Comparison of Esker Morphology and Sedimentology with Former Ice-Surface Topography, Burroughs Glacier, Alaska

Comparison of esker morphology and sedimentology with former ice-surface topography, Burroughs Glacier, Alaska

KENT M. SYVERSON* 1 STEPHEN J. GAFFIELD* > Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 DAVID M. MICKELSON J

ABSTRACT An esker observed melting out of the ice was the southeastern margin of Burroughs Gla- initially capped by flat fluvial terraces exhibit- cier and use hydraulic head maps to com- Topographic maps of the Burroughs Glacier ing a braided channel pattern. A sharp crest pare esker paths, morphology, and sedi- ice surface and the surrounding land surface developed as the sediment slumped to either mentology to predictions derived by Shreve from 1948 to 1990 were used to generate sub- side of the ridge. (1985b). glacial hydraulic head maps and compare esker paths to hydraulic gradients. Hydraulic INTRODUCTION Esker Genesis gradient at the glacier bed is controlled pre- dominantly by ice-surface slope, but it is also Purpose of Study The term esker in this paper is defined as a influenced by the slope of the glacier bed. Es- long, narrow ice-contact ridge, commonly kers at Burroughs Glacier have been observed Burroughs Glacier is located in the north- sinuous, and composed chiefly of stratified forming in subglacial and englacial tunnels. ern part of Glacier Bay National Park and sediment (Flint, 1971). Eskers have been de- Most of the eskers formed in subglacial stream Preserve, between lat. 58°57' and 59°1'N and scribed and research on them has been sum- tunnels oriented parallel to the calculated hy- long. 136°13' and 136°22'W in southeastern marized by Charlesworth (1957), Flint (1971), draulic gradient. Where the former ice-surface Alaska (Fig. 1). Glacier Bay is a region with Price (1973), Embleton and King (1975), Sug- slope and slope of the bed do not coincide, the a maritime climate characterized by small an- den and John (1976), and numerous other relative influence of these factors on esker nual and daily temperature fluctuations, high journal articles and texts. Eskers generally paths is analyzed. One esker is oriented par- relative humidity and cloudiness, and heavy form parallel to the ice-flow direction in two allel to the land-surface contours and perpen- precipitation (Loewe, 1966). main ways. First, eskers form where engla- dicular to hydraulic head contours, implying A long historical record of rapid déglacia- cial or subglacial streams discharge into an esker path controlled by the ice-surface tion at the southeastern margin of the Bur- standing water at the glacier margin. A series slope. In another area, a set of subglacially en- roughs Glacier makes it possible to compare of small deltas form a segmented ("beaded") gorged eskers trends perpendicular to the land esker paths, morphology, and sedimentology ridge as the ice retreats. Eskers formed in this contours (directly down the slope of the former to known past ice configurations. Shreve way are time transgressive, with the older glacier bed) and parallels the calculated hy- (1985a, 1985b) described the Katahdin esker segments located in the downstream direc- draulic head contours. These eskers formed system in Maine, an area with topography tion (De Geer, 1897; Baneijee and McDon- beneath thin ice in tunnels that were air-filled similar to that surrounding the Burroughs ald, 1975; Hebrand and Amark, 1989). much of the time, and thus the slope of the Glacier. He used land-surface topography The second mode of origin is deposition in glacier bed controlled the esker paths. In a and an estimated ice-surface slope to explain subglacial tunnels (Flint, 1971; Shreve, third area where the hydraulic gradient and esker paths and sedimentology observed. He 1985a, 1985b; Ashley and others, 1991). glacier bed slope in the same direction, a sub- stated that rapid melting was associated with Many of this type of esker, commonly glacially engorged esker trends down the land viscous heating of flowing water in nearly formed in the lee sides of nunataks, have slope and the calculated hydraulic gradient. level or descending tunnels within ice at the been observed melting out of the ice at the Most of the eskers at Burroughs Glacier are pressure melting point. This caused large southeastern Burroughs Glacier during the <6 m high, are sharp crested, commonly cross flows of basal ice and entrained debris into past 25 yr (Mickelson, 1971; Syverson, 1992). small hills, and contain poorly sorted, poorly the tunnel and produced sharp-crested eskers Price (1966) noted that the eskers at Case- stratified gravel and sand. The lack of sedi- that contain poorly to moderately sorted, ment Glacier (—15 km east-southeast of Bur- mentary structures implies high sediment in- poorly stratified sand, gravel, and boulders roughs Glacier) also formed on the lee sides flux rates to the ice tunnel during formation with rock types similar to the adjacent till. He of nunataks. The nature of the conduit and/or rapid deposition late during esker for- also postulated that large amounts of melting through which the stream flows and the site mation. "Anticlinal" bedding is not common. ice within the tunnel could cause changes in of deposition control esker sedimentation the location of the main subglacial stream, (Price, 1966; Koteff, 1974; Baneijee and Mc- eroding sediment and forming discontinuous Donald, 1975; Hebrand and Amark, 1989). *Present address: Syverson: Department of Ge- bedding and angular unconformities. In this Steady-state conditions in the conduit are de- ology, University of Wisconsin, Eau Claire, Wis- study we describe the morphology and sedi- consin 54702-4004; Gaffield: BT2, Inc., 3118 Wat- fined by water pressure controlled by the ford Way, Madison, Wisconsin 53713. mentology of recently exposed eskers near weight of overlying ice, water discharge, ice-

Geological Society of America Bulletin, v. 106, p. 1130-1142, 8 figs., 1 table, September 1994.

