene ELA depression and consequent temperature depression cannot reliably be ascertained from the calculated ELA surfaces.

INTRODUCTION existed in alpine regions (Charlesworth, Glacial equilibrium-line altitudes (ELAs) 1957; ostrem, 1966; Flint, 1971; Andrews, have been widely used to infer present and 1975). These indices have not been rigor- Pleistocene climatic conditions. Many ously investigated in a single area to see studies in arctic and alpine areas demon- which best represents past ELAs or to de- strate that the regional trends of modem termine how much error is associated with and Pleistocene ELAs relate to modern each index. Methodological comparison precipitation patterns, which are in turn in- studies by Andrews (1975), Gross et al. fluenced by atmospheric circulation, oro- (1977), and Hawkins (1980) are limited graphic effects, and distance from the sea either by small sample size, by lack of an (Porter, 1964, 1975a, 1975b, 1977; Qstrem, accuracy statement, or by choice of indices 1964, 1966; Wahrhaftig and Birman, 1965; that relate to modern rather that past Grosval’d and Kotlyakov, 1969; Peterson ELAs. Commonly used ELA determination and Robinson, 1969; PCwe and Reger, 1972; techniques will be evaluated in the Col- Andrews and Miller, 1972; Miller et al., orado Front Range for rapidity and subjec- 1975; Trenhaile, 1975; Hawkins, 1980). In tivity of data collection, and for magnitude addition, Pleistocene air temperatures have of local error from regional ELA trends. often been inferred by applying modern These techniques include altitudes of rem- lapse rates to the vertical distance between nant cirque floors, altitude of the glaciation present and Pleistocene ELAs (Peterson et threshold, maximum altitude of lateral al., 1979; Table 1). moraines, median altitude of reconstructed Equilibrium-line altitudes of past glaciers glaciers, and area1 relationships between cannot be directly measured, so a number accumulation and ablation zones of recon- of indices have been developed as surro- strutted glaciers. The “best” method of gates for ELAs, particularly those which representing regional ELAs will then be 289 0033-5894/82/060289-22$02.00/O Copyright 0 1982 by the University of Washington. All rights of reproduction in any form reserved. TABLE 1. ESTIMATES OF ELA SURFACE AND TEMPERATURE DEPRESSION FOR LATE PLEISTOCENE GLACIAL STAGE IN THE WESTERN UNITED STATES Altitude Temperature depression depression Reference Location Method (ml (“Cl Flint (1971) Front Range, Colorado Cirque-floor depression 1250 Legg and Baker (1980) Front Range, Colorado Tree-line depression 500 5 Richmond (1965) Southern Rocky Mountains Depression of ELA from THAR method 1400 8-9 Cirque-floor depression 1200 Maher (1961) San Juan Mountains, Tree-line depression 670 Colorado Wendorf (1961) Northwest Texas Plant distribution change 9-11 Oldfield and Schoenwetter Southern High Plains Depression of vegetation (1964) belts >1200 Martin and Mehringer Southwestern U. S. Depression of vegetation (1965) belts 900- 1200 Antevs (1954) Southwestern U. S. Cirque-floor depression 900- 1300 6 Leopold (195 1) Northern New Mexico Cirque-floor depression 1500 9 Dahl(1964) Northern New Mexico Plant distribution change 4 Wright, et al. (1973) Chuska Mountains, Tree-line depression New Mexico, Arizona Galloway (1970) Sacramento Mountains, Depression of periglacial Arizona geomorphic activity XM- 1500 10-11 Wahrhaftig and Birman Sierra Nevada. California Depression of glaciation wm threshold 850 Carver (1972) Oregon Cascades Depression of ELA from AAR method 1000 Scott (1977) Oregon Cascades Depression of ELA from AAR method 900-950 Porter (1977) Washington Cascades Depression of glaciation threshold 800- 1000 4-7 Flint (1971) Glacier National Park, Cirque-floor depression > 1300 Montana Waddington and Wright Yellowstone National Park, Tree-line depression 500 (1974) Wyoming Baker (1976) Yellowstone National Park, Tree-line depression 600 Wyoming EQUILIBRIUM-LINE ALTITUDES IN COLORADO 291 used to infer some aspects of Front Range niques because Pleistocene glacier extents paleoclimates. are well known (Fig. 1). The former glaciers Evaluation of each method is based on have been reconstructed from field and the assumption that regional ELA trends in aerial-photo mapping of tills by many au- a relatively small mountain range can be thors (Table 2), and by aerial-photo map- approximated by simple (first- or second- ping of glacial-erosional trimlines in upper order) trend surfaces. This assumption has valleys by this author (Fig. 2). Using terms also been expressed in studies on regional from the Wind River Mountains of Wyo- trends of glacierization (Andrews et al., ming (Richmond, 1960, 1%5, 1974; Madole, 1970), cirque-floor altitudes (Peterson and 1976b), the Front Range chronology is rep- Robinson, 1969; Andrews et al., 1970; resented by at least three major Pleistocene Trenhaile, 1975; Meierding and Birkeland, glaciations (pre-Bull Lake, Bull Lake, and 1980), and median altitudes of present and Pinedale) and multiple minor Holocene ice reconstructed Pleistocene glaciers (Haw- advances. Radiometric ages of tills have kins, 1980). An advantage of trend-surface only been obtained at a few sites (e.g., Ben- mapping over traditional hand-generalized edict, 1973; Madole, 1976a, 1980b; Nelson contouring of ELA surfaces is that the re- et al., 1979; Richmond, 1960) because of sidual error from the surface is readily ex- the absence of datable materials, so tills pressed. Residual error in this report is root have primarily been correlated with one mean square error, calculated as another and assigned ages by relative dating techniques, such as amount of post- (N - 1)-l i (Pi - 0,)’ 1 o’5 9 depositional weathering or erosional mod- L 1 i=l ification of till (Meierding and Birkeland, where Pi is the value predicted by the trend 1980). Correlation of tills in different val- surface and Oi is the observed value at a leys is often uncertain, particularly where sampling point. Both the trend-surface and tills are old. Sheet-like ridgetop diamictons RMSE are computed by the SYMAP pro- were previously thought to represent pre- gram (Dougenik and Sheehan, 1975). The Bull Lake tills (Ives, 1953a), but they are RMSE is a more appropriate statistic for now classified as Tertiary deposits of un- expressing absolute data variation than known origin (Madole, 1976b). The pre-Bull “percent variance explained” by the sur- Lake stage, which probably consists of face, as reported by Petersen and Robinson multiple glaciations, is represented by (1969), Andrews er al. (1970), and Hawkins deeply weathered tills with subdued or no (1980). Unlike variance, the RMSE is in the morainal form. Pi-e-Bull Lake till in the same metric as the data and is not depen- North St. Vrain Creek valley is younger dent on the trend-surface gradient or on the than 720,000 yr B.P., based on paleo- area covered by the trend surface. The magnetic data from lake sediments associ- RMSE expresses both the sampling error ated with the till, but it predates Bull inherent in each method and natural varia- Lake till, according to weathering and mor- tions due to local microclimatic influences phologic criteria (Madole, 1976b; Madole on glacial accumulation and ablation. If and Shroba, 1979). Pre-Bull Lake valley natural ELA error from a trend surface is a glaciers were only slightly more extensive constant for a given type of glacier over an than subsequent glaciers, and their deposits entire region, then the method that mini- are either buried or have been largely re- mizes residual error provides the “best” moved by erosion in most valleys. Bull representation of the regional ELA surface. Lake glaciers were almost as widespread as FRONT RANGE pre-Bull Lake glaciers and their tills are be- GLACIAL CHRONOLOGY lieved to correlate with tills at West Yel- The Front Range is a favorable region for lowstone, Montana, which are dated by ob- evaluating past ELA reconstruction tech- sidian hydration at 130,000- 150,000 yr 292 THOMAS C. MEIERDING

