Interstate State Park: A Brief Geologic History
please note that this is a very brief geologic history, if you wish to learn more please read "The Geology of the St. Croix National Scenic Riverway," by Adam Cahow
1.2 Billion Years Ago- 1.0 Billion Years Ago
Situated along the St. Croix River places Interstate State Park in a unique location due to the areas dynamic geologic past. The entire length of the St. Croix River is underlain by an ancient rift zone in the Earth's Crust that stretches from the Lake Superior region to northern Kansas. This unique geologic feature known as the Keweenawan or Mid-Continent Rift is also associated directly with a high gravity anamoly. This anamoly is known as the Mid-Contient Gravity High. These were both created when large fractures in the Earth's crust developed (commonly called rifts) which allowed for extensive volcanic activity to occur in the area. Hundreds of large extensive lava flows began between 1.2 and 1.0 bya, most of these flows involving non-explosive volcanism. These flows, or flood basalts, were very fluid and spread out over large areas leaving deposits of nearly equal thickness. The rift zone also caused a synclinal structure to develop in the Earth's crust. This structure later underwent extensive faulting and caused massive subsidence in the rift valley. Sediments were deposited on top of these sinking areas at about the same rate as the subsidence. Today a veritcal tilt can be seen in the surronding rock which tilts toward the Southwest at 15 degrees from horizontal in Interstate State Park. The cliff rocks that are exposed in Interstate State Park are the remnant of these ancient flood basalts which were incised into by glacial meltwaters exposing dramatic basaltic cliffs almost the same as what is seen today. The Dalles, as this section of the St. Croix is called, were continually buried and eroded after the Keweenawan events, but the actual formation of much of the exhumed landscape was created in the Early Cambrian period before the Late Cambrian Sea covered the area in more sedimentary deposits. This landscape was only to be exhumed again by the remergance of the river eroding out the sedimentary deposits which had covered the buried erosional landscape. Eagle Peak is an example of one of these exhumed features which has been buried by sedimentary deposits only to remerge because of erosional processes.
(Figure 1 Left, Keweenawan Rift. In this image the Keweenawan Rift or Mid-Continent Rift can clearly be seen by the dark linear area extending from Lake Superior to Northern Kansas. This rift spilled out large basalt flows into the surrounding locations creating large pancake shaped formations. Evidence of these Basalt flows can be seen in the rock cliffs in Interstate State Park. From www.geo.msu.edu/geo333/rift_valley)
(Figure 2 Right, Eagle Peak. In this image the remnants of the precambrian basalt flows can be seen in the rocks. These are rocks are part of the exhumed landscape that makes up Eagle Peak.)
- 515 Million Years Ago
About 515 million years ago the entire St. Croix region was submerged beneath a shallow sea. This sea changed level many times but deposited 100's of feet of sediment along the area. This buried the Precambrian landscape that had been formed and carved out at Interstate State Park. Sea level changed numerous times for about the next 100 million years. Until the area was raised above sea level around 345 million years ago. After this period the landscape was under constant erosional processesdue to this uplift until the Ice Age began.
- 70,000 - 10,000 years ago
It is important to recognize that there have been many glacial advances and retreats over the last 2.0 million years, many of which glaciated much of modern day Wisconsin. About 70,000 years ago the last major ice advance began and is the most influential on todays modern landscape. This glacial stage, known as the Wisconsinin, was the greatest influence on the glacial landforms we see today. At its farthest reach the Laurentide Ice Sheet stretched from the Arctic Circle to Iowa and the Ohio River. There were many lobes that extended southward from the ice sheet but the main lobes that concern the formation of the landforms we see in Interstate State Park were the Superior, Chippewa, and Granstburg (The Granstburg Lobe is considered a sublobe). During the Wisconsinin the St. Croix glacial advance, which occured 20,000 years ago and ended around 15,000 years ago, extended from Lake Superior southwest reaching the modern day area of Minneapolis and St. Paul at its farthest extent. This advance had the largest impact on the landscape surrounding the St. Croix River Valley including Interstate State Park. Eventually this advance stopped and began to retreat across the landscape. There were various advances and retreats during this time creating numerous moraine deposits and other features across the landscape. The esker just north of Interstate State Park, on the Wisconsin side of the river, is a remnant from this glacial lobe. Also the Ice Walled Lake Plain that makes up the Turtle Lake area is a remnant of this as well. Interstate State Park was completely overlain by glacial ice during the Wisconsinin and evidence of this can be seen in the Keweenawan age basalt. Straitions and Chattermarks can be seen scattered across the rock faces all pointing in the direction of southwestward flow. The Grantsburg sublobe also played a large role in contributing to the landscape in the park as it dammed the St. Croix River just north of the park. This inturn formed glacial Lake Granstburg which partly drained through the Dalles area. As all of these glacial lobes retreated huge amounts of meltwater spilled out of glacial lakes and into river valleys. In the case of the St. Croix Dalles at Interstate State Park, both the draining of glacial Lake Duluth and glacial Lake Grantsburg supplied the tremendous amount of water needed to erode the gorge that is present today. The flooding would have been so immense that the carving of such a gorge would have been insignificant to the total amount of energy the river had at the time. Much of this erosion would have been due more to breaking off of rock rather than through simple abrasion. This is due to this basalts unique jointing. Also, the erosion worked at a much faster rate vertically than horizontally causing the deep gorge with steep cliff walls that are present today.
(Figure 2. Left, Ice Age Deposits of Wisconsin. Wisconsin has experienced numerous glacial advances and retreats. Evidence of this can be seen by the deposits that have been left behind. The Wisconsinin Stage, being the last major advance, left most of what we see today and shaped much of the modern Wisconsin landscape.)
(Figure 3. Right, The path of meltwaters from glacial Lake Duluth as it passed through St. Croix River Valley through the Dalles carving out the tremendous and scenic gorge we see today. )
This unique geologic history made this spot impossible to pass up for the Wisconsin State Park serivce. Created in 1900, Interstate State Park is the oldest park in the state of Wisconsin. Visited by thousands of visitors each year Interstate State Park remains one of the unique attractions of the St. Croix River Valley and to the states of Minnesota and Wisconsin. Today you can visit parks on both the Minnesota and Wisconsin side of the river and enjoy the area and its sites. For more information on the park today visit either Interstate State Park of Minnesota or Interstate State Park of Wisconsin.