Lake Pend Oreille
Throughout the earth's history the climate has fluctuated and at times the temperatures have been cooler than they are now. This change in temperature can last anywhere from 2 to 10 million years and is referred to as an ice age. During an ice age the snow begins to accumulate at higher elevations in the mid to upper latitudes. Due to the cooler temperatures during the ice age, the snow does not completely melt during the summer months. The snow accumulates, compacting under its own weight and forming glaciers that flow like slow moving rivers of ice down the valleys and into the plains, spreading out across the continent. Comment: during the last Ice Age (Pleistocene Epoch), which lasted about 2 million years, interglacial conditions – like today’s, prevailed for about half the time.
Lake Pend Oreille in northern Idaho is a fascinating geomorphic feature. This lake, at about 1158 feet is the deepest by far in the region, lies in a basin formed by Cordilleran glaciations immediately below the site of the ice dam that repeatedly formed Pleistocene Glacial Lake Missoula. The Cordilleran Ice Sheet extended farthest along major south-trending valleys and lowlands, forming several composite lobes segregated by highlands and mountains. Looking south to southeast from the posted coordinates of this Earthcache, you are provided with a vista of the intersection of the Clark Fork valley and the Purcell Trench as well as the area of the Clark Fork ice dam and Lake Pend Oreille trough.
The Purcell Trench, a major topographic depression in British Columbia and northern Idaho, is incised into the margin of a metamorphic and granitic complex, formed during Mesozoic convergent tectonics, but later subjected to Eocene extension. The trench is bounded on the west by the Selkirk Range and on the east by the Cabinet Mountains. The Purcell Trench has been the site of several drainage reversals. Prior to the Miocene time, ancestral river ranges flowed within the Purcell Trench from the Canadian border south to the Coeur d’Alene area in a meandering pattern following the least resistant rock exposures and fault zones. This pattern is still apparent in the present day meandering shape of Lake Pend Oreille.
During Miocene time the Columbia River Basalt Group invaded the southern part of the trench and formed a lava-dammed lake with water levels up to 2400 ft above sea level. However, the basalt dam eventually was eroded, restoring southward drainage. During the Pleistocene era, the Cordilleran ice advancing into Lake Pend Oreille repeatedly blocked the Clark Fork drainage and formed Glacial Lake Missoula.
The terminus of the glacial extended to the southern end of the lake and upon each failure of the ice dam, much of the catastrophic flood discharge was directed through the lake basin and into the Rathdrum Prairie (as seen in the photo below). Paleomagnetic analysis of the lakes filling and emptying cycles near the dam and in Lightning Creek shows secular variation in each of the lake filling cycles, but the analysis shows none within the instantaneous flood cycles.
After the latest flood (15-12,000 years ago), the Pend Oreille basin was again reoccupied by a glacier but this advance did not result in catastrophic lake drainage. Terminal deposits of till at the southern end of Lake Pend Oreille are undisturbed. Proglacial deltas and kame terraces in the Clark Fork valley left by this advance are intact and therefore postdate the catastrophic Missoula floods.
There have been at least five major ice ages in the past one billion years. The most recent, the Pleistocene Ice age, began about 2 million years ago. Glaciers did not continually cover the earth during this time; there have been interglacial periods where temperatures warm slightly and the glaciers melt and retreat.
In the most recent advance, glaciers reached their maximum extent about 18,000 years ago and had completely receded by 12,000 years ago. It was during this last glacial advance that a finger from the glacial ice sheet moved south through the Purcell Trench in northern Idaho, near present day Lake Pend Oreille, damming the Clark Fork River creating Glacial Lake Missoula. But that’s another subject.
Basalt: A dark volcanic rock. The most common type of lava.
Cordilleran: Referencing the extensive chain of mountain or mountain ranges, especially the principal mountain system of a continent.
Eocene Epoch: An interval of geologic time. The second epoch of the Tertiary Period, from about 58 to 37 million years ago.
Glaciation: the process of covering part of the earth's surface with glaciers or masses of ice.
Geomorphic: Of or resembling the earth or its shape or surface configuration. Pertaining to landform creation.
Mesozoic Era: The interval of geologic time from about 245 to 65 million years ago.
Miocene Epoch: The fourth epoch of the Tertiary Period, from about 24 to 5 million years ago.
Paleomagnetic: The study of the record of the Earth's magnetic field preserved in rocks and sediment through time.
References and pictures:
Website -Ice Age Flood Institute - http://www.iafi.org/index.htm
The Restless Northwest – A Geological Story – Author, Hill Williams
Roadside Geology of Idaho – Authors, David D. Alt & Donald W. Hyndman
On the trail of the Ice Age Floods – A geological field guide to the Mid-Columbian Basin - Author, Bruce Bjornstad
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