The oldest rocks at Interstate Park belong to the Keweenawan Supergroup, a thick sequence of volcanic and sedimentary layers that filled a segment of the Midcontinent Rift System roughly 1.1 billion years ago. At least ten separate basaltic lava flows are exposed within the park, remnants of an ancient rift valley that once tore across the continent's interior. Later, during the Cambrian period between 530 and 470 million years ago, a shallow sea deposited sandstone and siltstone atop the basalt. Scientists originally named this epoch the Croixian because the layers exposed here served as its type locality -- the reference standard for the entire geological classification. The most recent chapter belongs to the Cenozoic: as the Laurentide Ice Sheet retreated between 19,000 and 14,000 years ago, glacial Lake Duluth released massive outburst floods down the Brule outlet and into the St. Croix River, carving the deep inner channel known as the Dalles and drilling the park's famous potholes through sheer hydraulic force.

Steamboat service to Taylors Falls began in 1838, and by the time a railroad connection arrived in 1880, the Dalles had become a popular tourist destination. A travel agent named George Hazzard saw both a natural wonder and a political opportunity. He launched a campaign to protect both sides of the gorge, rallying newspapers, local landowners, and influential figures like W.H.C. Folsom. The Minnesota Legislature eventually introduced a bill calling for the creation of the State Park of the Dalles of the St. Croix, urging cooperation with Wisconsin. Hazzard and his allies faced stiffer resistance across the river, but in 1900 they succeeded: Wisconsin designated its side as its very first state park. The collaboration made Interstate Park the first parkland in the nation shared between two states -- a boundary-crossing achievement that matched the boundary-crossing geology beneath it.

The 1930s transformed Interstate Park from scenic curiosity into a developed recreation area. U.S. Route 8 was blasted through the Minnesota side in 1931, and the state built stone overlooks and guardrails along the highway. In 1935, Civilian Conservation Corps Company 633 arrived and spent years constructing roads, trails, picnic grounds, a beach and beach house on Lake O' the Dalles, and retaining walls built from local stone. Their work earned recognition as significant examples of National Park Service rustic design. The CCC and WPA Historic District on the Minnesota side preserves six contributing properties, including a 1939 Refectory that now serves as the visitor center, a 1938 retaining wall at the Glacial Gardens, and two rustic restrooms from the 1920s that actually predated and inspired the federal program's design choices.

Among the park's rock formations, one landmark stood out for generations: the Devil's Chair, a basalt pinnacle on the Minnesota side that resembled a high-backed throne. In April 2005, the formation collapsed. An investigation determined that vandals had used crowbars and possibly a hydraulic spreader to topple the ancient pillar. Despite a reward for information, the culprits were never identified. The loss underscored what makes Interstate Park irreplaceable -- its formations took glacial floods and a billion years of geological time to create, and they cannot be rebuilt. Today, all potholes and sensitive rock areas are off-limits to climbing. Located just an hour's drive from the Minneapolis-Saint Paul metropolitan area, the park draws visitation numbers comparable to many national parks, a level of popularity it has maintained since the 1930s.