Geology

About 1.1 billion years ago, the area that is now Straight Lake State Park was part of a mid-continental rift. Magma oozed out of the rift forming basalt, which can still be seen in several areas of the park. Many years later, during the last Ice Age, many of the park's features, as seen today, were formed. 

While visiting Straight Lake State Park, look out for these unique glacial features found along the trails. This information is a summarized from a geology hiking guide for the Ice Age Trail, courtesy of the UW-Eau Claire Geology Department. 

Tunnel Channel 

The most distinctive glacial feature in the park is the tunnel channel, formed when pressured water flowed under the glacier to cut the channel. The channel is a long valley that extends 7.5 miles and contains Straight Lake, Rainbow Lake and the Straight River. You can see the channel on the Ice Age Trail to the east of Rainbow Lake. 

Boulder Valley

Along the Ice Age Trail in the park is a valley strewn with large boulders, many up to 5 feet in diameter. Most of these are basalt: a dark volcanic rock. Glacial meltwater eroded the valley and removed finer sediments, leaving the heavy boulders.

Look closer! Basalt rock often contains small holes (former air pockets in lava) that have been filled with other sediment over time. When you come across this dark gray rock, look for these pockets. 

Glacial Erratics

Erratic rocks can be found throughout the park. These rocks were transported by glaciers and range in size from pebbles to large boulders. Glacial erratics don’t match the bedrock naturally found here. Some come from as far as Canada. 

Kettles and Hummocks

Hummocky topography is the most common terrain in the park. Hummocks are small hills. They are often interspersed with kettles: depressions or pits in the land that can be filled with water (called kettle lakes) or remain dry. These are both features created by the glaciers. 

Pitted Outwash Plain

South of 270th Ave., the trail runs through a pitted outwash plain, which consists of a gently sloping plain with scattered kettles (pits). A pitted outwash plain forms as meltwater flows away from the edge of the glacier and deposits sand and gravel over broken-off ice blocks. When the buried ice melts, the sediment collapses to form the “pits” on the sloping surface.

Moulin Kame

A moulin kame is a distinct, small conical hill that rises nearly 20 feet above the ground. Look for it north of State Highway 48. The moulin kame formed as meltwater and sediment flowed down a vertical shaft (moulin) in the ice and deposited sediment to form a cone-shaped moulin kame on the glacier bed.

Esker

The Ice Age Trail north of State Highway 48 follows an esker; a narrow, raised snake-like strip of land. This 50-foot high esker is part of a 5-mile long esker between Straight Lake and Big Round Lake. Sometime after meltwater eroded the Straight Lake tunnel channel, water flow decreased, and sediment was deposited in a smaller channel.