1.1 billion years ago, a series of volcanic eruptions from the Midcontinent Rift System sent at least ten separate lava flows across the region that now includes Interstate Park. These flows hardened into basalt, which fractured at several joints. In the Cambrian period, between 530 and 470 million years ago, the region was covered by a shallow sea which deposited sandstone and siltstone atop the basalt. At the end of the last glacial period 10,000 years ago, torrents of water flowed south from the melting glaciers, carving the St. Croix River Valley. The soft Cambrian sediments were easily eroded into a wide valley, but the hard basalt was resistant to erosion and when it reached the hard basalt floor, the river was channeled into a deep, steep-walled gorge. The river switched courses over time, carving several small ravines. At one stage the flow entered the current channel on the Wisconsin side below the modern gorge, hollowing out an area which is now a backwater lake called Lake O’ the Dalles. When the river began carving the main gorge, it intersected an old joint running through the basalt. The water naturally followed this easier path, resulting in the sharp bend in the St. Croix River at Angle Rock.The walls of the Dalles rise from 50 to 250 feet (76 m) above the river.
The Glacial St. Croix River carved a series of terraces along its banks. Sand and gravel were occasionally caught in eddies or whirlpools and were swirled around with such force that they drilled holes straight down into the rock. Larger rocks caught in the spin would polish the shafts smooth. The Glacial Gardens area on the Minnesota shore contains more than 80 of these glacial potholes, the greatest concentration in the world. The volume of the St. Croix River eventually lessened, exposing many of the potholes to the air. Dirt, vegetation, and rainwater have collected in them, disguising their true depths. Some have been excavated; one, the Bottomless Pit, is 10 feet (3.0 m) wide and 60 feet (18 m) deep, the deepest explored pothole in the world. Other unexcavated potholes in the park are even wider, suggesting that they may be deeper as well.