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of the study area, in ca. A.D. 1700 (Fig. 1). The ice remained at its maximum until sometime between 1817 and 1842 (McKenzie, 1970). Changing climatic conditions near the end of the Neoglacial ice maximum caused a neg- ative glacier mass balance and ice-margin retreat. Calving occurred at the narrow mouth of Glacier Bay near Bartlett Cove during the Neoglacial maximum. The calving rate accel- erated when the ice margin retreated north- ward to the wider and deeper parts of Glacier Bay (Fig. 1), and the glaciers rapidly decreased in areal extent and thickness. The earliest maps of the Burroughs Glacier area, published by Cushing (1891) and Reid (1896), show a thick ice mass called the Cushing Pla- teau covering all but the highest peaks. Mick- elson (1971) suggested that ice was 650-700 m thick in the Burroughs Glacier region during the Neoglacial maximum. The historical record indicates that an area of —510 km2 has been deglaciated in the Burroughs Glacier region since 1900. During this time, the thinning Cushing Plateau separated into several small glaciers (called the Burroughs, Plateau, Cushing, and Carroll Glaciers) as nunataks and ridges emerged from the ice (Fig. 1). Lar- son (1978) reported ice-surface lowering of 7.1 m/yr at —125 m above sea level in Figure 1. Maps showing locations of (A) Glacier Bay and (B) the study area (arrow). Bu, 1972-1973. Burroughs Glacier; W, Wachusett Inlet; C, Carroll Glacier; Cu, Cushing Glacier; Q, Queen The deglaciation of the Burroughs Glacier Inlet; Be, Bartlett Cove—location of the ice margin during the Neoglacial maximum in the early region has been closely monitored since the 1800s. Modified from Smith (1990). early mapping by Cushing and Reid. Photo- graphs and observations made by the Amer- ican Geographical Society since 1926 have been summarized by Field (1947,1959). Field surface topography, bed topography, ice gested that ice-tunnel sediment deposition studies on the glaciology of the Burroughs temperature, and bed permeability (Shreve, occurs at the ice margin or within 3-4 km of Glacier by Taylor (1962, 1963) and Gaffield 1972; Paterson, 1981). it during déglaciation and is time transgres- (1991), glacial geology by Mickelson (1971) Eskers have been used by numerous work- sive. It is likely that final deposition in eskers and Syverson (1992), and glacial hydrology ers to interpret the déglaciation history of at Burroughs Glacier was late during by Larson (1977,1978) supply additional de- large areas (Shreve, 1985a; Aylsworth and déglaciation. tailed information that forms the basis of this Shilts, 1989; Ashley and others, 1991); how- modern process study. Land- and ice-surface ever, the timing of final sediment deposition DEGLACIATION maps of the southeastern Burroughs Glacier within ice tunnels is problematic. Most re- region in 1948,1960,1970, and 1990 provide searchers agree that final esker sedimentation The Neoglacial (Porter and Denton, 1967) detailed control on the rapid deglaciation in takes place during the waning stages of de- was a time of marked glacier expansion in this area (Fig. 2). The 1948 topographic map glaciation. Shreve (1985b) suggested that the coastal Alaska. A stump pushed over by the (Fig. 2A) shows that nunataks were not ex- 150-km-long Katahdin esker system in Maine ice at the base of Nunatak A (Fig. 2D) indi- posed at the southeastern Burroughs Glacier, formed simultaneously, citing evidence such cates that Neoglacial ice reached this eleva- but they were exerting much control over the as the increase in esker size downstream, es- tion in the Burroughs Glacier area —2500 yr ice-surface topography. Photographs from kers crosscutting later ice-margin positions, B.P. (Mickclson, 1971). Evidence for multi- 1950 show that Nunataks A, B, and D and the lack of kame deltas and outwash fans. ple Neoglacial advances is lacking in the (Fig. 2B) had emerged from the ice. By 1960 Ashley and others (1991) agreed that the Wachusett Inlet region (Goodwin, 1988), so it (Fig. 2B) six nunataks were exposed (A, B, Katahdin esker system was well integrated appears that the Burroughs Glacier region C, D, E, and F, named by Taylor, 1962). during déglaciation, but they suggested that has been covered by ice continuously since These nunataks absorbed heat, increased ab- final deposition took place in subglacial tun- that time. According to Goldthwait (1966), lation, and led to rapid ice-surface lowering nels near the ice margin and in marine fans. the Neoglacial ice mass reached its maximum and ice-margin retreat in this area (Mickel- Banerjee and McDonald (1975) also sug- extent in the Bartlett Cove area, 65 km south son, 1971; Syverson, 1992).

Geological Society of America Bulletin, September 1994 1131

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/9/1130/3382083/i0016-7606-106-9-1130.pdf by guest on 01 October 2021 N 1948 Glacier Margin A Nunatak 7 Photo Station

scale in meters

(Contour Interval: 100 feet) - approximately 30.5 meters *

N I960 Glacier Margin A Nunatak 7 Photo Station

scale In meters

(Contour Interval: 25 meters)

^acYvttS Figure 2. Topographic maps of the southeastern Burroughs Glacier and the surrounding land surface. The capital letters represent nunataks named by Taylor (1962), and the numbers represent photograph stations. (A) 1948, part of the Mount Fairweather (D-l) quadrangle with an original contour interval of 100 ft. (B) 1960.

1132 Geological Society of America Bulletin, September 1994

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/9/1130/3382083/i0016-7606-106-9-1130.pdf by guest on 01 October 2021 N 1970 Glacier Margin A Nunatak 7 Photo Station

acale in matera

(Contour Interval: 25 meters)

• 1990 Lake HI 1990 Burroughs Glacier A/ Elevation Contour A Nunatak 7 Photo Station •El Named Esker

scale in meters

(Contour Interval: 25 meters)

