4mi NW of Helena

The Scratchgravel Hills are composed of the intruding Cretaceous granodiorite satellite pluton of the Boulder Batholith, flanked by metasedimentary rocks of the Belt Super group, sedimentary rock, and alluvium. (Herrin et al., 1987)

The Scratchgravel Hills occupy an area about 3 miles wide and
4 miles long from north to south that begins about 2 miles north
of Helena. They stand out prominently in the western
part of Prickly Pear Valley, and except for a low ridge that extends
to the foothills of the Continental Divide on the west, they are completely detached from the neighboring mountains. On other sides
they are surrounded by the nearly level floor of Prickly Pear Valley,
to which they present an abruptly rising east front. The highest
summit is a peak 5,250 feet above sea level, or about 1,500 feet
higher than the valley.

The northern part of the Scratchgravel Hills is underlain by shale,
sandstone, and limestone of Algonkian (Belt) age. These rocks are
folded, and adjacent to the granite described below they are altered
to quartz-mica schist and other metamorphic products. Extremely
altered patches of them along the east and south borders of the hills
consist largely of contact-metamorphic silicate minerals, such as
garnet, hornblende, and epidote. Along the west side of the hills
is an area of dark-gray diorite-gabbro, which weathers to a dark-red
or brown soil. It consists chiefly of feldspar and a black ferromagnesian mineral and, as determined microscopically by Clarence S.
Ross. The central and southern parts of the Scratchgravel Hills are
composed of quartz monzonite, an intrusive igneous rock that is
popularly known throughout the region as granite. It is a warm-
gray crystalline rock appearing to the unaided eye as if made up
chiefly of white and pale-pink feldspars, with scattered grains of
black ferromagnesian minerals.
(BULL 842, METALLIFEROUS DEPOSITS OF THE GREATER HELENA MINING REGION, J.T. PARDEE AND F.C SCHRADER)

Fossils

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Localities in this Region

Other Regions, Features and Areas that Intersect

References