Major down-cutting that created the St. Croix River gorge and the potholes at Interstate State Park were formed by the tremendous force of the glacial meltwater.
We know that torrents of water rushed through the valley as the glacier left the area and continued to melt to the north. The large volume of meltwater filled and overflowed the Lake Superior basin creating glacial Lake Duluth, because the eastern end (the current outlet of Lake Superior) was still blocked by the glacier. The water level is believed to have been 400-500 feet higher than the current level of Lake Superior. The down-cutting and pothole formations could possibly have happened when glacial Lake Duluth suddenly lowered by as much as 30 feet in a very short time when a new outlet for the water opened up and rushed down what is now the St. Croix River valley. It also could have happened when other glacial lakes to the west or in the Upper Peninsula of Michigan suddenly drained into Lake Duluth and then down the valley.
The date of such a mega-flood can't be determined because the water swept away the deposits that may have provided some clues. We do know that the flood water reached at least 900 feet in elevation because the rock of Eagle Peak (in the park; located to the south) has been eroded by the river.
The potholes found at Interstate State Park were drilled into the 1.1 billion year old lava flow of basalt; (see GC5NEFD - It's Earth's Fault). The potholes, however, are only about 10,000 years old.
Imagine the amount of water that a mile-thick sheet of ice would release when it melted. Then imagine all the water rushing down the river at this location. Water alone would not be able to drill into the basalt to create the potholes. The water also carried a huge amount of silt, sand, gravel, and rocks. With enough water power and debris, eddies in the water will gradually carve a shallow hole in the rock at the bottom of the river. If a larger stone gets into the shallow hole, the swirling sand and silt will carve away at it until it gets smooth and round enough to roll around. This larger stone then drills further into the basalt and gets worn down in the process.
When the river finally became slower and smaller, the wearing process stopped and 
the pothole and ball-shaped "grindstones" were left high and dry. Such grindstones are frequently found in potholes. Some, are also found miles away, possibly because they were valued and exchanged by Native Americans in the past. The stone shown here was found in the Town of Seymour, Brown County, Wisconsin in 1943. It is approximately nine inches in diameter. Some have called it a "game ball." Not sure what game would have been played with a heavy stone such as this.
After viewing the potholes in the area, move on to the St. Croix River view waypoint listed and look at the river to the west and a little north. You will see turbulence of the water. This is the same type of turbulence that created the potholes, but they were created when there was much more water and far greater turbulence. Never-the-less, small potholes are likely being formed below the water surface today where you see the eddies.
Before you leave the park, be sure to stop by the Interpretive Center to see some grindstones by the monument outside. There is also a video inside the Center that describes pothole formation. The Center is open most days of the year from 9 am to 4 or 5 pm; winter hours may vary.
IATCC is the cache designation to highlight a series of EarthCaches along the Ice Age National Scenic Trail grouped into a special category called "ColdCache."
The Ice Age Trail is one of eleven National Scenic Trails designated by the National Park Service. This unique trail is entirely within the state of Wisconsin and follows along the terminal moraine of the most recent glacier which retreated about 10,000 years ago.
This project is supported by the Ice Age Trail Alliance (IATA). The goal is to bring more visitors to the trail and promote public awareness, appreciation, and understanding of Wisconsin’s glacial landscape.
The IATA has created an awards program to encourage visits to the trail and ColdCache sites. This awards program is separate from, and in addition to, any other Geo- or EarthCache awards program.
A current list of approved ColdCache sites can be found on the “IATCC Bookmark List”. More information on the Ice Age Trail Atlas, the Companion Guide, the ColdCache Project and Awards Program can be found on the “ColdCache Webpage”.
DNR Permission:
The Geocache Notification Form has been submitted to Kurt Dreger of the Wisconsin DNR. Geocaches placed on Wisconsin Department of Natural Resource managed lands require permission by means of a notification form. Please print out a paper copy of the notification form, fill in all required information, then submit it to the land manager. The DNR Notification form and land manager information can be obtained at: www.wi-geocaching.com/hiding
Resources:
Geology of the Ice Age National Scenic Trail; by David M. Mickelson, Louis J. Mahler Jr., and Susan L. Simpson
Minnesota Department of Natural Resources; Department of Parks and Trails; Interpretive Services, 2009, by Dave Crawford
Geology Walking Tour; Wisconsin Interstate Park; St. Croix Falls, Polk County, WI, by Ryan Peterson and Dr. William S. Cordua; University of Wisconsin-River Falls, Department of Geology
To claim this find:
Compare the large pothole to those seen from the "Pothole Overlook."
1. Describe the potholes in terms of elevation, size, depth, and direction (if any) of the drill-holes.
2. Based on this information; what, if anything, can you say about the grindstones and/or the action of the water at the time the potholes were created.
3. How large are the grindstones found on display near the Visitor Center?