1133 Geological Society of America Bulletin, September 1994

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The Map Overlay Process forms quickly enough to maintain a water- filled tunnel much of the time. Rothlisberger (1972) states that ice < —50 m thick is not ca- 1948 Glacier pable of deforming rapidly enough to main- Ice Thickness Surface, H 48 tain a water-filled tunnel. For this reason we H-z 1960 Glacier assume that tunnels beneath ice <50 m thick are air-filled much of the time. Surface, H 60 Hydraulic Head The second assumption is that the pressure 1970 Glacier in ice surrounding the tunnel equals the over- h = 0.9 (H-z) + z Surface, H 70 burden pressure. This requires that all other 1990 Land stress components are small compared to the Surface, z overburden pressure. Burroughs Glacier ice slows as it approaches the margin, setting up Figure 3. Geographic information system procedure used to calculate ice thickness and sub- a weakly compressive flow regime. This can glacial hydraulic head. Ice thickness was calculated at each grid point by subtracting the 1990 be observed in aerial photographs displaying land-surface elevation (z) from the elevations of the glacier surface (H) in 1948,1960, and 1970 weakly developed longitudinal crevasses (see Fig. 2). The ice thickness (H — z) for each year and the land-surface elevation (z) were then near the glacier margin (Taylor, 1962, 1963); inserted into the equation to calculate total hydraulic head (h). however, these stresses are small compared to the ice overburden pressure (Gaffield, 1991), so the second assumption holds quite GEOGRAPHIC INFORMATION SYSTEM Ice thickness was calculated by subtract- well. METHODS AND RESULTS ing the 1990 land-surface elevation from the Subglacial hydraulic head was calculated elevations of the ice surface in 1948, 1960, at each grid point with an ice-thickness value Glacier bed elevations and past ice-surface and 1970 (Fig. 3). The data were compiled in for 1948,1960, and 1970. Hydraulic head val- elevations were used to construct hydraulic tabular format, and calculations were per- ues were contoured in ARC/INFO, and hy- head maps in the southeastern Burroughs formed in a spreadsheet program. Ice thick- draulic head maps for 1948, 1960, and 1970 Glacier region for the years 1948, 1960, and ness could not be calculated at grid points were generated (Figs. 4A-4C). Esker paths 1970. Topographic maps of the 1948, 1960, inside the 1990 ice margin where the bed to- were then compared with the glacier bed to- and 1970 ice surfaces and the 1990 land sur- pography was not known or outside the 1948 pography, hydraulic head gradients, and ice- face (Fig. 2) were digitized and incorporated ice margin. The number of grid points at margin positions. Below we discuss the char- into a GIS (geographic information system) which ice thickness was calculated for 1948, acter and hydraulic setting of each esker by Gaffield (1991, Appendix B). The main- 1960, and 1970 were 1792,1147, and 614, re- system. frame version of ARC7INFO (written by the spectively. Ice thickness was used to calcu- Environmental Systems Research Institute, late the total hydraulic head for water in sub- NUNATAK F ESKER (El) Inc.) was used. glacial tunnels using the technique described Six topographic highs located on each of by Shreve (1972) and Paterson (1981). At Description of Esker the four maps served as reference points that each grid point hydraulic head was calculated were used to transform the maps to the same using the formula h = 0.9(H - z) + z, The esker south of Nunatak F (El, scale and coordinate system. The six refer- where h is the total hydraulic head (in Fig. 2D) is sharp crested, —275 m long, 5-20 ence point positions on the 1948 U.S. Geo- meters), H is the ice-surface elevation (in m wide, and trends north-south (Mickelson, logical Survey quadrangle were recorded in meters above sea level [a.s.l.]), and z is the 1971). The crest of esker El ranges from 0.5 Universal Transverse Mercator (UTM) coor- elevation of the glacier bed (in meters a.s.l.). to 3 m above the surrounding land surface. dinates, and then each of the maps was trans- The term H-z is the ice thickness. The southern part of the esker is located in a formed to the UTM system. This allowed This method of calculating hydraulic head 0.5-m-deep channel eroded into till. The sur- overlaying the maps with a standard devia- uses two main assumptions. First, the water face that the esker crosses is highest at the tion of the horizontal position of —21 m. Tri- pressure in the tunnel must equal the pres- upstream end, descends 5 m to its lowest el- angulated irregular networks (TINs) were sure in the surrounding ice. This requires that evation near the esker midpoint, and then generated from the digitized elevation data the tunnel remain full of water at all times. rises 2 m in elevation to the downstream ter- for 1948, 1960, 1970, and 1990. A TIN is a The flow of water in englacial and subglacial mination point of the esker. three-dimensional model of a surface that is tunnels is not steady state because of daily The esker contains very weakly stratified composed of triangular facets that connect and annual melting cycles and weather fluc- sand and cobble gravel at the northern end. three data points. Elevations were sampled tuations. Even beneath ice as much as 100 m In the low, central portion, the esker contains by overlaying a rectangular grid on the TINs. thick, ice cannot flow into the tunnel fast laminated sand and silt. At the southern end, The horizontal spacing of the grid was 100 m enough to keep the tunnel full of water con- it lies in a channel in the till that is lined with in both the north-south and east-west direc- stantly (Rothlisberger, 1972; Shreve, 1985b), a cobble lag at the base, overlain by 0.3-0.4 tions, and the total number of grid points in so this assumption does not hold at all times. m of laminated silt and sand, the lower part of the study area was 9152. The TIN elevations Shreve (1985b) states that ice -100 m thick which is draped over the cobbles. The lami- were sampled at each grid point, except can expand or contract within a few days to nae range from 1 to 5 mm thick, and the thick- where the grid extended beyond the bound- adjust to changes in discharge. Thus, in this est laminae are graded. Several counts indi- aries of a map. article we assume that ice >100 m thick de- cated 3-8 laminae/cm, so —150-320 laminae

