Looking up at Cultus Mountain you can see the expansive concave depression.
This was shaped by an ancient landslide event that took place sometime during the past
15,000 years after the last Ice Age. The full range of this landslide event has been
detected through LIDAR mapping. The landslide flowed westward between Pigeon Creek and
Cold Spring Creek, splitting into two sections that stretched for more than 1.5 miles.
A later, more recent landslide evident through LIDAR mapping flows south across the older
landslide. The slide is believed to have resulted from a failure of the weak metamorphosed
sedimentary rock, Darrington Phyllite; landslide debris also includes glacial sediments.
Landslide events in this area may have been triggered by seismic activity along the Devil’s
Mountain Fault Zone located south of Fire Mountain Scout Camp.
The Devils Mountain Fault Zone, located just south of Fire Mountain Scout Camp, is an
approximately 78 mile long fault line that runs east to west, through the town of
Darrington and westward to Vancouver Island, within 2 miles of downtown Victoria,
British Columbia. It is an active fault zone on which at least one large earthquake has
taken place within the past 2,000 years. Recent research estimates that this fault has the
potential to generate a strong earthquake of as large as magnitude 7.5 on the Richter scale,
generating catastrophic impact across the Puget Sound region, western Washington, and
Crystal Falls, located at Fire Mountain, offers a beautiful and vivid representation of the
fascinating geology of the Walker Valley. The falls are formed by an andesite escarpment,
showing exposed coal and sandstone. Beyond the falls in the foothills of the Cultus Range
is the Devil’s Rock Garden, an isolated talus field with massive boulder formations. Located
on land owned by the Washington Department of Natural Resources, the Devil’s Rock Garden was
the site of Fire Mountain’s rock climbing program during the 1980s.
The Walker Valley is internationally famous for its mineral specimens, including large geodes
containing crystals of amethyst, clear quartz, calcite, siderite, and gem-quality agate.
It is the only place in its surrounding volcanic region in which geodes have been found,
reflecting the Valley’s unique geologic history. Geologists have suggested, based on the
Walker Valley’s minerology, that the following sequence of events led to the creation of
the geode zone:
First, sandstone, conglomerate, and siltstone of the Chuckanut Formation were
deposited along a river flood plain about 40 million to 55 million years ago,
before the emergence of the North Cascades mountain range.
Later, magma intruded along a fault that developed when these sedimentary rocks
were uplifted as the North Cascades rose, potentially related to activity along
the Devils Mountain Fault Zone.
As the molten rock cooled, cavities called vesicles developed from the release of
water vapor and other gases. The rock eventually became andesite, and during its
final stages of cooling, shrinkage caused narrow fractures to develop that
intersected the vesicles, allowing warm water filled with minerals to percolate
through them. Walker Valley mineral deposits primarily contain minerals composed of
silica, calcium, and iron.
Later, a large hydrothermal event, potentially triggered by seismic activity, created an extensive zone of breccia rock.
Over time, the surrounding sedimentary rock eroded, leaving the andesite as a ridge.