LONOMONT -

4 0

FIG. 1. Locations of reconstructed Pleistocene glaciers in the Colorado Front Range. EQUILIBRIUM-LINE ALTITUDES IN COLORADO 293

TABLE 2. MAJOR REPORTS CONCERNING FRONT RANGE PLEISTOCENE GLACIERS References Glaciated region Kiver (1972) Laramie River drainage basin Eschman (1955) Michigan River drainage basin Ray (1940) Cache La Poudre River drainage basin Richmond (1960) East slope, Rocky Mountain National Park Madole (1%9), Madole and Shroba (1979) St. Vrain Creek drainage basin Ives (1938, 1953a, 1953b), Gilbert (1968), Boulder Creek drainage basin Bonnett (1970), Gable and Madole (1976), Madole (1976a, 1980a) Ives (19381, Ray (1940), Richmond (1974), Upper Colorado River drainage basin Madole (1976a), Meierding (1977) Nelson et al. (1979), Millington (1980) Fraser River drainage basin

B.P. (Madole and Shroba, 1979; Pierce et ole, 1976b). At present, the limits of the al., 1976). Bull Lake tills are moderately early glaciations are not well enough de- weathered near the surface and retain pro- fined to be useful in the evaluation of ELA nounced morainal form, but with rounded, reconstruction methods. non-pitted crests (Richmond, 1965; Mad- By contrast, tills and trimlines of the

FIG. 2. Example reconstruction on stereo aerial photos of Pinedale (P) glaciers and additional areas covered by Bull Lake (BL) and pre-Bull Lake (PBL) glaciers in the northern St. Vrain Valley (modified from Madole and Shroba, 1979). Maximum altitude of outermost Pinedale lateral moraine is at LM. Smaller Pinedale glaciers have not been studied or mapped in the field, and their extents are not precisely known. 294 THOMAS C. MEIERDING

