The first volcanic event 29 million years ago formed intrusive rocks of the large Chilliwack batholith, which extends south into the U.S. state of Washington. In Canada, the batholith consists of rocks ranging from hypersthene quartz gabbro to albite granite. Three main plutons are present. The oldest and most extensive is an irregularly-zoned pluton composed of quartz diorite at the margins, grading inward through granodiorite to a small core of granite. This pluton is exposed on both sides of Chilliwack Lake, a narrow north-south trending lake in the Cascade Mountains. Two younger plutons, consisting of nearly homogeneous leucocratic biotite quartz monzonite, are situated north of Chilliwack Lake and east of Slesse Mountain.