1134 Geological Society of America Bulletin, September 1994

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are present. Dark colored laminae contain surface lowering in the area of the moulin was this area was parallel to the slope of the land 2% sand, 96% silt, and 2% clay, and the light- faster than bed erosion at the downstream surface. Calculated hydraulic gradients for colored laminae contain 52% sand, 47% silt, end of the tunnel. Changing hydraulic head 1948 and 1960 also are oriented parallel to the and 1% clay. The laminated silt and sand conditions caused water velocity fluctuations paths of the eskers. In 1948 ice above the grade upward into cross-bedded fine sand at and the deposition of laminated silt. eskers was 120-140 m thick (Table 1). Ice the surface. Most of the cross-bedding in the A nearly constant hydraulic gradient may pressure was high enough to cause rapid ice upper, more coarse-grained sediment indi- have been maintained as the ice surface low- deformation and theoretically maintain sub- cates flow from north to south in the central ered after 1964, but the filling of the tunnel glacial tunnels completely filled with water and southern parts of the esker (Mickelson, probably caused an increased velocity be- (Rothlisberger, 1972; Shreve, 1985b). By 1971), although -10% dip to the north. cause of the decrease in tunnel diameter and 1960, the ice thickness was 40-60 m thick, so the deposition of fine to medium cross- the subglacial tunnels probably were air-filled Interpretation and Hydraulic Head bedded sand in the upper sections. Because at least part of the time (Rothlisberger, 1972). Considerations water was flowing uphill to deposit most of It cannot be determined from the sediments this sand, this implies that pipe flow contin- or morphology if all of these eskers formed Esker El is the only esker for which we ued until the tunnel nearly filled with sedi- during pipe-flow conditions, but the eskers have detailed data about its time of forma- ment or water stopped flowing. The esker with undulating longitudinal profiles formed tion. In 1960, a small moulin was located in was uncovered in 1967 or 1968, and the north in water-filled tunnels at the time of sediment the lee of Nunatak F at the present head of end (where the water flowed englacially) re- deposition. the esker, and water flowed out of a tunnel at mained ice cored. Collapse of the esker in this Two eskers trending north-south are also the southern margin of this ice where the ice-cored section has now produced a multi- located in this area (E2, the northern slope of downstream end of the esker is now located. crested ridge. station 14, Fig. 2D), and these are interpreted Water at that time was evidently flowing en- as subglacially engorged eskers (Mannerfelt, glacially and subglacially in the present loca- NUNATAK D ESKER SYSTEM (E2) 1945). A subglacially engorged esker, as de- tion of the esker. Water still flowed out of the scribed by Mannerfelt, is deposited by sub- channel in 1961 and 1963, but by 1964 and Description of Eskers glacial streams flowing in tunnels from the ice more so in 1965, ponding had occurred at the margin directly down the slope of the glacier channel mouth (Fig. 41 in Mickelson, 1971). The Nunatak D esker system contains five bed independent of the ice-surface slope. Es- Although no photographs are available, rates subparallel, interconnected eskers located on kers E2 on the northern slope of station 14 are of retreat indicate that the ice cover was re- the lee side of Nunatak D (E2, Fig. 2D). The oriented parallel to the slope of the land sur- moved from the esker by late 1967 or 1968. eskers originate at a kame terrace and trend face, but they trend approximately perpen- Thus, we believe that esker El formed in this southeastward, parallel to the former ice- dicular to the hydraulic gradients calculated tunnel sometime between 1960 and 1967. surface and bed slopes. These eskers, first for 1948 and 1960 (Figs. 4A and 4B). The ice Esker El trends parallel to the 50 m land described by Mickelson (1971), are sinuous, over these eskers in 1948 was 100-140 m contour (Fig. 2D). The hydraulic gradient cal- sharp crested, 2-4 m high, 7-10 m wide, and thick. These eskers are located slightly higher culated in this area for 1948 is parallel to the 130-175 m long (Fig. 5). Long profiles of the than the 1960 ice margin, so they probably 1948 ice-surface slope (Figs. 2A and 4A). esker crests undulate up to 5 m. Flat crests formed in submarginal chutes beneath ice Photographs of this area in 1960 (the 1960 1-3 m wide are present in a few places. <50 m thick shortly before 1960. Because of map does not cover this area) show that the The eskers generally contain poorly low deformation rates associated with thin ice-surface slope was parallel to the path of sorted, poorly to moderately stratified sandy ice, these tunnels would have been air-filled the esker. Thus, it is likely that the ice- pebble and cobble gravel. Bedding is 2-20 cm much of the time, and water flow would have surface slope was the primary control of the thick and horizontal when present. Beds are been controlled by the slope of the bed. subglacial hydraulic gradient at this locality. sharply defined yet massive where stratifica- Coarsening-upward sequences may repre- Following Mickelson (1971), we propose tion is observed. Cross-bedding 15 cm high sent the final stages of esker formation, where that the ice tunnel opened englacially at its was observed in one locality, but most of the rapid sediment deposition decreases the tun- upstream end and subglacially near its mouth eskers are devoid of paleocurrent structures nel cross-sectional area faster than ice can sometime during or before the summer of or bedding. Most outcrops do not show ver- melt. Flowing water then has a smaller cross- 1959. Water flow in the tunnel was governed tical grain size trends, but coarsening-upward sectional area through which to pass, and by the subglacial hydraulic gradient, because sequences are observed in some places. In thus it moves faster, increasing the compe- the central part of the esker is lower than ei- the upper 0.75 m of one of the eskers, mas- tence of the water and allowing transport and ther end, and there is no buried ice or sug- sive sandy pebble gravel with a granule ma- deposition of coarser material. gestion of collapse to show that the central trix grades upward to a sandy cobble and Shreve (1985b) states that sharp-crested portion was built on ice. The tunnel floor was boulder gravel containing subangular to eskers form in association with large amounts first eroded in the middle and downstream rounded clasts. of melting in nearly level or descending segments leaving a cobble lag, probably be- reaches of esker paths. He also states that the tween summer 1959 and late summer 1964 Interpretation and Hydraulic Head poorly sorted sediment and lack of well- when water velocity was rapid. In late 1964 Considerations defined bedding are caused by the influx of the hydraulic head in the tunnel was insuffi- debris from ice flowing into the ice tunnel. cient at times to force water over the south- The E2 eskers in the lee of Nunatak D The eskers in the lee of Nunatak D are sharp ern tunnel threshold, and ponding occurred. trend southeast within a bedrock valley. Dur- crested and lack sorting and sedimentary This presumably happened because the ice- ing déglaciation, the ice-surface slope over structures, although it is doubtful that the ice

Geological Society of America Bulletin, September 1994 1135

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/9/1130/3382083/i0016-7606-106-9-1130.pdf by guest on 01 October 2021 • 1990 Lake IH 1990 Burroughs Glacier

N 1948 Glacier Margin N 1948 Hydraulic Head Contour A Nunatak 7 Photo Station /El Named Esker

mla in IWHil 0 750 It»

(Contour Interval: 10 meters)

1990 Lake

E£S 1990 Burroughs Glacier N 1960 Glacier Margin N 1960 Hydraulic Head Contour A Nunatak 7 Photo Station /El Named Esker

i;ala in meter*

o 7 so 1500 (Contour Interval: 10 meters)

Figure 4. Subglacial hydraulic head maps of the southeastern Burroughs Glacier (modified from Gaffield, 1991). (A) 1948. (B) 1960.

