Alhedens Mergelforsyning mined the marlstone and borrowed 20 prisoners from Horsens to do the hard work, and later 10 more prisoners were added to the workforce. Because of this, a new branch of Horsen prison was built in Kølsen.
The marlstone was mined by hand. Every day, 20 carts were loaded with 10 tons each. During the 28 years the marl pit was open, more than 1,2 million tons of marlstone was manually loaded into the carts. The pit closed in 1934. In this way, the hill Klinthøj was spread all over the heath. Today, only the wardens house remains, along with a flat piece of land.