Most of Mount Slesse is made up of granitic rocks related to the Chilliwack batholith, which intruded the region 26 to 29 million years ago after the major orogenic episodes in the region.
The majority of Mount Slesse is comprised mainly of granitic rocks linked to the Chilliwack batholith, which entered the region between 26 and 29 million years ago after a series of major mountain-building episodes.