sentences:  Through their full extent in Otter Tail County, the hills have steeply sloped sides covered by prairies and deciduous forests, primarily maple and oak.The hills are formed by the convergence of the eighth and ninth marginal moraines formed by the Wadena and Des Moines lobes of the Wisconsonian glaciation.This moraine forms an arc about ten to twenty miles (16 -32 km) wide and 200 miles (320 km) long.