Big Trip to Big Rock Spokane Valley, Washington. Jon Bromley, Koby Sonnabend, Alex Deutsch. Figure 1: Weathered biotite muscovite granite schist. Introduction.
Jon Bromley, Koby Sonnabend, Alex Deutsch
Figure 1: Weathered biotite muscovite granite schist
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 samplesand 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
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 Supergroupdating back to the Precambrian (Griggs A.B,1973).
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
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).
Start of Hike
Figure 4: Map of site location (Derkey, 1999)
Figure 5: Quartz feldspar paragneiss
Figure 6: Biotite-muscovite granite
Figure 7: View of outcrop 3 a biotite-muscovite granite from the west looking east.
Figure 8: Well foliated layers of Cambrian biotite muscovite granite with sillimanite, plagioclase and grey quartz (Griggs A.B,1973)
Figure 9: View of outcrop of BR-4; biotite muscovite granite
With goodfoliation at the bottom of the outcrop; yet foliation isn't visible above 50 feet, leading usto believe this is a schist.
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
Figure 11: boulder of biotite muscovite granite schist
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
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. Werecommend 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.
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