Bull Lake Research Articles

Quaternary evolution of Cedar Creek alluvial fan, montana

Cedar Creek alluvial fan is a textbook example of an alluvial fan because of its fan shape with smooth, concentric contours and excellent symmetry. Similar planimetric shapes have been used to infer uniform fan deposition; however, Cedar Creek alluvial fan is composed of four fan deposits of Quaternary age, Qf1 (oldest) to Qf4 (youngest), indicating that fan deposition was nonuniform in both time and space. Field studies indicate that deposition of Cedar Creek alluvial fan is related to glaciofluvial outwash activity during the Pleistocene and upper-fan entrenchment and lower-fan deposition during the Holocene. Qf1 and Qf2 deposits are sub-horizontally bedded, clast-supported sandy gravels uniformly imbricated upfan. Comparison of soil profiles developed in these deposits to radiogenically-dated chronosequences within the region indicates that Qf1 and Qf2 are correlative with Bull Lake and Pinedale-age deposits, respectively. These relationships are substantiated by physical correlation of Qf1 and Qf2 with Bull Lake and Pinedale moraines, respectively, in the Cedar Creek drainage basin. The sedimentology and timing of Qf1 and Qf2 indicate deposition in high-energy, proglacial, braided streams. Furthermore, the present morphology of Cedar Creek alluvial fan was established largely during aggradation of Qf1 and Qf2 when sediment supply to the fan was sufficient to activate 60% to greater than 90% of the total fan area. During Bull Lake glaciation, the apex of Qf1 deposition formed the apex of Cedar Creek alluvial fan as Qf1 covered more than 90% of the present fan area. During Pinedale glaciation, Qf2 deposition shifted downfan; Qf2 is inset into Qf1 above the intersection point, but below the intersection point it eroded and/or buried Qf1 as it activated as much as 60% of the fan area. Qf3 and Qf4, comprising 21% of the fan area, are inset into Qf2 in the lower fan area. Soil development in Qf3 and Qf4 deposits indicate episodic deposition and entrenchment beginning in early Holocene and continuing to present. A post-glacial decrease in sediment supply to Cedar Creek alluvial fan is indicated by sediment storage within the Cedar Creek drainage basin. Decreased sediment supply to the fan resulted in upper-fan entrenchment of Qf2 and deposition of Qf3 and Qf4 in the lower-fan area.

Winter movements of burbot (Lota lota) during an extreme drawdown in Bull Lake, Wyoming, USA

AbstractWinter movements of burbot (Lota lota) were monitored in Bull Lake, Wyoming during a record low water storage period in the winter of 1985–1986. Burbot with surgically implanted ultrasonic transmitters were commonly sedentary during the drawdown. The fish that did move were most active during dusk and at night in winter and during the day in spring. Burbot did not appear to occupy well defined home ranges or show discernible movement patterns. Movement into shallow water during winter was attributed to colder water temperatures and the onset of the spawning season. Burbot were always found associated with silt substrates during the drawdown. Aerial photographs of the exposed bottom revealed that virtually all suitable spawning and nursery habitat for burbot was eliminated by the 13.4‐m drawdown.

Soils Developed in the Glacial Deposits of the Type Areas of the Pinedale and Bull Lake Glaciations, Wind River Range, Wyoming, U.S.A.

The degree of soil development in glacial deposits in the Fremont Lake area (FLA) and Bull Lake type area (BLTA) on opposite sides of the Wind River Range of western Wyoming is chiefly influenced by the ages of the parent materials although other soil-forming factors are important. Soil morphology, clay content, and calcium carbonate content are useful in distinguishing moraines of the Bull Lake glaciation (about 140 to 150 ka) from those of the Pinedale glaciation (about 14 to 35 ka) in these areas. In the FLA, soils in Bull Lake deposits have an average Profile Development Index (PDI) of 39 index-cm and average 15% clay and 7% calcium carbonate (CaCO3), and soils in Pinedale deposits have an average PDI of 25 index-cm and average 6% clay and 1% CaCO3. In the BLTA, soils in Bull

Obsidian-hydration dating of fluvially reworked sediments in the West Yellowstone region, Montana