Pinedale glaciation are so fresh and the the ELA surface for former cirque glaciers geologic record is so complete that this (Charlesworth, 1957; Flint, 1971; Andrews, glaciation is favorable for reconstruction of 1975). Cirque topoclimates are usually past ELAs in the Front Range. Pinedale standardized in the Northern Hemisphere tills are characterized by slight surface- by sampling only north- to east-facing boulder weathering, poorly developed cliques (Flint, 197 1). Small north-facing soils, and sharp-crested moraines which glaciers receive little short-wave it-radiance, commoniy contain undrained depressions however, so they should represent the re- (Richmond, 1965; Madole, 1976b). A buried gional ELA at an altitude at least 100 m Pinedale till in a lateral position along the lower than that derived from south-facing Fraser River Valley is older than lake sedi- cirque glaciers (Trenhaile, 1975) and per- ments radiocarbon-dated at 30,050 ? 1200 haps 50 m lower than large glaciers with yr B.P. @I-2912), but end moraines of that relatively flat gradients. till were probably buried by till from Although the use of cirque floors as a sur- younger, more extensive Pinedale ice ad- rogate for ELA is simple in conception, vances (Nelson et al., 1979; Millington, many subjective decisions influence the de- 1980). Radiocarbon dates of 22,400 + 10701 rived ELA. For example it is difficult to -1230 yr B.P. (DIC-870) and 12,180 identify cirques at the low end of their al- 4 240 yr B.P. (GaK-4834) were obtained for titudinal range. They are usually poorly basal and upper sediments, respectively, at formed compared to high-altitude cirques glacial Lake Devlin, a lake dammed by a (Fig. 3) because they were occupied by ice Pinedale lateral moraine (Madole, 1980a; a shorter time, and the ice was thinner and Madole and Shroba, 1979). Neither date less erosive, than were higher cirques. Add- conclusively demonstrates the age of the ing to the confusion, headward portions maximum extent of Pinedale ice. A basal of stream valleys in some Front Range bog date of 13,820 rt 810 yr B.P. (GaK- granitic terranes resemble glaciated valleys 4537) in the Colorado River Valley is the of altitudes far below Pleistocene ELAs. oldest limiting date for a Front Range Poorly-formed, low altitude cirques often Pinedale terminal moraine (Madole, 1976a). have sloping, rather than level floors, and it The moraine itself is thought to be several is not certain which contour should be ac- thousand years older than this date because cepted to represent the cirque-floor altitude of the time necessary for vegetation to be- (Fig. 3a). The more cirques that are sam- come well established on freshly deposited pled in a given region, the higher the de- till. Most Front Range terminal moraines rived ELA because there is a lower limit to have not been radiocarbon-dated, so it must cirque formation but. no upper limit other be assumed that Pinedale glaciers in all than that caused by topography. In some valleys were at or near their maximum ex- studies, all cirques in a region have been tent at about the same time, perhaps sampled and their average altitude com- 16,000- 18,000 yr ago. Radiocarbon dates puted (Peterson and Robinson, 1969; at the glacier source areas in the Front and Andrews ef al., 1970; Trenhaile, 1975), but Park Ranges (Benedict, 1973; Madole, this sampling method may reflect the height 1980b) demonstrate that Pinedale deglacia- of the topography more than it does any tion was complete by 10,000 to 11,000 ELA surface. In other studies, only the yr B.P. lowest cirque within each topographic map is sampled (Porter, 1964; PewC and Reger, CIRQUE-FLOOR ALTITUDES 1972). However, few contour maps in cen- Pleistocene cirque-floor altitudes are ters of heavily glaciated mountain ranges rapidly obtained from contour maps and include Pleistocene cirques that fully con- they have been widely used to approximate tained their glaciers, so cirque-floor al- EQUILIBRIUM-LINE ALTITUDES IN COLORADO 295

FIG. 3. Cirque forms (floors marked with arrows) on Front Range contour maps: (a) poorly formed cirques probably fully contained their respective glaciers and their floor altitudes approximately represent ELAs of the glaciers that formed them (from Mt. Richtofen 1:24,000 quadrangle): (b) well-formed, high-elevation cirques at the head of Big Thompson valley have floors well above the Pinedale ELA (from McHenry’s Peak 1:24,000 quadrangle). titudes are also influenced by topography Subjectivity of sampling and high natural under this sampling scheme. variability of cirque-floor altitudes render Sampling of cirque-floor altitudes in this the method an unreliable ELA indicator study was restricted to the single lowest over a region as small as the Front Range. north-to-east facing cirque which appears The method probably overestimates ELAs to have fully contained a glacier on each for cirque glaciers, with the magnitude of 24,000 quadrangle (Fig. 4a). For cirques the overestimate dependent on the sampling with continuously sloping floors, the floor scheme. Subjective sampling decisions may altitude was sampled at the location with explain differences between authors in re- the lowest downvalley gradient within the ported Front Range ELAs. Mean Front cirque. The lowest Front Range cirques are Range cirque-floor altitudes are reported to assumed to have been occupied by Pinedale be 3450 m by Richmond (1965), 3230 m by ice because Pinedale glaciers were almost Madole (1976b), 3350 m by Meierding and as extensive as older glaciers, but the actual Birkeland (1980), and 3 161 m by Meierding time period over which these low-elevation (this report). Variability of cirque-floor cirques were carved is likely to represent a altitudes from the first-order trend surface “composite” age (Porter, 1964; Richmond, is moderate (RMSE = 109 m, Table 3), in 1965). Only one Front Range author differ- part due to the limited sample size (n = 12). entiated cirques formed by Bull Lake cirque The true RMSE from the regional cirque- glaciers from those carved by Pinedale floor altitude surface is probably closer to glaciers (Bonnett, 1970) and the reasons for the 135 m error reported by Andrews et that differentiation were not explained. al. (1970) for 49 north-facing cirque floors 296 THOMAS C. MEIERDING TABLE3. COMPARISON OF METHODSFORDETERMININGPASTANDPRESENT ELAs IN THEFRONTRANGE Mean altitude First-order Second-order Number of trend-surface trend-surface Rapidity Subjectivity of data of data of data Method samples (m) RMSE (m) r2 RMSE (m) r2 generation generation Reconstructed Pinedale glacial stage Altitude of small 12 3161 109 0.44 69 0.77 Rapid Highly north- to east-facing subjective cirque floors Median altitude of small 65 3085 97 0.30 91 0.39 Moderate Moderately glaciers subjective Glaciation threshold 13 3388 186 0.18 135 0.36 Rapid Highly subjective Maximum altitude of 45 3188 148 0.15 129 0.37 Time- Highly lateral moraines consuming subjective Percent vertical distance Moderate Slightly between toe and headwall subjective on large glaciers (THAR) 0.50 24 3265 82 0.36 71 0.51 0.45 24 3212 80 0.38 70 0.52 0.40 24 3161 80 0.39 71 0.51 0.35 24 3107 79 0.42 72 0.52 0.30 24 3056 82 0.41 76 0.49 0.25 24 85 0.38 78 0.48 Accumulation-area ratio Time- Slightly for large glaciers (AAR) consuming subjective 0.50 24 3298 92 0.50 66 0.75 0.55 24 3257 86 0.59 64 0.80 0.60 24 3209 85 0.56 57 0.80 0.65 24 3163 81 0.65 55 0.83 0.70 24 3118 89 0.58 58 0.82 0.75 24 3076 88 0.59 59 0.81 Present Median altitude of 16 3665 111 0.42 110 0.42 modem glaciers Mean summer freezing 23 4704 194 0.08 188 0.25 altitude 298 THOMAS C. MEIERDING with active glaciers in Okoa Bay, Baffin England and Baffin Island, respectively. Island (Table 4). PCwC and Reger (1972) used a THAR of 0.66 to map modern Alaskan snowlines. MEDIAN ALTITUDE OF SMALL The median glacier altitude method can PINEDALE GLACIERS also be used to represent ELAs of past re- Equilibrium-line altitudes of small circu- constructed glaciers. Median altitude was lar glaciers in Europe (Charlesworth, 1957) computed for each of 68 small (<3 km2) re- and in the Pacific Northwest (Porter, 1964; constructed Front Range glaciers of as- Scott, 1977) have been represented by a sumed Pinedale age. Most of these glaciers toe-to-headwall altitude ratio (THAR) of extended outside their cirques, but all were 0.50, whereas Manley (1959) and Andrews confined within a single cirque-valley sys- (1975) considered a THAR of 0.40 more ap- tem. There is considerable potential error propriate for cirque glaciers in northwest (+50 m?) in median glacial altitude deter-

