Transcript Document

Big Trip to Big Rock Spokane
Valley, Washington
Jon Bromley, Koby Sonnabend, Alex
Deutsch
Figure 1: Weathered biotite muscovite granite schist
Introduction
On Saturday the 24th of
May our group set out to
Big Rock at 12:30 pm and
the purpose of our field
trip to Big Rock was to
observe the rocks, take
samples and photos for a
virtual field trip. At Big
Rock There are very large
rocks sticking into the air,
over a hundred feet in two
outcrops.
Figure 2: view of outcrop 4 from the west
History of the Area
This conservation area contains well over 1,450 acres of land
that is some of the most distinct in the Spokane area. The land
was declared a conservation area in 1966 and was established to
protect wildlife, native plants, and recreation. A major landmark
of this conservation area is the distinct outcrops commonly
referred to as Big Rock. These outcrops were the ones that our
trips attention was directed towards.
The outcrops and the conservation area of the Dishman Hills are
believed to belong to the Prichard Formation Undivided which is
the oldest formation in the Belt Supergroup dating back to the
Precambrian (Griggs A.B,1973).
Directions
Big Rocks’ outcrop location can be accessed from two different
trail heads.
1.) Iller Creek Trail Head, 9001 E Holman Rd. Spokane WA 99206
2.)Steven Creek Trail Head, S. Stevens Creek Rd. Spokane WA
99223
Our group used the S. Stevens Creek trailhead as the location for
beginning our hike.
Figure 3:Map showing route used by group
Site Location
Our group viewed a total of 5 outcrops on our trip. Taking us
through the NW 1/4 of Section 18 onto the SE 1/4 of Section 7 to
the center then out of the NW 1/4 of that same section in the
T24N, R44E within the Spokane SE 7.5-Minute Quadrangle
(Figure 4).
BR-4
BR-3
Pretty View
BR-5
BR-2
Start of Hike
Figure 4: Map of site location (Derkey, 1999)
The Rocks: BR-1
• Located about .3 miles up from the
Steven’s Creek Hiking Trail head.
• This outcrop is a light to medium
gray color. Crystals of quartz,
feldspar and biotite are about 1-2
mm in length. This outcrop is
foliated and is weathered well.
• We believe this outcrop is a quartz
feldspar paragneiss.
• It appears to be tilted at about 2030 degrees dipping to the NW.
Figure 5: Quartz feldspar paragneiss
The Rocks: BR-2
• Located ~200 feet north of BR-1
and ~70 feet higher in elevation.
• Light to medium dark color, fine to
medium grained biotite muscovite
granite.
• Has K-feldspar & plagioclase as well
as small amounts of grey quartz
• This outcrop is well weathered and
even shows rounded weathering of
bedrock suggesting spheroidal
weathering.
Figure 6: Biotite-muscovite granite
The Rocks: BR-3
• This outcrop is light to
medium gray in color.
• well over 100 feet tall.
• well foliated with
fractures throughout
and weathering.
• the outcrop is made up
of granite with biotite,
muscovite, grey quartz
crystals 1-3 mm and
small amounts of
sillimanite with foliation
and few garnets.
Figure 7: View of outcrop 3 a biotite-muscovite granite from the west
looking east.
• Showing the
foliation of BR3
• You can see
the sillimanite
in the granite
layers
Figure 8: Well foliated layers of Cambrian biotite muscovite granite
with sillimanite, plagioclase and grey quartz (Griggs A.B,1973)
The Rocks: BR-4
• The outcrop is the
largest outcrop in
size, over 100 feet
high and over 100
feet in diameter; this
outcrop is very large.
• The outcrop shows
fractures as well as
weathering.
Figure 9: View of outcrop of BR-4; biotite muscovite granite
• With good foliation at
the bottom of the
outcrop; yet foliation
isn't visible above 50
feet, leading us to
believe this is a schist.
• The mineral
composition is
identical to BR-3.
Figure 10: view of BR-4 from the west looking east; biotite muscovite
granite 130 feet tall from north side, 200 feet tall from south side
Figure 11: View from BR-4 standing about 50-60ft tall, looking NW towards BR-5
BR-5
The Rocks: BR-5
Figure 11: boulder of biotite muscovite granite schist
• BR-5 is a site
composed of two
features.
• One feature; the
large boulder
stands about 1015ft tall and is
about 8ft wide.
• Consists of
foliated, coarse
grained granite
mica schist seen
near the bottom
of outcrops BR3,
BR-4. Same
mineral patterns
with more garnets
and plagioclase.
• Image of a unique weathering pattern on BR-5.
• This portion of BR-5 is made of mica granite schist that is
identical to what the large boulder from the previous slide
was composed of.
Figure 12: View of outcrop BR-5; mica granite schist boulder on the right and mica granite schist outcrop
on the left.
Figure 13: closer look at physical weathering of biotite
muscovite granite schist
Concluding Thoughts
From when we started we moved from quartz
feldspar paragneiss to a biotite muscovite
granite schist. Throughout the whole trip there
were the similar mineral patterns; white to grey
quartz, k-feldspar, plagioclase, sillimanite,
biotite, muscovite, with some weathered
hornblende found on the ground and few
garnets.
A very interesting hike with interesting terrain, a
definite change in scenery for hikes that you
normally see in Spokane. There isn't normally
100+ foot outcrops of rock that aren't basalt in
this area. We recommend anyone to go there
and explore, you know where it is, how to get
there, history of the conservation area and a
summary of the geology of the outcrops.
Our last stop was a pretty view, here are pictures
of that view.
Figure 14: Looking north east towards Spokane Valley, from last point on site location
map
Figure 15: panoramic view south towards the Palouse from last point on site
location map
References
• PUBLISHED AND IN-PROGRESS 7.5-MINUTE QUADRANGLES IN
WASHINGTON BY NAME (n.d.): n. pag. Http://quimpergeology.org.
Washington State Department of Resources, Nov. 2010. Web. 7 June
2014.
<http://quimpergeology.org/ger24kmappingstatus.pdf>. Spokane
Southeast: Derkey and others, 1999, DGER OFR 99-6
• "Dishman Hills Conservancy - About Us." Dishman Hills Conservancy About Us. Dishman Hills Conservancy, n.d. Web. 7 June 2014.
<http://www.dishmanhills.org/Default.aspx?pageId=1054136>.
• "Google Maps." Google Maps. Google, 2014. Web. 7 June 2014.
<https://www.google.com/maps/%4047.5584216%2C117.260569%2C13z>.
• Griggs, A.B., 1973, Geologic map of the Spokane quadrangle,
Washington, Idaho, and Montana: U.S. Geological Survey
Miscellaneous Investigations Series Map, I-768, 1 sheet, scale
1:250,000, (Reprinted 1981) [http://ngmdb.usgs.gov/
Prodesc/proddesc_9548.htm]