They straddle the Dalles of the St. Croix River, a deep basalt gorge with glacial potholes and other rock formations.

[2][3][4] In the Cambrian period of the Paleozoic Era, between 530 and 470 million years ago, the region was covered by a shallow sea which deposited sandstone and siltstone atop the basalt.

This epoch of the Cambrian, the Furongian, was originally called the Croixian in North America because the layers exposed in this area were its type locality.

[5] The presence of older glacial deposits south of the Interstate Park demonstrates that the Laurentide Ice Sheet repeated glaciated it and surrounding areas over the Pleistocene Epoch.

Within Wisconsin, these older glacial deposits consist of remnants of, highly weathered dark-gray loam till and lake sediment with reversed magnetic polarity and a deeply weathered, pre-Sangamonian Stage, reddish-brown, sandy loam till with normal magnetic polarity.

Both the landforms and deposits related to these earlier glaciations have been either eroded or buried by the latest advance of the Laurentide Ice Sheet over this area of the Last Glacial Maximum.

As the Lake Superior Lobe retreated, the Brule outlet opened and the higher Moose Lake was abandoned as a lower Duluth level was quickly established by massive and sudden outflow through the lower Brule outlet and down St. Croix River.

In and surrounding Polk County, Minnesota, geomorphic and stratigraphic relationships evidence exists for at least two drainage events.

This lag layer overlies an unconformity eroded into older glacial till, lake sediment, or bedrock.

[8] The deep inner channel, which includes the Dalles, that forms the modern St. Croix River valley was excavated by a second drainage event.

This demonstrates that the St. Croix valley is slightly younger than the Kettle River valley and Moose Lake outlet and was cut by water flowing out of the lower and younger Brule outlet of the Duluth level of glacial Lake Duluth.

[5][10] When the massive flow of water through the Brule outlet and down the St. Croix River ceased, the potholes became exposed to the air.

[12] Most of the vegetation today is second-growth forest, with some sections dominated by maples and basswoods and others by eastern white pines.

[14] The first Europeans to pass through the Dalles were Daniel Greysolon, Sieur du Lhut, and his 1680 expedition, though he made no particular mention of the site Fur traders used the river extensively, and a French fort was located near the Minnesota campground in the early 18th century.

A crew of 175 men working 24 hours a day under electric lights took six weeks to break the jam, during which time several mills downstream went out of business.

[14][15] To control the water flow and prevent further catastrophic jams, the Nevers Dam was built upstream in 1890 in what is now Wild River State Park.

[15] In the 1860s businessmen from St. Paul proposed mining the basalt of the Dalles to make gravel, a plan which galvanized interest in protecting the area.

[16] A travel agent named George Hazzard became the leading advocate for a park, and gained the support of newspapers, several landowners in the area, influential people like W.H.C.

[18] In 1906 the commissioner of the Minnesota park asked a family from Stillwater to conduct boat tours of the Dalles.

[21] Using basalt quarried in the park by the CCC, they built restrooms, picnic shelters, water fountains, and retaining walls.

These historical structures are clustered in two separate areas of the Minnesota park: in the campground and near the Glacial Gardens.

The two contributing structures are the 1937 stone curb in the parking lot and a 150-foot-long (46 m) retaining wall built in 1938 at the south end of the Glacial Gardens.

The Combination Building was based on a design also used in Whitewater State Park in 1938, though the use of different local materials gives them a very different appearance.

An open-sided picnic shelter with a free-standing metal fireplace was actually built in 1980 and is considered a non-contributing property to the district.

An investigation found that the basalt pinnacle was toppled by vandals using crowbars and possibly a hydraulic spreader.

[10] A long-standing concession offers scenic boat cruises and canoe and kayak rentals with shuttle service back from near Osceola, Wisconsin, and William O'Brien State Park.