This study evaluates obsidian-hydration dating in postglacial fluvial terraces cut into an outwash plain near West Yellowstone, Montana. Fluvial transport fractures obsidian grains. However, some old hydration rinds may be preserved, thus, a grain may record several fracturing events. The most recent fracturing event at West Yellowstone is recorded in surface sediments from all of the terraces, which were cut in a shorter period of time than the technique can discern. They formed about 19,000 ± 1000 yr ago, using published hydration-rate estimates and a mean rind thickness of 6.34 ± 0.14 μm (1 SE). Alternatively, the application of published hydration-rate constants for the Obsidian Cliff flow with an estimated effective hydration temperature of 1.4°C yield an age of 24,400 ± 1100 yr (1 SE). Thicker rinds record fracturing during Bull Lake glaciation and cooling cracks from the emplacement of several source flows. Much of the observed spread in rind thicknesses (6.34 ± 1.69 μm: 1 SD) is probably the result of chemically induced variations in hydration rate. Terrace ages based on a single rind would range from 13,000 to 39,000 yr (±1 SD). Therefore, it is inappropriate to (1) use a set of hydration-rate constants determined from a single sample to calculate ages for multiple artifacts or geological samples, (2) date an archaeological or geological event on the basis of a single artifact, or (3) generate a chronostratigraphy on the basis of individual dates as a function of depth. Multiple evaluations of source chemistry and hydration rates and multiple rind measurements are required to date fracturing events.

The nature of groundwater flow in fractured rock: Evidence from the isotopic and chemical evolution of recrystallized fracture calcites from the Canadian Precambrian Shield

The isotope geochemistry of fracture calcites in three Precambrian plutons on the Canadian Shield has been investigated in order to understand the paleohydrogeological conditions in fractured crystalline rock. Fracture calcites of ancient hydrothermal origins in the Chalk River and East Bull Lake plutons exhibit 18O enrichment and 13C depletion trends resulting from recent low-temperature calcite recrystallization under open-system conditions for oxygen, but semiclosed for carbon, and under extremely variable time-integrated, water/rock (calcite) ratios. This process causes recycling of elements with calcite distribution coefficients > 1 (rare earths, manganese, and possibly iron) from the precursor calcite to younger calcites as well as calcite control over the 87Sr 86Sr ratio of the groundwater within the Chalk River pluton. The large but variable water/rock (calcite) mole ratios calculated from the shifts in the stable isotopic compositions of fracture calcites are compatible with fracture flow models that invoke flow channeling within single fractures that also contain regions of immobile porosity. Quantification of isotopic shifts resulting from recrystallization requires that the initial isotopic composition of the precursor hydrothermal calcite be well constrained. Unlike the Chalk River and East Bull Lake plutons, hydrothermal fracture calcites in the White Lake pluton, which intrudes marble-rich country rocks of the Grenville Supergroup, have highly variable δ 13C and δ 18O values. This is attributed to mixing of carbon from magmatic and sedimentary reservoirs, and to oxygen isotopic exchange between hydrothermal fluids and carbonate country rocks at the time of intrusion. Convective circulation of meteoric groundwater from the surrounding 13C-rich carbonate rocks (up to 6.6%.) resulted in δ 13C values as heavy as 3.0%. for the fracture calcites in the pluton.

SOILS DEVELOPED IN LATE PLEISTOCENE TILL, MEDICINE BOW MOUNTAINS, WYOMING

Detailed morphological descriptions and physical and chemical analyses were made for four soils developed on Pinedale and Bull Lake tills in the Medicine Bow Mountains, south central Wyoming. Means of identifying the age of the soils from morphology and chemistry were evaluated. A quantitative model was applied for distinguishing between Pleistocene soils of different ages. By employing a clay accumulation index, CAI = d(Bclay – Cclay) * Thickness(cm)), developed in Pennsylvania (Levine and Ciolkosz 1983), a linear regression model (Soil Age) Y = a + b * (CAI) was developed which effectively differentiated between soils formed in various Late Pleistocene tills. Logarithmic regression models provided lower significance than the linear model. Additional pedological characteristics (lithology and clay mineralogy) assisted in the relative chronological verification of the model's predictions for Medicine Bow Mountains soils. Data from other studies were used, with data from this study, to build a model for the estimating ages of soils in the Rocky Mountains. Again, a linear model provided the best fit to this expanded data set.