TABLE 4. COMPARISON OF METHODS FOR DETERMINING PAST AND PRESENT ELAs OUTSIDE THE FRONT RANGE” Number RMSE from of first-order Method samples trend surface Reconstructed Late Wisconsin glacial stage Maximum altitude of lateral moraines 33 108 from large glaciers in Baffin Island (data from Hawkins, 1980) Percent vertical distance between toe 14 70 and head wall = 0.50 on large glaciers in BaRin Island (data from Hawkins, 1980) Accumulation-area ratio = 0.65 for large 34 94 glaciers in Bat&r Island (data from Hawkins, 1980) Present Late summer transient snowline in 129 104 southeastern British Columbia and Alberta (data from Gstrem, 1973) North-facing cirque floors containing 49 135 glaciers in Okoa Bay, Baffin Island (Andrews er al., 1970) Glaciation threshold in southeastern 40 146 British Columbia and Alberta (data from Gstrem, 1966) Glaciation threshold in southern British 27 129 Columbia (data from Gstrem, 1966) Glaciation threshold in Cascade Range, 18 91 Washington (data from Porter, 1977) Accumulation-area ratio = 0.65 for small 34 139 glaciers in Baffin Island (data from Hawkins, 1980) Accumulation-area ratio = 0.65 for large 9 86 glaciers in Baftin Island (data from Hawkins, 1980) a Root mean square errors were computed here from data in listed reports. The sampled areas were all approximately the same size (about 3000 km*). EQUILIBRIUM-LINE ALTITUDES IN COLORADO 299 mination due to difficulty in establishing higher (Qstrem, 1966; Andrews and Miller, upper and lower ice limits. Ice was inferred 1972; Miller et al., 1975; Porter, 1975b; to extend upward in each valley to the al- Trenhaile, 1975). The method is easy to titude where the cirque headwall slope is use, so maps of present glaciation thresh- greater than 60” (the greatest slope attained olds have been prepared for the northern by ice at modem glacial headwalls). Morain- Atlantic region (Ahlmann, 1948), northern al deposits are generally too small to be Norway (Ostrem, 1964), southern British recognized on aerial photos in these val- Columbia and Alberta (Ostrem, 1966), leys, so the termini of the small glaciers western Greenland (Weidick, 1%8), Baffin were primarily mapped by photo interpre- Island (Andrew and Miller, 1972), the tation (Fig. 2) of the locations separating Queen Elizabeth Islands in Arctic Canada glacially eroded valleys (rounded in cross (Miller et al., 1975), the Cascade Range and section) from stream-eroded valleys (an- Olympic Mountains of Washington (Por- gular in cross section). Glaciers were not ter, 1977), and the southern Alps of New sampled if there was considerable doubt as Zealand (Porter, 1975b). Glaciation-thresh- to where the valley shape change occurred. old maps have also been produced for As was the case with cirques, most small reconstructed late Pleistocene glaciers Front Range Pinedale glaciers faced north in the Sierra Nevada, California (Wahrhaf- to east due to shading and were located on tig and Birman, 1965), the Cascade Range the east side of the range (Fig. 4b). and Olympic Mountains (Porter, 1977), and Advantages of the median small-glacier northern Norway (Anderson, 1968). altitude method over others are that it is The glaciation-threshold surface is better quickly applied by a skilled photo interpre- computed for a large region of rolling high- ter, and a large sample size can be gener- lands (Flint, 1971) than for a small, rugged ated for a detailed reconstruction of ELA mountain range like the Front Range with spatial patterns. Error from the first-order more steep ridges than isolated peaks. trend surface is slightly lower for median Trend and altitude of the Front Range altitudes of small Pinedale glaciers (RMSE Pleistocene glaciation-threshold surface = 97 m) than it is for cirque-floor altitudes could only be computed from the 13 out of (RMSE = 109 m). The mean altitude of 40 quadrangles in the range that contained small reconstructed glaciers over the Front suitable peaks with and without Pleistocene Range is about 100 m lower than it is for the glaciers (Fig. 4~). Data collection is highly methods based on the large glaciers that are subjective where a worker must not only assumed to give the best indication of Pine- decide if a small glacier existed on a ridge, dale ELAs (Table 3). This seems reasonable but must also decide to which peak along because of the northerly, shaded orienta- the sloping ridge the glacier belonged. Suit- tion of the small glaciers. able isolated peaks that did not harbor glaciers are also difficult to locate in the GLACIATION THRESHOLD Front Range. Therefore, it is not known how much of the high RMSE to the linear Regional snowline trends are often ap- trend