Documenting the sedimentology of an unusual set of gravel deposits exposed along Snowbowl Road Developed with funding from the Howard Hughes Medical Institute ABSTRACT This study focuses on documenting and interpreting the sedimentology of a set of gravel deposits presently exposed along Snowbowl Road approximately 1.5 kilometers above the intersection of Snowbowl Road with the Grant Creek Road. The gravel deposit is exposed on the north side of the road in a series of artificial exposures which were cut in the 1950’s when the Snowbowl Road was constructed. The deposit consists of a set of discontinuous exposures along ~100 meters of the road, and roughly 6 meters of stratigraphy is exposed. The main gravel deposit is dominated by pebble-sized clasts that are typically angular and oblate. Sand is present but not ubiquitous. Much of the gravel is characterized by an open framework. Pebble imbrication is locally present in some of the beds. Bed thicknesses are generally centimeter to decimeter scale and bedding contacts include both sharp and gradational varieties. The gravel deposits are also characterized by cut and fill structures that measure up to ~ 30 meters across and ~ 4 meters in height. In addition to the main gravel deposit exposed in the road cut is a smaller fan-shaped deposit located at the thalweg of a small drainage and composed of several layers of poorly-sorted, locally matrix-supported conglomerate. The smaller fan-shaped deposit is interpreted to have formed as a result of sediment transport during large flow events and deposition onto the flat roadway. The main gravel deposit is preliminarily interpreted to represent shoreface erosion and deposition along a gravel beach associated with glacial Lake Missoula. METHODS -Stratigraphic sections were measured to determine the thickness and sedimentology of layers and the nature of contacts between layers -Grain sizes and shapes were evaluated to determine the extent of reworking and -Imbrication direction was measured to determine flow direction Results This outcrop consists of a well-stratified open-framework gravel that includes abundant grains of quartz and argillite. Coarser-grained layers are usually capped by very fine grained sand or silt. Layers range in thickness from 2-20 cm There are a few sets of cross beds through out the outcrop, the biggest measures ~3 m vertical relief and ~5.5 m across. The quartz rich layers consist of grains ranging in size from.25 mm – 2 mm grains that are sub-rounded to angular. The fragmented mudstone layer appears to have been deposited by a higher energy flow. These grains range in size from 1 mm to 30 mm and are angular to sub-angular. Two dark layers are composed of dominantly green and gray mudstone fragments which are poorly sorted. Grains within the mudstone layers range in size from.5 mm to 10mm and are angular to subangular. Discussion This set of exposed beds was likely deposited from glacial Lake Missoula roughly 15,000 years ago. The open-framework nature of the gravel and the large-scale cross beds are interpreted to represent wave reworking along the shoreline of glacial Lake Missoula. The angular clast shapes indicate that much of the debris in this deposit was derived from first-cycle reworking of locally-exposed rocks belonging to the Belt Supergroup. Kyle Brangers Department of Geosciences Measured Stratigraphic Section Dark gray section dominated by mudstones Poorly sorted angular sediment displaying open-framework packing Crossbedding layers of poorly sorted sediment Lines show bedding style green lines point to corresponding layers (top). Picture of outcrop without interpretation (bottom)