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Welcome to the next edition of Doc.’s Roadside Geology Tour: Washington County !
The listed coordinates will bring you to a Geological Marker at a scenic overlook offering yet another breathtaking view of the beautiful St. Croix River. This particular site is known as Lookout Point and it marks the northern terminus or end point for Lake St. Croix.
Lake St. Croix is a riverine lake which formed as the result of the natural sediment deposition and damming process at the confluence of the St. Croix and Mississippi Rivers at Prescott, WI and extends northward past Stillwater, MN.
The Mississippi, Minnesota and St. Croix Rivers were carved over 11,000 years ago as proglacial lakes Agassiz and Duluth drained at the end of the last Ice Age. These glacial lake outburst floods were cataclysmic in magnitude. Torrents of water raged across this region for over a year before these giant lakes were drained. Deep gorges and valleys were carved. Upper substrate was eroded. The force of the these rivers cut deeply into the faultines down to the most resistant layers of bedrock.
These active glacial rivers are classified as a “young” or “youthful river”. A youthful river is a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. The gradient is controlled primarily by tectonics. The discharge is controlled by climate. The sediment load is controlled by various factors including climate, geology in the headwaters and the stream gradient. Fast moving water is capable of carrying, or suspending, a much greater volume of sediment than slow moving water, thus enhancing it ability to cut a path through varying stratas.
As time passed and the glacial lakes drained, the volume of water flowing through these channels has greatly reduced. These rivers are now classified as a “mature river.” The gradient has become less steep than those of youthful rivers and flows are slower.
A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper.
As the speed of the water slows, the ability to suspend particulate matter also declines. The original deeply incised channels of the river valleys begin to fill with sediment. Sediment begins to accumulate and form deltas where tributaries meet the main river flow. This ongoing cycle continues to alter the flow and course of the maturing rivers. These tributary fans dammed the main rivers, forming riverine lakes.
The gradient and flow rate of each river is unique and subject to weather and watershed. The flow rate of the Mississippi was so greatly reduced that the ongoing deposition of sediment nearly dammed the St. Croix River at the confluence at Prescott, WI. The accumulation of river debris and sediment is now known as Point Douglas Park. The hydraulic damming action of the reduced flow, and subsequent backfilling, caused the formation of Lake St. Croix which extends from Prescott, WI northward through Stillwater, MN.
As the flow of the St. Croix River was nearly dammed off by the continuing sediment deposited by the ever slowing Mississippi, the waters began to backflow, creating additional erosion off the limestone and sandstone walls, thus widening the river valley.. Evidence of a large eddy current creating lateral erosion is visible as you look downstream from the marker. The prominence you are standing on resisted the erosive forces of the backflow and created a natural stricture or teminus point for expanding Lake St. Croix as the volume of the river began to subside.
The Geological Marker reads as follows:
“This site of this tablet marks the northern limit of Lake St. Croix, impounded by the natural dam of sand and gravel, made by the Mississippi where it is joined by the St. Croix River, twenty miles below Stillwater. The valley, with it's steep banks, is typical of youthful topography of a young stream and it's size, compared with the river, indicates that a much larger volume of water flowed here when the St. Croix was an outlet of glacial Lake Duluth, the ancestor of Lake Superior. The highway and picnic grounds occupy a river terrace on which the river flowed at an earlier stage. The rock walls of the valley are chiefly sandstones formed in the sea when it covered Minnesota during the Cambrian period 500 Million years ago. Because of the thickness of the beds and the excellence of the exposures along the river, these formations, wherever they appear in North America, are known as the St. Croixian series."
The following tasks are required to log this EarthCache:
Please e-mail the following information:
1. While you are standing at the marker, please enter these coords to a point on the bluff across the river and tell me what the ancient river channel width was. N 45 04.702/ W 092 47.237.
2. Now please enter the following coords, which is the point of where the damming sediments of the Mississippi created what is now known as Point Douglas Park at the confluence of the Mississippi and St. Croix Rivers and tell me what the length of Lake St. Croix is. N 44 44.939 / W 092.48.557.
3. As you approach this geological marker on the highway, please tell me what type of stone you see in the roadcuts as you near this cache and what is the overall colour of the stone?
4. Please post a photo of the views of the St. Croix River or Lake St. Croix from this vantage point, as they are phenomenal and should be shared!
Thanks for taking the time to visit another special spot on Doc.’s Roadside Geology Tour.
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Coordinates are in the WGS84 datum