surface (186 m) is due to sampling proximated by reference to the glaciation methodology and how much is intrinsic to threshold, which is the average altitude the actual glaciation-threshold surface. A between the summit of the lowest mountain high RMSE (146 m) to a first-order trend with a glacier (usually a small cirque surface was also computed (Table 4) from glacier) and the highest suitable mountain Ostrem’s (1966) data on the modem glacia- without a glacier in a region (Charlesworth, tion threshold in southeastern British Co- 1957; Flint, 1971; Porter, 1977). The glacia- lumbia and Alberta-a continental location tion threshold parallels the ELA surface for which might be a modern climatic analogue valley glaciers, but is commonly 100-300 m to that of the Front Range Pleistocene, but 300 THOMAS C. MEIERDING which has fewer glaciation-threshold sam- elevation involves many problems. For pling problems. example, trees often obscure moraine In spite of data collection problems, the crests on aerial photos, especially near the glaciation-threshold trend-surface dips in ELA where lateral moraine crests are the same direction as surfaces based on small. Furthermore, bedrock and other de- small glaciers (Fig. 4a-c). Mean altitude of posits can be confused with till. Field the Front Range Pleistocene glaciation checking of these remote sites is too time threshold is about 200 m higher than the consuming to be feasible. In some valleys mean ELA data from the “best” recon- moraine crests are continuous upvalley struction methods (Table 3), in accord with from the terminal position to the upper limit findings of studies from other regions. of deposition, but in other valleys, major lateral crests are broken by tributary val- MAXIMUM ALTITUDE OF leys so that crests must be optically pro- LATERAL MORAINES jected across the voids on stereo aerial Ice in the ablation zone of a glacier flows photos. The longer the missing section, the outward toward ice margins with con- greater the chance of sampling the upper sequent deposition of till in the form of end- limit of a moraine crest deposited by a and lateral-moraine crests. After the glacier glacier other than the maximum Pinedale. recedes from its terminal position, the out- Pinedale lateral moraines are often absent ermost lateral moraines along the valley at high altitudes either because tills were walls may be left intact by the glacier if the not deposited on steep valley walls or be- valley walls are not too steep to hold the cause they have been eroded subsequent to till. Maximum upvalley altitude of these deposition. lateral moraines directly indicates where For the above reasons, maximum lateral the glacial processes changed from deposi- moraine altitude is among the least reliable tion to erosion, and these define the ELA at methods for determining Pinedale ELAs the time of maximum glacier extent (Fig. 2). (Table 3). A high RMSE (149 m) indicates Upper limits of lateral moraines deposited that elevation-data values vary widely from by Baffin Island Pleistocene glaciers were a linear trend surface. The method was also at the same ELA as that obtained by other the worst (RMSE = 108 m) of three used by methods in the Sulung Valley (Andrews, Hawkins (1980) for reconstructing Pleis- 1975), but were 60-80 m lower in the Mer- tocene ELAs in Baffin Island (Table 4). In chant’s Bay area (Hawkins, 1980). spite of the large error, mean altitude of Most Front Range valleys contain lateral Front Range lateral moraines (3188 m) is moraines that can be traced to a terminal almost the same as that derived by the Pinedale moraine (Fig. 4d). These lateral “best” methods. By contrast, the mean moraines were mapped from aerial photos, upper limit of Pinedale lateral moraines de- and altitudes of upper moraine limits were rived for the Front Range by Madole established from contour maps. Sampling (1976b) is 3290 m; the discrepancy between of maximum till altitude was limited to reports bears witness to subjectivity of the glaciers larger than 3 km2 because terminal method. and lateral moraines are difficult to discern on small Pleistocene glaciers. Lateral ALTITUDE RATIOS OF LARGE moraines of large Pleistocene glaciers can PINEDALE GLACIERS often be traced from the terminal moraine Richmond (1965) computed median al- up many tributary valleys. Only the altitude titudes (THAR = 0.50) between terminal of the highest outer Pinedale lateral moraines of large valley glaciers and the moraine was sampled in those cases. highest points on their headwalls to show Determination of maximum lateral till how ELA varies with latitude in the Rocky EQUILIBRIUM-LINE ALTITUDES IN COLORADO 301