1136 Geological Society of America Bulletin, September 1994

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68°68'S6" N 36°12'3B" W

1990 Lake IH 1990 Burroughs Glacier

N 1970 Glacier Margin A/ 1970 Hydraulic Head Contour A Nunatak 7 Photo Station El Named Esker

scale in meters

750 1500

(Contour Interval: 10 meters)

was thick enough during esker E2 formation Interpretation and Hydraulic Head EASTERN BRUCE HILLS ESKER (E4) for significant amounts of ice flow into the Considerations tunnels. Sediment deposition may have oc- Description of Esker curred too rapidly to develop well-defined Esker E3 follows both the slope of the gla- bedding. cier bed (Fig. 2D) and the calculated hydrau- Esker E4 is north of Calving Lake and lic gradients for 1948, 1960, and 1970 (Figs. northwest of the small 1990 Burroughs Gla- 4A-4C). Water flowed into the ice in a direc- cier ice remnant in the lee of the Bruce Hills SUBGLACIALLY ENGORGED ESKER tion contrary to the ice-surface slope direc- (Figs. 2D and 4C). The esker starts on a roll- (E3), BRUCE HILLS tion. Here the slope of the bed greatly ex- ing till surface near the Bruce Hills and fol- ceeded the slopes of the former ice surfaces, lows a sinuous path southwest to a point Description of Esker so the path of subglacial water flow was con- where marginal channels have eroded the trolled by the bed topography (Paterson, ridge. The esker is 260 m long, 0.5-1 m high, An esker interpreted to be a subglacially 1981). Aerial photographs of this area in 1970 1-3 m wide, and is located in a channel 0.3- engorged esker is located west-northwest of show a stream flowing from the Bruce Hills 0.5 m deep. Several 1- and 2-m-high hills are Calving Lake in a large valley on the Bruce and disappearing beneath the ice margin. Nu- crossed by the esker. The esker surface is Hills (E3, Figs. 2D and 4C). This esker is merous moulins in the photograph and land- covered with angular metasiltstone boulders sharp crested, sinuous, ~4 m high, 8-10 m surface maps indicate that the ice was thin and looks similar to the till surfaces on either wide, and 80 m long. Esker height is quite (<40 m thick) and the tunnel was open to the side. uniform, and the crest does not undulate lon- air. Apparently the subglacial tunnel was in The esker contains poorly sorted, poorly gitudinally. Esker E3 contains poorly sorted use before and during this time. Thus, esker to moderately stratified gravelly sand and and poorly stratified gravelly sand. Bedding, E3 formed sometime between 1960 and 1970 sandy gravel. Horizontal beds are 5-15 cm where present, is horizontal. Pebbles within when the ice thickness over the entire esker thick and lack internal structures. Cobbles the esker are subangular to angular metasilt- was <100 m (Table 1). As the ice thickness and boulders with long axes up to 30 cm are stone (the bedrock composing the Bruce approached 50 m, the subglacial stream distributed throughout the sediment. The en- Hills), indicating little transport and abrasion flowed directly down the bedrock slope in an tire esker is draped by 2-4 cm of laminated of materials. air-filled tunnel. sandy silt deposited within an ice-marginal

Geological Society of America Bulletin, September 1994 1137

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wide. The crest is flat and 3-5 m wide as the esker melts out of the ice, and numerous braided stream terraces are present on the crest. A sharp crest has formed as ice-tunnel sediment slumps to either side of the ridge. Hand-leveling of the uncollapsed esker crest on July 25,1990, indicates that the longitudinal esker profile undulated. The crest was nearly horizontal for the first 110 m, before gently rising to its highest point 1.6 m higher than the upstream end. From this point the esker was truncated by a series of channels and collapsed areas. The crest elevation decreased rapidly near the downstream end of the esker. Esker E5 will probably be destroyed as the underlying ice melts and the ridge collapses, although the esker was still intact in September 1993. Esker E5 contains moderately to well- stratified sandy gravel, gravelly sand, and sand. Unlike the eskers described previ- Figure 5. Esker in the lee of Nunatak D in 1970 (esker E2, Figs. 2D and 4). The esker is 4 m ously, this esker contains many sedimentary high. structures. Gravel and sand within the englacial esker exhibit cross-beds up to 1 m high. lake observed in the field in 1986. Clast contour at a small angle (Fig. 2D); thus, it Cross-sets dip southeastward at 20°-25° roundness was measured in the esker and ad- appears that the hydraulic gradient and path toward Burroughs Lake. Cross-sets contain jacent substrate in two different locations us- of the esker were controlled by the slope of interbedded sand and pebbly gravelly sand. ing the Krumbein (1941) scale. Mean clast the ice surface (Figs. 2A-2C). The ice above Sand units are usually ripple-laminated, and roundness at a depth of 5-15 cm within the this area was -220-240 m thick in 1948 and ripples climbing at subcritical angles toward esker is 0.44 (n = 100), slightly higher than 180-200 m thick in 1960 (Table 1). The ice in the southeast are frequently present. All pa- the mean roundness of 0.36 (n = 100) meas- this area was slightly > 100 m thick in 1970, so leocurrent indicators show water flowing ured in till 2 m from the esker sampling site. theoretically the subglacial tunnels at that southeastward toward Burroughs Lake. A Student's t test shows that mean roundness time would still have been water-filled most Just as the esker profile undulates along the values are not different (at a 95% level of sig- of the time (Shreve, 1985b). direction of flow, bedding undulations occur nificance) for material within the esker and till as well. One longitudinal section, —4 m high, adjacent to the esker, indicating little sedi- BURROUGHS GLACIER ESKER (E5) displays such a feature (Fig. 6). The top of the ment transport. section is a well-developed flat fluvial surface Description of Esker 2-3 m wide. Moderately well stratified pebble Interpretation and Hydraulic Head gravel and laminated medium sand are over- Considerations An englacial esker has been melting out of lain unconformably by a clast-supported cob- the Burroughs Glacier during the summers of ble gravel bed 0.7-1.0 m thick. The cobble- Esker E4 is parallel to the calculated hy- 1988-1993. The esker is located northwest of pebble/gravel contact rises —1.5 m over a draulic gradients in 1948 and 1960 (Figs. 4A Burroughs Lake along the northern ice mar- longitudinal distance of 15 m, truncating the and 4B), and it is nearly parallel to the hy- gin (E5, Figs. 2D and 4C). It trends north- underlying beds. Interbedded medium sand draulic gradient calculated for 1970 (Fig. 4C). northwest to south-southeast and is sinuous, and gravel are located stratigraphically above The esker crosses the 134 m land-surface 450 m long, 2.5-6.0 m high, and 10-15 m the cobble gravel.

