Gjern Bakker were formed as a buffer in front of a glacier, which was later crisscrossed by  large quantities of meltwater during the last ice age. Thus a subglacial stream trench from Gjern to the Gudenå was created.
The highest point is Åshøj which is 104 meters.
The hills were originally covered by forest, but most trees disappeared due to logging for firewood or for house-building. Later the area has been planted with mainly fir and pine