Mountains. This same method has long kins (1980, RMSE = 70 m), but are lower been used for ELA reconstruction in than RMSEs from any other method used in Europe (Charlesworth, 1957) because it is this report. Mean altitude of the Pinedale rapidly applied wherever ages of terminal ELA over the Front Range is about 3160 m moraines have been identified. Charles- if a THAR of 0.40 is used. worth points out many theoretical problems with the technique, but an empirical study ACCUMULATION-AREA RATIOS by Porter (1975a) shows that 20 modern The ELA of a former large glacier can be North American glaciers with normally approximated from a topographic map by distributed area-altitude curves have me- locating the altitude on the glacier surface dian altitudes within 50 m of the actual that places 0.65 of the glacial area in the ELA. Hawkins (1980) also found that the accumulation zone (Meier and Post, 1962; technique worked well for disclosing the Gross et al., 1977). This procedure has been ELAs of past glaciers in Baffin Island used to estimate modern ELAs in the (Table 4). Canadian Artic Islands (Andrews and Mil- The highest probable ice extent (inferred ler, 1972; Miller et al., 1975; Hawkins, to be the highest altitude on a cirque head- 1980), and Pleistocene ELAs in West Paki- wall that had a slope less than 60”) and the stan (Porter, 1970), the Cascades of Oregon lowest altitude along the terminal moraine (Carver, 1972; Scott, 1977), the Southern were obtained from contour maps for each Alps of New Zealand (Porter, 1975a), of 24 Pinedale valley-glacier systems in the Hawaii (Porter, 1979), and Baffrn Island Front Range (Fig. 4e). Sampled glaciers (Hawkins, 1980). were all larger than 3 km2 and were fully Surfaces of each of the same 24 recon- contained within valley channels. North structed Front Range Pinedale glaciers used Boulder Creek glacier was excluded be- for the THAR method (Fig. 4e and fj were cause much of the Pinedale glacier con- contoured with a 60 m interval. Contour sisted of thin ice on a bench high above the maps of existing Alaskan glaciers served as main valley, and South St. Vrain Creek a guide to the appropriate amount of sur- glacier was not included because it fre- face concavity near the source area and quently changed its terminal location over a surface concavity near the terminus of each wide Piedmont surface. Other glaciers reconstructed glacier. Surface area of the (Cache La Poudre, Monarch Valley, and glaciers was measured between every two small west-slope glaciers) were excluded contours, and a graph of cumulative per- because they were fed by high-level ice cent area at each altitude was constructed caps with multiple outlets. The Clear Creek for each glacier. Accumulation-area ratios and Fraser River Pinedale glaciers were not (AARs) of 0.50, 0.55, 0.60, 0.65, 0.70, and sampled because their terminal positions 0.75 were assigned to each glacier to see are not well established. which ratio provides the “best” estimate of Various toe-to-headwall altitude ratios, the Pinedale ELA surface for large glaciers including 0.50, 0.45, 0.35, 0.30, and 0.25, (Table 3). were successively computed to see which An AAR of 0.65 had the lowest RMSE produces the lowest RMSE (Table 3). (82 m) to the first-order trend surface, sup- THARs of 0.35 and 0.40 produced the low- porting the use of that ratio by previous est RMSEs (79 m) and their indicated ELAs workers. An AAR of 0.65 applied to 34 are lOO- 150 m lower than the ELA derived large BafEn Island glaciers (Hawkins, 1980) by the commonly used THAR of 0.50 (3265 generated a similar RMSE (94 m). The AAR m). The RMSEs are slightly larger for all = 0.65 method provides about the same THARs than those errors reported by Por- RMSE and mean Front Range ELA (3 160 ter (1975a, error range = t50 m) and Haw- m) as that obtained from the THAR = 0.40 302 THOMAS C. MEIERDING method. The high ELA correspondence PRESENT ELAs AND PINEDALE between the two methods on a glacier-by- ELA DEPRESSION glacier basis (r = 0.71, Fig. 5) occurs be- Modern ELAs are usually obtained from cause both use the same 24 glaciers and existing glaciers or estimated from summer most of these glaciers had the same basic freezing level if a mountain range is not high area-elevation relationships. Major excep- enough for glaciers to form. There is a large tions include West Chicago Creek glacier, disparity in altitude between these two which differed from others in that a large surfaces in the Front Range, and neither proportion of its surface was gently sloping provides a wholly satisfactory basis on at the upper end, and Roaring Fork glacier, which to estimate Pinedale ELA depres- which was steeper at the upper end and sion. more level at the lower end than most Approximately 16 small (