TABLE 1. ESKER SUMMARY

Esker (location) Parallel to Parallel to calculated hydraulic gradient? Ice thickness above esker (m) bed slope? 1948 1960 1970 1948 1960 1970

El (Nunatak F) No Yes Yes* Degl.' 120-140 <60 0 E2(SE of Nunatak D) Yes Yes Yes Degl.t 120-140 40-60 0 E2 (north slope Sta. 14) Yes No No Degl.' 100-140 <50 0 E3 (Bruce Hills) Yes Yes Yes Yes 100-140 60-100 <40 E4 (eastern Bruce Hills) No Yes Yes Yes5 220-240 180-200 120-140 E5 (Burroughs Glacier) N.D." N.D." N.D." N.D." 240-260" 180-220" 120-160"

* Esker El is beyond the 1960 map coverage, but aerial photos show that it was parallel to the slope of the ice surface in 1960. 'Site deglaciated by 1970. fiEsker E4 is subparallel to the hydraulic gradient in 1970. It crosses a hydraulic head contour at an angle of —60°. "No data because glacier bed not exposed beneath englacial esker E5. Hydraulic gradient not calculated for englacial tunnel. ''Esker E5 formed englacially below an ice thickness less than the total shown here.

1138 Geological Society of America Bulletin, September 1994

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gradient would be in the same direction (parallel to the ice-surface slope). The uncollapsed stream bed at the esker crest in Figure 6 shows that the undulating bedding in that section was not caused by post-melt-out collapse. Rather, ice-tunnel sediment deposition in this area took place in two major stages. First the lower pebble gravel was deposited within the englacial tunnel (Fig. 6). Increased water velocity or a shifting of the main conduit location eroded the underlying sediment and a 1.5 m undula- tion developed in the sediment at the base of the tunnel. High energy water deposited the unstratified, clast-supported cobble gravel. Laminated sand was deposited conformably above the cobble gravel during slower water flow. Truncation surfaces such as this are abundant within esker E5 sediment.

DISCUSSION Figure 6. Discontinuous bedding in esker E5 (Figs. 2D and 4), 1990. This englacial esker is melting out of the ice. See text for detailed description. Interpretation and Hydraulic Head Considerations

The ice-tunnel sediment displays poorly to the esker E5 region (Fig. 4). Esker E5 paral- The configurations of the glacier bed and well-developed "anticlinal" bedding in the lels this hydraulic gradient. However, esker the past ice surfaces are well documented at southern half of esker E5 (Fig. 7). At the lo- E5 did not form at the base of the glacier, so the southeastern Burroughs Glacier. How- cation shown in Figure 7, the esker is 6 m its path was not controlled by the calculated ever, the timing of esker formation is not well high and contains well-stratified sandy gravel values of hydraulic head, although the overall known. This is an important limitation to the and sand. Beds dip approximately parallel to the sides of the esker. Beds in the left limb dip at 35° away from the esker axis, although in one area beds dip at ~70°. Beds in the right limb dip at 45° and are overlain by unstratified cobble gravel. The apparent fold in bedding in the right limb is an outcrop pattern caused by an irregular outcrop surface and dipping beds. No faults are visible in the limbs of the "anticlinal" structure. Terraces at the esker crest display a braided channel pattern and contain sandy cobble gravel up to 0.7 m thick that unconformably overlies the "anticlinal" bedding (Fig. 7, where the men are standing). The massive cobble gravel above the right limb of the "anticlinal" structure in Figure 7 is 2 m thick and very similar to the cobble gravel near the crest. Sandy cobble gravel near the esker crest contains some vesicular silty sand lenses 1.0-1.5 cm thick. These lenses can be traced laterally for several meters before pinching out. Figure 7. "Anticlinal" bedding in sandy pebble gravel, esker E5 (Figs. 2D and 4) August 1989. Interpretation and Hydraulic Head A high velocity subglacial stream undercut the ice and led to ice collapse and the formation of a Considerations cross section. This 6-m-high outcrop is still confined by Burroughs Glacier ice on both sides. The apparent fold in the right limb is caused by an irregular outcrop surface and the dipping beds. Hydraulic head maps indicate that the sub- Men are standing on braided stream terrace containing cobble gravel that unconformably over- glacial hydraulic gradient sloped southeast- lies the pebble gravel. Cobble gravel above right limb is similar to gravel at crest. See text for ward throughout the time of déglaciation in discussion.