Effects of canopy formation by floating and submergent aquatic macrophytes on the water quality of two shallow Pacific Northwest lakes

Dense mats of aquatic macrophytes partitioned the littoral zone of two shallow lakes in Washington State into different habitats of varying water quality. Horizontal distribution of dissolved oxygen (DO) and pH in the lakes was associated with the patchy distribution of aquatic macrophytes in the lake, and the vertical distribution of DO and pH was associated with the location of the canopy in the water column. Elevated concentrations of DO and pH were observed in the canopies of submergent species, while lower DO concentrations and pH were seen both in the canopies of floating-leaved species and in the sub-canopy water of both growth forms. Open water was mixed to a greater extent than within macrophyte beds, resulting in higher sub-surface DO concentrations than beneath either submergent or floating plant canopies. Diel changes in DO and pH were associated with the growth form of the macrophytes that formed the plant canopy. Diel pH and DO changes were significant within the canopy of the submergent species that formed in the near-surface waters. Diel changes were not seen in either the surface of floating-leaved macrophytes, or in the sub-canopy of either growth form. In Keevies Lake, in Western Washington, dense canopies of overlapping floating leaves of Brasenia schreberi Gmel. reduced summer surface and sub-canopy dissolved oxygen (DO) to <2mg 1 −1 without significantly changing pH. In canopies of Potamogeton natans L., which has sub-surface, as well as floating leaves, the surface DO concentration was higher than in adjacent open water areas, but sub-canopy DO was as low as beneath B. schreberi canopies. In Bull Lake, in Eastern Washington, surface canopies of the submergent species, Ceratophyllum demersum L. and Myriophyllum exalbescens (Fern.) Jeps., regularly had DO concentrations of >30 mg 1 −1 and pH > 10. Beneath these canopies of submergent plants, DO of <1 mg −1 was common at 0.25–0.50 m, and pH was typically 1–2 units lower than at the surface.

Interpretation of ground VLF-EM data in terms of vertical conductor models

The very-low-frequency electromagnetic (VLF-EM) method has been used extensively for the detection and delineation of weak conductors such as those formed by water and/or clay-filled fracture and shear zones in the Precambrian rocks of the Canadian Shields as part of the Canadian Nuclear Fuel Waste Management Program. However, no satisfactory method of interpreting VLF anomalies over such conductors was available when the field data became available in the early 1980s. A study was undertaken to develop a method for quantitative interpretation of ground VLF-EM data over two-dimensional (2D) sheet-like conductors of low conductance, finite depth and depth extent, embedded in a resistive host rock of finite resistivity, using characteristic curves. Numerical modelling of the VLF response of such conductors revealed that the axes-ratio (inappropriately termed ellipticity in VLF literature) variations are much smaller than the corresponding tilt angle variations, and much less sensitive to the depths of the conductors. Extensive ground VLF-EM surveys over weakly conductive fractures and shear zones at several test areas in Ontario also indicated that the axes-ratio response is not only much smaller than the corresponding tilt angle response, it is often difficult to identify in the field data. An interpretation scheme was devised to determine the conductance, depth and depth extent of such weak vertical conductors located in a host rock of finite resistivity which depends on the peak-to-peak tilt angle response and the horizontal separation between the peaks. A complete interpretation of such conductors requires knowledge of the host rock resistivity and the approximate depth, if the depth extent of the conductor is required. The interpretation method was applied to interpret a VLF anomaly over a complex conductivity structure at East Bull Lake, Ontario which illustrated the limitations of the technique. A ground VLF anomaly from Chalk River, Ontario, was interpreted using the interpretation scheme described, which agrees well with the information from geological mapping and with the numerical forward modelling response.

The geochemistry, age, and origin of groundwater in a mafic pluton, East Bull Lake, Ontario, Canada