2900 : I I I I I I 2900 3000 3100 3200 3300 3400 3500

ELA from THAR = 0.40 (m)

FIG. 5. Graph demonstrating high correspondence between ELA determinations from THAR = 0.40 and AAR = 0.65 methods. EQUILIBRIUM-LINE ALTITUDES IN COLORADO 303

cause a high error (RMSE = 111 m) to the first-order trend surface for modern glaciers, which accords with computations of RMSEs (Table 4) of the late summer transient snowline in British Columbia (104 m; Qstrem, 1973) and modern small-glacier ELAs in Baffin Island (139 m; Hawkins, 1980). High snow accumulation in the southern end of the Front Range is evi- dently balanced by high ablation so that glaciers do not form there. Cirques in the northern end of the range are too low in altitude for glacier formation. Present Front Range glaciers are not comparable to small cirque glaciers of the Pleistocene because the poorly formed cirques at the altitude of the lowest Pinedale ELA surface were not as suitable for shading of glaciers as cirques now occupied by glaciers. Furthermore, unlike modern glaciers, the small Pleistocene glaciers were far from the range crest and probably re- FIG. 6. Mean annual snowfall in millimeters of ceived little snowdrift because they lack water-equivalent (diagonal lines show snowfall in ex- suitable catchment basins or passes to cess of loo0 mm), snowfall weather stations (small channel snowdrift from the west (Graf, dots), present cirque glaciers (large dots), and recon- 1976). Therefore, modem glaciers do not structed Pinedale glaciers (stippled). Snowfall data are reflect a regional ELA trend that would from Annual Summaries for Colorado by U.S. De- partment of Commerce, U.S. Department of Agricul- exist if there were a large population of ture, and National Oceanic and Atmospheric Admin- glaciers in the Front Range today, and the istration. 500 m vertical distance between ELAs of Pleistocene and present small glaciers un- tion, and exceptional showdrift input from derestimates the magnitude of Pleistocene the west with consequent cornice avalanch- climatic change. ing (Outcalt and MacPhail, 1965; Alford, Many older reports assume that ELAs are 1973). Local variations in these factors primarily controlled by the effect of air

4500- y* -17,622 +335.07 x n ~16

Rowe glacier . 4000- E . **...... se.*‘. : . :....;* = *5, ...... c.. T ...... ‘Y”“‘; l . l _m 3500 - ...... _.....-...... w . St. Mary’s glacier

3000 , I I 1 39.75 40.00 40.25 40.50 Latitude (‘N)

FIG. 7. Median altitudes 01 present glaciers along the Front Range crest (dashed best-fit line dips southward only because of anomalous glacier outliers). 304 THOMAS C. MEIERDING temperature on summer ablation, and that only to see if present Front Range regional ELAs are near the summer or July freezing temperature trends parallel Pleistocene level (Leopold, 1951; Richmond, 1965; ELA trends. Temperature data are from 23 Flint, 197 1). Recent work demonstrates stations with at least 15 yr of record (Na- that regional ELAs are also influenced by tional Oceanic and Atmospheric Adminis- or related to other climatic variables during tration, U.S. Department of Agriculture, the ablation season, including shortwave ir- and U.S. Department of Commerce Annual radiance (Paterson, 1969), spatial distribu- Summaries for Colorado; Haeffner, 1971; tion of cloudiness (Porter, 1975b), warm Barry, 1973). A lapse rate of 6.5WlOOO m is advection due to wind (Paterson, 1969), and computed from mean June, July, and Au- proportion of rain to snow (Miller et al., gust data (Fig. 8). Altitude of the summer 1975). Also important are winter accumula- freezing surface at each station is station tion factors, such as precipitation (Charles- altitude added to the quotient of mean worth, 1957, PCwe and Reger, 1972) and summer temperature divided by the lapse augmented snowfall in the form of wind rate. Radiosonde data for a single year have drift (Charlesworth, 1957). As a result, been used elsewhere to express the altitude mean summer temperatures at the ELA de- of the 0°C mean summer temperature sur- viate as much as 7°C from freezing. face (Leopold, 1951), but free-air tempera- Mean summer freezing level is computed ture data show less relationship to actual

5000

4500

2500

Air Temperature PC1 FIG. 8. Mean summer (June, July, August) temperature lapse rate with altitude at Front Range stations (uncorrected for latitude). EQUILIBRIUM-LINE ALTITUDES IN COLORADO 305 present and Pleistocene ground conditions Pinedale vertical distance than noted by (such as inversions) than do data projected most other reports (Table 1). High error and vertically upward from ground stations. For present uncertainties in the relationship comparison, radiosonde data at Denver, between temperature and ELAs preclude Colorado place the summer freezing al- direct lapse rate conversion of this surface titude 300 m lower than that calculated from depression to Pleistocene summer temper- ground data. ature change and cast suspicion on many Altitude of the summer freezing level ELA and air-temperature depression fig- (Fig. 9) dips northward at about the same ures calculated in the past. rate (350 m/degree of latitude) as the Pinedale ELA surfaces reconstructed by DISCUSSION AND CONCLUSIONS the best available methods (Fig. 4e and f), Six independent methods provide similar but it also includes a westward surface dip estimates of Pleistocene ELAs and ELA due to inversions in enclosed west-slope surface trends in the Colorado Front Range valleys. Inversions or other microclimatic (Fig. 4) and all produce surfaces that dip irregularities result in a RMSE of 197 m to northward at a gradient similar to that of the the trend surface, which is much larger than summer-freezing temperature surface (Fig. the error in the derived Pinedale ELA sur- 9). However, the methods vary in quality faces. The present summer freezing level is according to the number and location of about 1550 m above the Pinedale large- samples, the size of glacier sampled, and glacier ELA surface computed by the AAR the measurement methodology. Errors = 0.65, THAR = 0.40, and maximum from both the first- and second-order trend lateral-moraine altitude methods-a greater surfaces rank the ELA determination methods in the same order of quality provided that sample sizes are sufficiently large. Methods based on small (~3 km2) glaciers demonstrate greater local variation from the regional ELA surface than those based on larger glaciers (Fig. 10). Some of this altitudinal error can be attributed to the fact that local variations in topography affect mass balances of small glaciers more than they do large glaciers. That portion of the RMSE caused by natural processes in the Front Range is perhaps 290 m for small glaciers and 270 m for large glaciers. Er- rors larger than these are caused by methodological problems, which also appear to be greater for methods based on small glaciers than on large glaciers. For small reconstructed Front Range glaciers, the median altitude method involves somewhat less subjectivity and produces “better” ELA results from a larger number of samples than is the case with the glaciation threshold or cirque-floor methods (Table 3). FIG. 9. Modem mean summer freezing surface de- A slightly larger THAR than 0.50 for small rived from lapse rate data. glaciers would produce a trend surface at 306 THOMAS C. MEIERDING