Geological Society of America Bulletin, September 1994 1139

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application of Shreve's (1985b) method to the (which may later fill with subglacially en- crested eskers. According to Shreve, multi- Burroughs Glacier. The relationships be- gorged eskers), the water may flow above the ple-crested eskers tend to form in gently as- tween esker paths, hydraulic gradient, land- zone of debris-rich basal ice and not receive cending reaches of esker: paths. Esker E2 surface slope, and ice thickness are shown in much sediment. crest elevations ascend in Some areas, but the Table 1. This information can be used to Paterson (1981) states that in the ablation ground surface slopes uniformly in the direc- make some inferences about ice thickness at zone, the convex ice-surface profile tends to tion of water flow (Fig. 2D). the time of esker formation. Table 1 shows drive subglacial water away from the center Eskers at Burroughs Glacier are generally that some of the eskers at the Burroughs Gla- line of a glacier. In a valley, the cross-valley sharp-crested. However, esker E5, which cier probably formed beneath ice <100 m slope of the bed is probably large enough to melted out of the ice during 1988-1993, ini- thick where the assumptions of this method drive subglacial water toward the center line tially had a flat crest exhibiting numerous ter- do not hold, and the calculations are less even where the ice surface is convex (Pater- races underlain by cobble gravel up to 0.7 m valid. son, 1981). The southeastern Burroughs Gla- thick that unconformably overlies the ice- The paths of the esker systems described cier was not confined in a narrow valley, so tunnel sediment (Fig. 7). Braided patterns on at Burroughs Glacier generally do follow the eskers should radiate outward toward the the terraces and the fluvial cutbanks indicate calculated hydraulic gradients. Subglacially former glacier margins. With the exception of that these formed as a stream open to the air engorged eskers E2 on the northern slope of eskers that formed in submarginal chutes, eroded the ice-tunnel sediment. Such station 14 are the only eskers oriented per- they do follow such a pattern. However, the open-channel streams flowing atop eskers pendicular to the calculated hydraulic gradi- abundance of eskers in the lee of nunataks have been observed frequently during the ent. Here the eskers trend directly down the demonstrates that esker paths were con- past 25 yr at the Burroughs Glacier and could bedrock slope. Map evidence suggests that trolled primarily by the subglacial topography modify a sharp-crested esker into a broad- these formed in air-filled tunnels beneath ice and local variations in the ice-surface slope. crested esker. Broad-crested eskers have <50 m thick. Under these conditions, one In summary, the eskers generally follow also been observed in Maine and elsewhere would expect the water to flow directly down calculated hydraulic gradients at the south- (Shreve, 1985b). Esker E5 has become more the slope of the glacier bed. This shows the eastern Burroughs Glacier, reinforcing the sharp-crested as surrounding ice has melted problem of applying our method when one of work of Shreve (1985a, 1985b). However, and the sides of the esker have collapsed, de- the assumptions (a water-filled tunnel) is not some eskers formed beneath thin ice near the stroying the terrace morphology at the crest. met most of the time. The subglacially en- margin trend perpendicular to calculated hy- Esker E5 is the largest esker studied at Bur- gorged esker E3 trends parallel to the calcu- draulic gradients, so these would not be use- roughs Glacier and contains the most well- lated hydraulic gradient, but this is also par- ful for calculating paleo-ice-surface profiles. developed sedimentary structures. It formed allel to the bedrock slope. Two eskers (El in a larger ice tunnel several meters above the and E4) do not trend parallel to the bed slope, Esker Sedimentology and Morphology zone of debris-rich ice where sediment influx but they do trend parallel to the calculated to the ice tunnel caused by melting would hydraulic gradient. This suggests that the ice- Sediment observed within an esker may have been low. It is also likely that esker E5 surface slope controlled the hydraulic gradi- not represent conditions that existed in the was deposited during a longer time span, and, ent in those localities. Eskers that trend par- tunnel for a long period of time. Ice-tunnel therefore, the ice-tunnel sediment records allel to the bed slope, ice-surface slope, and sediment may have been deposited and more events in the form of bedding. calculated hydraulic gradient do not ade- eroded several times within the tunnel before "Anticlinal" bedding has often been ob- quately test the validity of this method of the final depositional event occurred. Most of served in esker cross sections. This has been head reconstruction. Eskers that trend uphill the eskers at Burroughs Glacier contain attributed to slumping and faulting of material would allow better evaluation of this tech- poorly sorted and poorly to moderately strat- as the ice walls supporting the esker melt nique, but there is only one such esker at Bur- ified sandy gravel and gravelly sand. Primary away (Flint, 1971; McDonald and Shilts, roughs Glacier. sedimentary structures are usually absent. 1975). Other workers have documented cases The absence of eskers that formed by flow Discontinuous bedding and angular uncon- where "anticlinal" bedding is a primary dep- up the stoss sides of nunataks and ridges at formities are common in longitudinal sections ositional feature (Sharp, 1953; Charlesworth, the southeastern Burroughs Glacier may be where the sediment is stratified. Shreve 1957; and Flint, 1971). "Anticlinal" bedding related to higher-than-average normal pres- (1985b) proposed that all of these sediment observed in esker E5 (Fig. 7) is not the result sure against the bed in those locations. Com- characteristics would be expected for eskers of post-melt-out collapse, for at the time of pressive flow against the bed on the stoss forming in nearly level or gently descending observation, the esker was still confined by sides of bumps may have closed subglacial reaches of esker paths beneath ice at the pres- glacier ice on both sides. The lack of faults passages, as described by Paterson (1981). sure melting point. Shreve (1985b) also sug- suggests that subglacial collapse did not play When the ice thickness decreased, eskers gested that melting of the tunnel walls would a major role in the formation of the "anticli- were deposited by subglacial streams that enhance the formation of discontinuous bed- nal" bedding. It appears the "anticlinal" bed- drained directly down the stoss sides of ding and angular unconformities, because the ding is either a primary depositional feature, bumps in submarginal chutes. Another pos- main tunnel location might shift to the sides, a folding feature, or some combination of the sible explanation is that water on the stoss leading to partial erosion and discontinuous two. Water flow in a full tube develops lon- sides of the nunataks appears to come to the bedding. gitudinal vortices (Schlichting, 1968; Shreve, ice margin through tunnels in clean ice, and Five subparallel, interconnected, sharp- 1972). Water flows down the sides of the tube this water carries little debris. Where ice is crested eskers southeast of Nunatak D (E2) and up in the center. It is suggested that lat- thin enough to allow water from the ice mar- contain poorly sorted sandy gravel. In Maine, eral vortices moving up the side of the esker gin to drain beneath ice in submarginal chutes Shreve (1985b) called such features multiple- can produce a rather steep sediment slope, a

