The northern part of the North Cascades National Park in northern Washington is north of the Skagit River between Mount Shuksan on the West and Ross Lake on the east. The area occupies approximately 500 square miles of steep mountains and thickly forested valleys centered on the precipitous Picket Range.
Old metamorphic rocks and young volcanic and sedimentary rocks are intruded by large masses of granitic rocks that together form a diverse, complicated, but well-exposed geologic section. The granitic rocks are the most abundant in the area; they intrude most of the other rocks, and they separate one suite of rocks in the eastern part of the area from a second suite in the western part. In the eastern part of the area, the oldest rocks are the Custer Gneiss of McTaggart and Thompson, a thick sequence of biotite and hornblende gneisses and schists. We have divided these rocks into three generalized units: light-colored gneiss, banded gneiss, and amphibole-rich gneiss. To the northeast of these rocks lies a metagabbro. This rock type is complex and is made up of several types of gabbro, diorite, amphibolite, ultramafic rocks, and quartz diorite that crop out along the Ross Lake fault zone. To the northeast of these rocks and also along the Ross Lake fault zone is the phyllite and schist of Ross Lake. These rocks are the highly sheared and metamorphosed equivalents of the plagioclase arkose and argillite sequence of Jurassic and Cretaceous age that is so widespread on the east side of Ross Lake. The Cretaceous Hozomeen Group of Cairnes lies along Ross Lake northeast of the phyllite and schist and consists mainly of slightly metamorphosed greenstones with subordinate chert and phyllite. The phyllite in this unit is similar to that in the underlying phyllite and schist of Ross Lake with which it appears to be interbedded. The youngest rocks in the eastern part of the area are the Skagit Volcanics a thick sequence of welded tuff-breccia with some flows and air-laid tuffs. These rocks, which are probably early Tertiary in age, overlie the Hozomeen Group and the Custer Gneiss along the Canadian border.
In the western part of the area the oldest rocks are greenschist and phyllite of Mount Shuksan. These fine-grained foliated and crinkled rocks commonly contain narrow lenses or layers of quartz. They are unconformably overlain by the Chuckanut Formation in the southern part of the area. This formation, which is of Paleocene and Late Cretaceous age, is made up mainly of gently dipping plagioclase arkose with some interbedded black argillite and conglomerate. The Hannegan Volcanics overlie the Chuckanut in the northern part of the area and the greenschist and phyllite of Mount Shuksan in the central part. The Hannegan Volcanics which are of early Tertiary age, consist principally of air-laid volcanic breccias and tuffs, but also include some flows and one small porphyry stock.
The Chilliwack composite batholith consists of several types of granitic rocks, which were intruded at different times in the Tertiary. The two principal rock types are granodiorite and quartz diorite, but small bodies of quartz monzonite diorite, and alaskite are found in many parts of the area. Contacts between the various rock types may be either abrupt or gradational. All rocks of the Chilliwack batholith are younger than the other rock types except the Skagit and Hannegan Volcanics, which are in part younger than rocks of the batholith.
At least two periods of deformation are indicated by the tight folding of the older Custer Gneiss and the greenschist and phyllite of Mount Shuksan and the gentle folding of the younger Chuckanut Formation. At least three periods of faulting occurred, one before and two after the intrusion of the Chilliwack batholith. The two largest fault structures are the Ross Lake fault zone and a long northeast-striking fault that extends for 20 miles from Mount Shuksan down the Chilliwack Valley. The Ross Lake fault zone is pro