4
GENERAL GEOLOGY

Terrace area lies along a part of the eastern contact zone of an
elongated body of intrusive rocks forming part of the so-called Coast
Range batholith. The Coast Range mountains, which delimit these intru-
sives, run in a northwest direction along the Pacific coast in a belt roughly
90 miles wide. The eastern contact of the intrusives is bordered in this
area by Mesozoic volcanic and sedimentary rocks. Rocks of three periods
are present, Triassic, Jurassic, and Cretaceous. Of these the Jurassic
sediments and volcanics, designated the Hazelton group, are the most
important as they cover almost half of the area. Outcrops of the older
Triassic rocks are scattered and cover only a few square miles. The
Skeena sediments of Cretaceous age may occupy Cedar River valley in the
northwest part of the area.

Table of Formations

Formation Lithology

Recent and Gravel, sand, silt, boulder clay, glacial

Pleistocene drift
Lower Cretaceous Skeena Shale, sandstone, conglomerate, coal
Early Cretaceous Dykes

to Jurassic

Coast Range Granite, granodiorite, quartz diorite,
intrusives diorite, ete.

Sedimentary Division:
Sandstone, argillite, quartzite, greywacke,
Jurassic Hazelton group slate, tuffaceous sandstone, conglomerate
Volcanic Division:
Andesite, breccia, tuff, rarely limestone,
and argillite

Triassic Cherty quartzite, argillite, crystalline
limestone, conglomerate

TRIASSIC

The Triassic rocks consist of heavy beds of crystalline limestone,
cherty quartzite, slate, and argillite with some associated beds of breccia
and tuff. A chert-pebble conglomerate and a limestone-boulder conglo-
merate also occur. The conglomerate, comprised of chert pebbles, also
contains a few pebbles of volcanic rock. The boulders of the other conglo-
merate are large and well rounded and are believed to have been derived
from a Carboniferous or Permian formation.