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4. Esker E5 initially exhibits braided stream terraces at its crest, and later becomes sharp-crested as collapse occurs. Discontin- uous bedding and fluctuations in the esker- crest elevation indicate that the esker tunnel undulated longitudinally. This esker has better-developed bedding than other eskers studied at Burroughs Glacier, perhaps because of its larger size, slower sediment- supply rates to the englacial tunnel, slower sediment deposition rates, and longer time of formation. 5. "Anticlinal" bedding is rarely present. "Anticlinal" bedding in esker E5 is either a primary depositional feature, a compres- sional feature, or a combination of the two.

ACKNOWLEDGMENTS

This paper has been made possible by the assistance of many persons and organiza- Figure 8. Massive subglacial ice-tunnel sediment located near esker E3 (Figs. 2D and 4), 1978. tions. We thank the National Park Service The sediment outcrop is ~3 m across and is surrounded on the top and sides by ice of the and Jim Luthy, M/V Nunatak, for transport- Burroughs Glacier. ing equipment and supplies to Wachusett In- let. Erik Silvola, John Whedon, and Nelson Ham, Jr., assisted us in the field. Peter G. convex depositional surface, and "anticli- the formation of bedding and associated "an- Thum, of the Land Information and Com- nal" bedding. Figure 7 shows sandy cobble ticlinal" structures. puter Graphics Facility at the University of gravel at the esker crest and above the right Wisconsin-Madison, assisted with the GIS limb of the "anticlinal" bedding. The area CONCLUSIONS analysis, generation of figures, and text. Dis- filled with the cobble gravel may represent cussions with John Attig and Charles Bentley the shape of the water-filled tunnel occupied 1. Eskers at the southeastern Burroughs and reviews by William Warren and Christo- by flow vortices while esker sedimentation Glacier formed in subglacial and englacial pher Paola improved this paper. This project was taking place. When this water flow tunnels. Esker paths generally trend parallel was supported by major funding from the Na- ceased, a stream open to the air deposited to subglacial hydraulic gradients calculated tional Geographic Society (Burroughs Gla- cobble gravel in this region. The steeply dip- for a 22 yr interval. This reinforces the con- cier Expedition, grant 4087-89). The Univer- ping beds (—70°) in one part of the left limb of cepts of Shreve (1972, 1985a, 1985b) used to sity of Wisconsin Graduate School, the esker E5 (Fig. 7) suggest that compression reconstruct ice-surface profiles using esker Department of Geology and Geophysics at caused by ice flow may have been at least paths. University of Wisconsin-Madison, and the partially responsible for the "anticlinal" 2. Subglacially engorged eskers E2 on the Geological Society of America also provided bedding. north slope of station 14 are oriented perpen- monetary support for this project. Some fig- Most of the eskers at the Burroughs Gla- dicular to the calculated hydraulic gradient ures were produced by Gene Leisz at the cier lack "anticlinal" bedding, unlike the and indicate that the calculated gradient is in- University of Wisconsin at Eau Claire Media sharp-crested eskers described by Sharp correct in this location. Rather, the eskers Development Center with funding from the (1953), Flint (1971), Shreve (1985b), and oth- trend parallel to the slope of the former gla- UWEC School of Graduate Studies and Of- ers. The eskers at Burroughs Glacier com- cier bed. These eskers formed beneath ice fice of University Research. monly contain poorly sorted, massive to <50 m thick in tunnels that were usually air-

poorly stratified sandy gravel. Figure 8 shows filled. Here assumptions of our method were REFERENCES CITED a filled subglacial stream tunnel in 1978 be- not met, and the eskers are not oriented in the Ashley, G. M„ Boothroyd, J. C., and Borns, H. W., Jr., 1991, neath ice very close to the present location of direction predicted by the hydraulic head Sedimentology of late Pleistocene (Laurentide) deglacial- phase deposits, eastern Maine: An example of a temperate esker E3 (Fig. 2D). The tunnel is completely calculations. marine grounded ice-sheet margin, in Anderson, J. B., and surrounded by ice and shows little evidence Ashley, G. M., eds., Glacial marine sedimentation: Paleocli- 3. Eskers are generally sharp-crested and matic significance: Geological Society of America Special of bedding. The lack of sedimentary struc- contain poorly sorted and poorly stratified Paper 261, p. 107-125. Aylsworth, J. M„ and Shilts, W. W„ 1989, Glacial features around tures including bedding and "anticlinal" bed- sediment. Lack of bedding is caused by high the Keewatin Ice Divide: Districts of Mackenzie and Kee- watin: Geological Survey of Canada Paper 88-24, 21 p. ding may be a function of esker size. Eskers influx rates of material to the tunnel and/or Baneijee, I., and McDonald, B. C., 1975, Nature of esker sedimen- studied at Burroughs Glacier were <6 m tation, in Jopling, A. V., and McDonald, B. C., eds., Gla- rapid deposition in a single event before pipe ciofluvial and glaciolacustrine sedimentation: Society of Eco- high, much smaller than many eskers studied flow ceases. These sediment characteristics nomic Paleontologists and Mineralogists Special Publication No. 23, p. 132-154. elsewhere. Sediment may have been depos- are associated with ice tunnels nearly level or Charlesworth, J. K., 1957, The Quaternary era, with special reference to its glaciation: London, Edward Arnold, 2v., 1700p. ited almost instantaneously in the small ice descending through ice at the pressure melt- Cushing, H. P., 1891, Notes on the Muir Glacier region, and its tunnels at Burroughs Glacier, not allowing ing point, as suggested by Shreve (1985b). geology: The American Geologist, v. 8, p. 207-230.

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1142 Geological Society of America Bulletin, September 1994

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