Origin of an Ice-Rafted Bergmound in the Walla Walla Valley

Cindy Bartlett

Department of Geology, Whitman College, Walla Walla, WA 99362

Iceberg-transported clasts are scattered about the Walla Walla Valley below the 380 m elevation, the level of the highest floods from glacial Lake Missoula. These boulders and cobbles are found singly, in groups, and in one large (37 x 20 m) bergmound. The bergmound is at 310 m elevation on a basaltic ridge on the north flank of the Horse Heaven Hills just south of the Oregon-Washington border, 6 km south of Touchet, Washington. In theory, icebergs carried by the Missoula floods could have originated anywhere along the southern Cordilleran Ice Sheet from Flathead Lake, Montana, to the northwestern Columbia Plateau, Washington. The potential source areas have a variety of plutonic, volcanic, metamorphic, and sedimentary lithologies. Establishing the source area for the iceberg was determined by correlating the lithologies of the Touchet Bergmound to an area that was covered by late Pleistocene glaciers during the time of the last series of Missoula Floods. The combination of lithologies suggests that the iceberg which deposited this mound was a part of the Okanogan Lobe of the Cordilleran Ice Sheet near Grand Coulee Dam.


Return to the Geology Department Homepage
Return to the Whitman College Homepage