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Interstate Park

Home Ice-Walled Lake Plain	The present landscape at Interstate Park, which is located on the border of Minnesota and Wisconsin in the cities of Taylors Falls and St. Croix Falls, respectively, is the result of glaciations that occurred during the last Ice Age. The most recent period of glacial advances and retreats is known as the Wisconsinan Stage which began around 70,000 years ago and ended roughly 10,000 years ago. As the glaciers overlying northern Wisconsin retreated, the resulting meltwaters and debris altered the landscape, forming a wide variety of glacial landforms and features that can be observed today.
Esker History of Interstate Park Eagle Peak Trail River Bluff Trail Potholes Cascade Falls
	Dalles of the St. Croix River
	The steep-walled gorge known as the Dalles of the St. Croix River is one of the most striking and scenic areas of Interstate Park. As the last of the great continental ice sheets receded 10,000 years ago, meltwaters surged from the massive ice lobe that excavated Lake Superior, forming Glacial Lake Duluth at the ice margin (see map at right). The waters of Glacial Lake Duluth drained into the St. Croix River where they collided with layers of ancient volcanic bedrock. The rock at the bottom of the river broke away in pieces faster than the rock from the sides due to the tremendous pressure of the torrents of water. The combination of the erosive character of the glacial meltwaters from the retreating ice sheets and the St. Croix River carved the deep, steep-walled gorge through the bedrock that attracts both tourists and scientists alike.
	Downstream view of the St. Croix River from the Dalles of the St. Croix.	Upstream view of the Dalles of the St. Croix from the Minnesota side of Interstate Park.

	Presence of Potholes and their Formation at Interstate Park
	One of the most unique features at Interstate Park is the quantity, size, and variety of potholes that occur in the basaltic bedrock on either side of the Dalles of the St. Croix. As the glacial meltwaters drained into the St. Croix River, they flooded the present river valley, at times deviating from the main current and swirling to form small whirlpools or eddies. Sand, gravel, and rocks became trapped in the spinning whirlpools and spiraling eddies, and over time, the grinding action of these abrasive materials drilled circular or cylindrical holes known as *potholes* into the underlying bedrock. Over 100 potholes can be seen at Interstate Park, some of which are more than 100 feet above the present river level.
	Diagram of the evolution of pothole formation.
	The specific abrading materials which carve the potholes during their formation are called *grinders* and are frequently found in the bottom of the pothole. With the continued downward spiraling action of the grinder, the pothole becomes deeper and the once angular surface of the grinder is smoothed (see diagram at right).
		Above, a view inside a pothole is shown. The grinder which formed the pothole is visible at the base and the corkscrew pattern on the walls of the pothole was cut by the sprialing current of water. At left, smoothed grinders are depicted in a display at the Interstate Park Information & Ice Age Interpretive Center. A field notebook is included for scale.
	The size of the potholes carved into the basaltic bedrock at Interstate Park varies greatly, but generally the larger and deeper potholes are located on the Minnesota side of the park to the west of the St. Croix River. The considerable range in size includes small, shallow depressions of a few inches in both diameter and depth, to large, deep, cylindrical holes measuring up to 25 feet in diameter and 80 feet in depth.

	Compilation of potholes of varying sizes seen at Interstate Park. A pen and field notebook are placed in some potholes for scale.
	Comparsion between a shallow pothole (at left) and a much deeper pothole (at right).		Example of a pothole with a very large diameter. The dashed yellow line highlights the rim of the pothole which extends beyond the photograph.

	Some potholes are found grouped in clusters in the basaltic bedrock. When potholes formed close together sometimes they merged into one larger pothole as the grinders and swirling water cut them larger and larger. The increase in size wore away the wall between them, forming a single pothole.

			At left is a view of a "half-pothole" cross-section. This pothole was carved deep into the bedrock but because other potholes were located nearby, the glacial meltwaters wore away the wall between them leaving what we see today. The blue lines represent the original extent of the pothole before it merged with others. This pothole is located in Devil's Parlor on the Minnesota side of Interstate Park.




Created By: Jennifer Mikolajczyk mikolaja@uwec.edu December 11, 2006