Glaciation and Forming the Missouri River Trench
— Double Ditch State Historic Site —
During the most-recent Ice Age, a massive, one-to-two-mile-thick ice sheet blanketed North America. Its center was located west of Hudson Bay. Glaciers are not static blocks of ice and, during the last ice age, glaciers advanced and retreated many times as temperatures cooled and warmed. The glacial ice scoured and gouged landscapes, picked up, and then deposited large quantities of sediment. When the glaciers melted, they released large amounts of water, some of it gradually as small streams, some quickly, through newly-formed valleys into lakes and rivers.
About 25,000 years ago, glaciers flowing toward the south and west dammed rivers that had flowed northeast. A glacier eastof today's Washburn dammed the Knife River. Water backed up along its banks, forming a reservoir lake. This lake eventually spilled over on its southeast corner, possibly in a spectacular flood. The water quickly carved a trench southward to today's Bismarck forming the segment of valley you see here. Similar events happened on many of western North Dakota's preglacial rivers. New diversion trenches directed meltwater to the south, eroding relatively deep valleys, and linking preglacial and glacial valleys together to form what we know today as the Missouri River. Those parts of the Missouri River that flow eastward are in wide, broad, older valleys-leftovers from river valleys that had carried preglacial drainage into Canada. River segments that flow south tend to be in narrower, steeper trenches. In a few places, such as the former river bed east of Washburn, the ancient wide valleys remain, but no streams flow in them.
The drainage pattern in North Dakota prior to glaciation. Preglacial rivers flowed northeast into Hudson Bay.
The extent of glaciation in North Dakota.
The valley you see here was formed when an ice- dammed lake in the old Knife River Valley overflowed from a point near Washburn, carving the north- south segment to the Bismarck area.
The route of the preglacial McLean River. When the McLean River was blocked by a glacier, a proglacial lake formed to the west of the glacier. When the proglacial lake overflowed it eroded a narrow trench from Riverdale to past Stanton.
Erected by State Historical Society of North Dakota.
Topics. This historical marker is listed in this topic list: Waterways & Vessels.
Location. 46° 56.412′ N, 100° 54.137′ W. Marker is near Bismarck, North Dakota, in Burleigh County. Marker can be reached from Double Ditch Loop half a mile north of Edgewood Drive, on the right when traveling north. Marker is on skids so location may vary slightly. Touch for map. Marker is in this post office area: Bismarck ND 58503, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within walking distance of this marker. Potande and the Mandan Fishery (within shouting distance of this marker); Bullboats (about 400 feet away, measured in a direct line); Hunting and Gathering (about 500 feet away); Mandan Origin Stories (about 700 feet away); Mandans and the Practice of Farming (approx. 0.2 miles away); Square Buttes (approx. ¼ mile away); Double Ditch State Historic Site (approx. ¼ mile away); Stone Shelter (approx. ¼ mile away). Touch for a list and map of all markers in Bismarck.
Credits. This page was last revised on January 13, 2021. It was originally submitted on January 10, 2021, by Connor Olson of Lemmon, South Dakota. This page has been viewed 34 times since then. Photo 1. submitted on January 10, 2021, by Connor Olson of Lemmon, South Dakota. • Mark Hilton was the editor who published this page.
Editor’s want-list for this marker. A wide shot of the marker and its surroundings. • Can you help?