The area is underlain by a roof pendant of metamorphic rocks, with north-south steep to vertical foliation and steep joints striking north-east and north-west. These are primarily metavolcanic and metasedimentary amphibolite, hornfels, quartzofeldspathic schists, gneiss and a transition zone of mixed rocks near the eastern contact with the pluton. Granodiorite and variable tonalites intrude the metamorphic rocks. A boulder conglomerate unconformably overlies these basement rocks in the east-central and west-central part of the area. Mappable lithologic units are plotted at a scale of 112,400 dovering an area of 4.1 square miles.