The geochemistry of groundwater in the East Bull Lake gabbro-anorthosite pluton near Elliot Lake, Ontario, Canada, has been investigated. Three chemical types of groundwater are present to depths of about 650 m, and are controlled by the nature of the groundwater flow systems and the water/rock interactions in the fractures. Ca HCO 3 water is present in the recharge area of the local flow system and evolves rapidly along the direction of groundwater flow to a high-pH, Na HCO 3 water primarily by cation exchange reactions. Saline Na Cl water is present below a depth of about 350 m. On the basis of its chemistry and hydraulic head measurements, the Na Cl water is believed to be part of a deep regional groundwater flow system. Carbon-14 measurements of the Na HCO 3 water suggest that this water was recharged shortly after the last Pleistocene deglaciation. This is supported by δ 18O values that are as much as 6%. lighter than shallow modern groundwater. Despite the high Mg content of the enclosing rocks, Mg 2+ concentrations in the groundwater of this unit are <1.0 mg L −1, and may be controlled by present-day serpentinization or formation of talc. Laumontite is thermodynamically stable in the Na HCO 3 and Na Cl waters and hence could still be forming under the present low-temperature conditions (<25°C). The stable isotope composition of the Na Cl water suggests that this water type is not a geochemically evolved product of the overlying waters, although present-day albitization of the anorthosite could be responsible for the increased Ca Na ratio in the Na Cl water compared to the other water types. The salinity of this water appears to have been derived outside the pluton, possibly as an ancient infiltration into crystalline basement rocks of saline formation waters present in Paleozoic sedimentary rocks, which formerly overlay the pluton, or from nearby Proterozoic formations. This hypothesis is supported by Br Cl ratios that are similar to sea water and many present-day formation waters, and the isotopic composition of the dissolved sulphate. The He “age” of the saline water is of the order of 10 8 years. Therefore both the saline water and many of its solutes appear to have ancient origins.

Radionuclide sorption on primary and fracture-filling minerals from the East Bull Lake pluton, Massey, Ontario, Canada

The East Bull Lake gabbroic pluton near Massey, Ontario, has been studied as part of the underlying research for the Canadian Nuclear Fuel Waste Management Program. The pluton has undergone a complex, multiple alteration history. The dominant feature of this alteration is replacement of the pyroxenes by calcic amphiboles and by a variety of other alteration minerals, particularly in fracture zones. These fracture zones would play a dominant role in the eventual transport of radionuclides from a nuclear waste disposal vault through the host rock. Interactions between radionuclides in solution and minerals lining the fractures may retard migration of these radionuclides. A comparative study of the sorptive capacity of fracture zone minerals for radionuclides representing alkaline earths ( 90Sr), alkali metals ( 137Cs), rare-earth elements ( 144Ce), actinides ( 241Am) and two anionic-forming fission products ( 75Se and 95mTc) was carried out using thin sections from drill core material obtained from fracture zones. The distribution of sorbed radionuclides on thin sections was determined by autoradiography. In general, alteration minerals show enhanced sorptive capacity in comparison to primary rock-forming minerals. Increased K content significantly increases Cs sorptin, while increased Mg and Fe content plays a positive role in the removal of Sr from solution.

Results of geoscience research in the Canadian Nuclear Fuel Waste Management Program: Introduction

Canada, along with other countries that are considering the permanent disposal of high-level radioactive wastes from nuclear power generation, is undertaking a program of research into deep geologi...

Geology of the East Bull Lake gabbro–anorthosite pluton, Algoma District, Ontario

The East Bull Lake Pluton, a layered gabbro–anorthosite intrusion 90 km west of Sudbury, Ontario, is in an inward-dipping lopolith and is 780 m thick in the centre and elliptical in plan view (13.5 km × 3.5 km). It intruded Archean metavolcanic and metaplutonic rocks of the Superior Province during the Early Proterozoic (2480 Ma).The intrusion consistes of a basal anorthositic unit, succeeded by rhythmic-layered gabbro, troctolite, layered gabbro, and massive and dendritic gabbro units. It is offset by the west-northwest-striking, subvertical Folson Lake fault. South of this fault, only anorthosite and massive and dendritic gabbro are exposed. North of the fault, subsurface lithologies intersected by me boreholes correlate with surface lithologies. Troctolite, the most distinctive marker that can be correlated between boreholes and surface exposures, confirms the general attitude and shape of the layers and lopolith.Chemical composition of the intrusion ranges from high-Mg tholeiite to calc-alkaline high-alumina basalts.Fractures occur in several preferred orientations, the most common being northwesterly, subparallel to the Folson Lake fault, numerous mafic dykes, and topographic lineaments. Complex fracture-filling and alteration mineralogies formed under a wide range of P–T conditions representative of epidote-amphibolite – greenschist facies, pumpellyite–prehnite facies, zeolite facies, and low-temperature rock–water interaction processes.The last movement on the Folson Lake fault was a dextral strike slip of up to 3.0 km that postdates most mafic dykes.

