Wedge Hill was once an island in the glacial St. Croix River, with the surrounding area more than 40 feet underwater as trillions of gallons of glacial meltwater carved the wedge-shaped hill and a river channel whose west bank lay one-half mile away and east bank more than one mile away. As the glacial waters retreated, Wedge Hill remained on a terrace above the present-day St. Croix River, where over thousands of years native plant communities took hold and periodic wildfires favored deep-rooted flowers and grasses while limiting most trees and shrubs except occasional oaks with fire-resistant bark, creating an oak savannah later altered by agriculture, the steel plow, cultivation of prairie soil, and the introduction of non-native plants as the area became known as Wedge Hill Farm. Carl and Ethel Johnson moved onto the 190 acre farm in 1934 and worked it until Ethel's death in 1950, emphasizing soil conservation through crop rotation, contour plowing, and liberal use of animal fertilizer to make the light sandy soil productive for a dairy herd of about twenty cows as well as poultry, pigs, oats, corn, and alfalfa. Cattle grazing among the oaks kept other trees and shrubs from taking over, and the family also used Wedge Hill for winter recreation, with sliding, downhill ski runs through pasture gates, and skating rinks cleared on ponds. Today Wedge Hill is part of William O'Brien State Park.