Along the ICE AGE FLOODS
NATIONAL GEOLOGIC TRAIL

Green Monarch Ridge Ice Age Floods National Geologic Trail THIS IS WHERE IT ALL BEGAN! The Green Monarch Ridge viewpoint, just south of the Hope Fault, is positioned along the edge of the Purcell Trench Ice Lobe. Here it was responsible for damming the mouth of the Clark Fork drainage basin. This damming effect resulted in the formation of Glacial Lake Missoula, the source of the waters for much of  the PNW Pleistocene Ice Age Floods. The Purcell Trench Ice Lobe originated in Canada and flowed south into Idaho, guided by the structural control of the Purcell Trench. Following the path of least resistance into the basin now occupied by Lake Pend Oreille, it was impeded by the Green Monarch Ridge, located approximately 10 miles south of this viewpoint. Grinding and scraping, the Purcell Trench Ice Lobe backed up behind the Green Monarch Ridge, eventually building an ice plug up to 4,000 feet thick, and scouring out the deepest parts of Lake Pend Oreille. The Purcell Trench Ice Lobe divided into four sub-lobes: one went approximately west (US Highway 2), two went south (US Highway 95 and Lake Pend Oreille) and one went east (SR 200). All four sub-lobes contributed to blocking the Clark Fork drainageQuick Facts MANAGED BY:US Forest Service – Idaho Panhandle National Forest

Ice Age Floods National Geologic Trail 12,000 to 17,000 years ago a 4000′ tall ice dam blocked the path of the Clark Fork River creating glacial Lake Missoula. At the end of the last ice age this ice dam failed, releasing more water than is held in modern day Lake Ontario and Lake Erie combined. 600 cubic miles of water rushed through this area and down Lake Pend Oreille destined for the Pacific Ocrean.One of the most intriguing questions about the catastrophic flooding is how the ice dam failed. Various mechanisms for glacial outburst floods have been proposed: Ice erosion by overflow water, subglacial failure by flotation, deformation of ice by water pressure, and erosion of subglacial tunnels by flowing water. One model suggests a self-dumping phenomenon. In this mechanism, floodwaters are released when the lake level reaches nine-tenths the height of the ice. At this depth the ice becomes buoyant, subglacial tunnels form and enlarge, and drainage occurs until hydrostatic pressure is decreased and the ice again seals the lake. The self-emptying model is used to explain the numerous cycles in the rhythmite deposits and to interpret each cycle as a separate flood. Even so, only the total collapse of the ice dam can explain the largest of the catastrophic foods. Sub-glacial tunneling and enlargement due to thermal erosion progressing to collapse have also been proposed, as well as catastrophic failure due to water pressure. All are dependent on the configuration of the ice dam and structure of the ice. Quick Facts Location:Clark Fork, Idaho