B 8 % %

\ I I 1 I . 1 I I 0 20 40 60 80 100 120 140 160 180 200 ROOT MEAN SQUARE ERROR FIG. 10. Root mean square error (meters) from first-order trend surface for ELAs derived from techniques involving (A) small (<3 kmz) and(B) large (>3 km2) glaciers. Method abbreviations include M, modern; P, Pleistocene; GT, glaciation threshold; THAR, toe-to-headwall altitude ratio; TS, late summer transient snowline: CF. ciraue-floor altitude; AAR, accumulation-area ratio; LM, maximum altitude of lateral moraines. the same altitude as those derived from a result of a greater area of topography suit- large glaciers instead of 100 m lower, al- able for small glacier formation. though the lower surface could be a real The only direct method of determining phenomenon due to greater shading of ELAs for large Pleistocene glaciers in the small north-facing glaciers compared to Front Range (maximum altitude of lateral large glaciers. moraines) suffers from excessive error, so In spite of error differences, all three Pinedale ELAs are best expressed by the methods based on small glaciers provide indirect AAR = 0.65 and THAR = 0.40 nearly identical northeasterly linear trend- methods. The dual correspondence of mean surface dips (Fig. 4a-c), and this indicates ELAs (3160 m) and RMSEs (80 m) between that small Pleistocene glaciers were lower the two methods lends credence to the by 50 to 100 m on the east side of the Front technique of defining ELAs by minimizing Range than on the west side. Large glaciers residuals to a computed trend surface. This were not similarly affected (Fig. 4d-f). The Front Range mean ELA is also supported east-west altitude difference may be due to by the lateral moraine method (3188 m). the fact that small northward-facing valleys The AAR method has a more satisfactory with an overall eastward component had physical basis than the THAR method be- less total irradiance than those with a cause it integrates surface areas, absolute westward component (Alford, 1973). On elevations, and gradients of ice accumula- the other hand, the northeasterly regional tion and ablation, whereas the THAR trend-surface dip may only be a result of method utilizes only the latter two. The random effects due to limited sample size, AAR method is time consuming to apply particularly on the west slope. The larger because of the lengthy mapping procedures number of small glaciers on the east slope is necessary to reconstruct and measure areas EQUILIBRIUM-LINE ALTITUDES IN COLORADO 307 of Pleistocene glaciers. It is suggested that the more easily applied THAR = 0.40 method be used for initial reconnaissance of ELAs in mountain ranges where ages of Pleistocene terminal moraines are known and where glaciers have a dendritic plan shape similar to larger Front Range glaciers. However, an important reason for eventually determining correct AARs of large glaciers of many mountain ranges by the method of residual error minimization is that it may be possible to gauge relative worldwide variability of Pleistocene glacier mass balances. For example, regions where AARs were found to be high would presumably have had low mass balances, slow flow rates, and low erosion rates. Second-order trend surfaces from the lateral moraine, AAR, and THAR moraine methods (Fig. 11) all indicate that Pinedale glaciers toward the latitudinal center of the range had ELAs 100 m lower than a linear trend surface predicts, and the low ELA FIG. 11. Second-order trend surface 1‘0 r altitudes zone is in the same location as modern where AAR = 0.65 on large reconstructed Pinedale cirque glaciers. Winter storms from the west glaciers. were presumably funneled into the Front Range crest center through low-lying Mid- dle Park to the west of the Front Range, so mature pending a clearer understanding of snowfall and wind drift would have been ELA temperature relationships. high in the center of the mountain range. Higher temperatures in the southern Front ACKNOWLEDGMENTS Range, and lower topography and lower Computation of reconstructed glacier areas and total snow accumulation on glaciers in the collection of climatic data were performed by James northern Front Range, could explain the Davis, Lynda Hall, Greg McCabe, and Gay Schlicht- ing. Helpful discussion on statistical and computer high ELAs in those locales. techniques was freely given by Cort Willmott and Broad-scale or regional trends in Pleis- Anne Webster. High-altitude aerial photos of the Front tocene ELA surfaces can be mapped and Range were purchased by the University of Delaware climatic interpretation made from any of Geography Department. Low altitude photos and the six methods in this report provided the working space to interpret them was provided by the U.S. Forest Service, Hot Sulphur Springs; National altitudinal range of one ELA surface sub- Park Service, Estes Park; and U.S. Geological Survey, stantially exceeds the error associated with Denver. I especially thank John Andrews, Kenneth a given method. Pleistocene ELA depres- Pierce, and Stephen Porter for adding useful sugges- sion should only be computed if present and tions and perspectives to the manuscript. past glaciers can be approximately stan- REFERENCES dardized for size, shape, and surrounding Ahlmann, H. W. (1948). Glaciological research on the topography, and if the same ELA determi- north Atlantic coast. 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