Anderson Canyon – A Classic Area in South-Central Idaho for Teaching Alpine Glacial Geology

The well-preserved record of alpine glacial geology displayed in Anderson Canyon, south-central Idaho, is a potential source of diversity and enrichment for summer field programs that visit the Rocky Mountain west each year. This valley provides a record of regional glacial history within a relatively small area (9 square km or 3.5 square miles), and is suitable for a short student mapping project. The moraines and outwash in Anderson Canyon define two major glaciations (the Copper Basin and Potholes) that are generally equivalent to the Bull Lake and Pinedale glaciations of adjacent Rocky Mountain areas. Associated lacustrine deposits document impoundment and drainage of two glacial lakes. Reconstruction of ice margins, fluvial patterns, and lacustrine systems provides a basis for correlation with regional Pleistocene events. The access road to Anderson Canyon is located midway along an improved road that connects Ketchum-Sun Valley on Idaho 75 with U.S. 93, approximately 27.3 km (17 miles) north of Mack...

The use of hornblende etching, clast weathering, and soils to date alpine glacial and periglacial deposits: A study from southwestern Montana

Many techniques for relative dating of surficial deposits suffer from (1) the need to make judgments and (2) poor reproducibility of data. Furthermore, many of the techniques generate data that do not permit the use of statistical analysis. In contrast, assessing the etching of hornblende grains yields objectively obtained, reproducible data that can be subjected to statistical analysis. In a study of the deposits of the Bear Gulch basin, Tobacco Root Range, southwestern Montana, the hornblende-etching data proved superior to the clast-weathering data and the soil-profile data in making interpretations of age relationships. The mean etching of hornblende grains decreases as a logarithmic function of depth. Hornblende grains from older deposits are more etched, especially near the surfaces of the profiles. The collective evidence supports a record of four first-order glacial and periglacial events, which have been correlated with (1) the youngest pre-Bull Lake glaciation, about 280,000 yr B.P.; (2) the Bull Lake glaciation, about 140,000 yr B.P.; (3) the Pinedale glaciation, consisting of two second-order advances, the first and most extensive between 26,000 and 20,000 yr B.P. and the second about 12,000 yr B.P.; and (4) a mid Neoglacial episode of periglacial activity. In addition, a major landslide blocked the valley in pre-Bull Lake time, and an episode of flood erosion along the valley floor is believed to have truncated the late Pinedale terminal moraine about 1500 yr B.P. In a study area for which numerical ages of the deposits are available, hornblende etching can be expressed mathematically as a function of the time since deposition, with the resulting relationship used to date deposits wherein ages are unknown.

Thermal diffusivity of igneous rocks at elevated pressure and temperature

Thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400°C and pressures of 200 MPa. The measuring method was based on the concept of cylindrical symmetry and periodic heat pulses. The seven rocks measured were Westerly (Rhode Island) granite, Climax Stock (Nevada) quartz monzonite, Pomona (Washington) basalt, Atikokan (Ontario, Canada) granite, Creighton (Ontario, Canada) gabbro, East Bull Lake (Ontario, Canada) gabbro, and Stripa (Sweden) granite. The diffusivity of all the rocks showed a positive linear dependence on inverse temperature and, excluding the East Bull Lake gabbro, showed a linear dependence on quartz content. (Quartz content varied from 0 to 31% by volume.) Diffusivity in all cases rose or remained steady with increasing confining pressure. The pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 MPa, depending on rock type. The pressure effect (measured as a percentage change in diffusivity) is stronger in the four rocks of granitic composition than in the three of basaltic composition. Our results agree well with existing thermal diffusivity measurements at atmospheric pressure.

Morphological and chemical characteristics of soil catenas on pinedale and Bull Lake moraine slopes in the Salmon River Mountains, Idaho

Morphological and chemical properties of soils developed on moraines of granitic composition, and forested with lodgepole pine, in Bear Valley, Idaho, change significantly with age and slope position. Soil development on Pinedale and Bull Lake moraine slopes of similar curvature and steepness was assessed at the summits, shoulders, backslopes, and footslopes of both catenas, and at the toeslope of the Bull Lake catena. Many soil properties show trends in development with both age and slope position. These properties include clay content, clay film development, color, plagioclase weathering, and dithionite-citrate-extractable iron (Fed). The degree of development of all these properties is greater on the Bull Lake catena relative to that on the Pinedale catena. This trend reflects increased soil development with age. On both catenas, development of soil properties is commonly highest at the footslope or toeslope sites and minimum at the backslope site. Downslope changes are attributed to both colluviation and pedogenic processes, including sorting of sediments during downslope transport, greater accumulation of eolian materials transported to downslope sites by surface processes, and greater in situ weathering of mineral grains at the footslope and toeslope sites.

