Gjern Bakker hills emerged as a rim of land in front of a glacier, which was cut through by large volumes of meltwater during the last Ice Age, creating a tunnel valley from Gjern to the River Gudenå. The highest point, Åshøj, is 104 metres. The hills were originally covered by forest, but most of it disappeared due to logging for firewood and house-building. The hills have since become covered with heather. All that remains today of the original forest is oak thickets, especially on the slopes. Due to the extremely hilly terrain and the sandy soil, there has been very little farming on the hills. In 1893, local residents therefore suggested to the Danish Heath Society that the hills should be planted up, as they believed that, due its outstanding natural beauty, the area deserved a better fate. The area has since been planted with mainly spruce and pine.