Tuesday, August 30, 2011

Geology of Granite Lake: Term Project by Alex Grider

View from the road facing east toward Granite Lake. 

From the same vantage, but facing south.

Located about five miles outside of Cheney, Wa, Granite Lake is one of several lakes in the area. I wasn't entirely sure what I would find near the lake, except for it's namesake, Granite. There is a road that hugs the side of the lake and a small bridge that traverses it. This is where I parked and did my field work. I tried to ignore loose rocks on the surface, though they appeared to all come from indigenous bedrock and had been deposited there by the road construction. With a hammer present, I broke off several samples of rock, though later I determined that they all belonged to two rock types: granite and gneiss.
The road had cut into this formation of gneiss which stands about 10 feet high.
This photo was taken near the shoreline. I noted large, continuous deposits of gneiss at the shoreline, with a layer of granite intruding in the middle. This led me to believe that the gneiss had formed underground and later on granite intruded in between, giving the rocks a light, dark, light color pattern. I believe this granite formed in the cretaceous, along with the mountains of British Columbia and Montana and then was intruded into the much older rocks of the Belt Supergroup  
  I predicted I would find a thick layer of weathered granite into which a series of small lakes had formed. I also expected to find basalt as this area is in the midst of the Channeled Scablands. Well, I found granite, but not as much as I expected, and I didn't find any basalt, which I cannot account for. Interestingly, I found much more gneiss than I did granite. The rock face face exposed by the road was mostly gneiss as were most of the rocks by the shore. 
This is a chunk of gneiss that was taken off of the cliff face.
Within the Gneiss is found plagioclase fedspar and pyroxene.   

This piece of granite dug up by the shoreline. Similar granite was found intruding within gneiss in the same area. The minerals within the granite were quartz, black dots indicating biotite, and feldspars, both plagioclase and potassium.
On Interstate 90 going East toward Spokane, take exit 270. Take a right off the ramp onto Hwy  904. Drive a half mile and take a right onto Medical Lake - Four Lakes Road. Drive another half mile and take a left onto Silver Lake Road. Granite Lake is another half mile down this road.


-EWU Geology Dept. http://www.geology.ewu.edu/map.htm
Alt, David and Donald W. Hyndman. Northwest Exposures: A Geologic Story of the Northwest. Mountain Press        Publishing Company. Missoula Mt. 1995. Print.