Low-Energy Seismic Survey of Quaternary Materials, Rocky Mountain National Park, Colorado

One of the greatest needs in Quaternary stratigraphy is for information regarding subsurface units and the relations between them Low-energy (hammer) seismic surveying has the potential to provide that information inexpensively and unobtrusively In reconnaissance studies m Rocky Mountain National Park, Colorado, the Pinedale and Bull Lake tills, colluvium, alluvium, and weathered and fresh bedrock can be differentiated m the shallow subsurface (30 m) by this method Seismic velocities with units are constant *5%, and are generally constant _+ 10% for deposits present at several localities The ability of the method to discriminate among unconsolidated sediments of various ages and origins suggests that rt has potential for pedologic, geomorphic, and environmental applications

A petrochemical study of calcic amphiboles from the East Bull Lake anorthosite-gabbro layered complex, District of Algoma, Ontario

Calcic amphiboles are ubiquitous in the East Bull Lake anorthosite-gabbro complex, northeastern Ontario. The mode of occurrence suggests the amphiboles replaced clinopyroxene throughout the stratigraphic levels of the pluton, but they are also prolific in the top younger units. Three types of amphiboles, namely (1) tremolite-actinolite, (2) actinolitic hornblende and (3) hornblende, were identified. The composition of the amphiboles shows that they define a continuous series without any obvious compositional-miscibility gap. The analyses further indicated that the amphiboles evolved from actinolite to hornblende through coupled substitution involving edenite and tschermakite end members. Among the three coexisting amphiboles, chlorine is concentrated in hornblende. This is due to preferential location of hornblende along grain boundaries and/or its favourable structure.

87Sr/ 86Sr values in groundwaters of the East Bull Lake pluton, Superior Province, Ontario, Canada

The East Bull Lake pluton is a 2.48 Ga-old, layered mafic intrusion located on the southern margin of the Superior Province, Precambrian Shield in Ontario, Canada. The groundwaters have been studied as part of a detailed geological mapping and geochemical study of the pluton for the Canadian Nuclear Fuel Waste Management Program. All groundwater samples are dilute—the most concentrated Na Cl type water contains 3.1 g/l total dissolved solids. Present day 87Sr/ 86Sr values of the rock samples varies from 0.705 to 0.714 and the groundwaters from 0.711 to 0.720. The Ca HCO 3 recharge waters show the greatest range in values, in contrast to the deeper Na HCO 3 and deep Na Cl waters where a narrow range of 0.712–0.713 is found. Fracture-filling laumontite, calcite and gypsum have the same isotopic signature as the deep water. The Ca HCO 3 water is the youngest component of the groundwater system and its spread in isotopic values reflects local conditions at the surface. The Na Cl water is considered to be the oldest component and its Sr isotopic ratio value indicates equilibration with silicate fracture mineral phases. The isotopic data suggest that laumontite, calcite, and gypsum fracture phases have either crystallized from this water or are currently controlling its isotopic composition by dissolution. No age can be assigned to the deep waters, except that the Na Cl component may date from the Early Paleozoic, based on geological arguments or, more likely, be 10 4–10 6 years old based on kinetic studies of rock-water interaction and other isotopic data. If seawater was one component of the Na Cl water history, there is no record of this in the 87Sr/ 86Sr ratios.

A PINEDALE/BULL LAKE INTERGLACIAL PALEOSOL AND ITS IMPLICATIONS, CENTRAL LEMHI MOUNTAINS, IDAHO

A calcium-rich paleosol, preserved on buried till and buried colluvial surfaces, was discovered at several locations in the central Lemhi Mountains. The strati-graphic and pedologic characteristics of the paleosol assign its formation to the Interglacial between the Pinedale and Bull Lake Glaciations, between about 127,000 and 50,000 years before present. Pollen extracted from the paleosol reveal, in association with pedologic data, the horizons above the B2t level were decapitated by subsequent glacial and colluvial erosion. Interpretation of the pollen assemblage reveals that the lower treeline rose a minimum of 120–150 m during the interglacial, which must have been considerably warmer and drier than at present. [Key words: paleosol, Pinedale/Bull Lake Interglacial, paleoenvironmental reconstruction, pollen analysis, Lemhi Mountains, Idaho.]