- Our families land and house in Glide. The Little River is to the left of the image behind the trees.
About a year ago this month I wrote about the geology of Glide, Oregon. For those unfamiliar with the town, Glide is a few miles east of Roseburg in the Umpqua Valley of southern Oregon. It pretty much lies in the foothills off the Cascade mountains and it’s one of the last towns you’ll see heading out to Crater Lake on OR 138. My partners grandparents live down there and it’s been somewhat of a vacation spot of ours for the past four years.
When I wrote that post I’d only taken two general geology courses and a three day field trip to central Oregon. I knew just enough about geology to sound intelligent to the lay person, but grossly misinformed to anyone who knew the difference between dacite and andesite.
One of many serpentinite outcrops in the river bed
Regardless of my ignorance, I tried to write as thorough of a post as possible based on some of my observations. I also included pictures taken with my old eye-fone 3GS to illustrate my explanations. To be sure, there were a lot of things I couldn’t explain. Take for example the masses of green serpentinite in the river bed behind my grandparents house. I knew that it was a metamorphic rock that only formed at great depths. So, how did it find it’s way to the surface of the earth? The only explanation I could conceive was that the river carved it’s way through the valley, and with the aid of uplift, the serpentinite was exposed. It hadn’t really occurred to me that I was looking at an ophiolite- sea floor rocks exposed at the surface of the earth. (For a great explanation of an ophiolite, I highly recommend Evelyn Mervine’s O is for Ophiolite blog post.)
A very sneaky piece of serpentinite masquerading as gneiss
However, I didn’t let that stop me from trying to figure out the geology of the area. Every time we went down there, I brought my rock hammer, hand lens and a little bottle of hydrochloric acid to look for the signature fizz of a carbonate rock. I found the usual assortment of sedimentary river rock, more serpentinite, and rocks that developed a calcite band or crust from the flow of the river. One of the more unusual rocks I found was what I thought to be a piece of gneiss. It displayed the usual banding and as such I always referred to it as gneiss. However, on this last trip, I took a closer look at the rock and realized it was another piece of serpentinite sporting some gneiss-like banding.
Since that first blog post, I’ve had geomorphology, mineralogy and petrology. The latter of which did more to help my understanding of geology than anything I had taken up to that point. It was from the petrology field trip to eastern Oregon that I learned about the power of observation- and by this I mean the capacity to unravel the mysteries of a landscape by looking at the various rocks and landforms of the area. It was on that trip and on this one to Glide that I realized geology is a balancing act. It’s equal parts understanding the land for what it is and what it was. Focus too much on one or the other and you lose out on the majesty and greatness of what you’re studying.
An outcrop of heavily weathered serpentinite. Apparently it gets quite a few visits from other geology students.
With that experience and knowledge in hand, I decided to take a look at some of the outcrops around our families land. A few of the outcrops I figured were basalt. This was more assumption than observation. 9 times out of 10, if you guess a rock is basalt in the state of Oregon, chances are you’re right. After some whacking with the rock hammer, I realized that the outcrop was composed of serpentinite. In fact most of the road cuts in the area were composed of serpentinite. And since this is ophiolite country, the odds are good that the rest of the hills are composed of unexposed sheeted dikes, gabbros and, dare I say it, basalt. Just not of the Columbia River kind.
I’ve been fortunate enough to have visited Glide during all seasons and see the way the Little River affects the rocks and surrounding area. During Summer months, the river runs at it’s lowest and is great for swimming and finding rocks. In the Spring the river tends to run at it’s highest and possesses a deafening roar. One can see the cycle of the river by looking at the tops of the outcrops in the middle of the river. On this last visit I noticed tree trunks on these outcrops that stand at least 20 feet.
That large tree trunk wasn't there last summer
Scour marks left behind from gravel in fast flowing water
When the water is low enough you can start to see scour marks and pot holes in the surrounding rocks. These are left by the pebbles and gravel of the river as they churn about in eddy’s from the fast flowing winter waters. The first time I saw those scour marks I was immediately reminded of the marks left behind by the advance of glaciers. Unlike glaciers though, rivers can also create pot holes in the underlying rock. I’m not terribly sure how it works, but I think it has to do with sediment getting trapped in the current and slowly being driven into the rock, much like a never ending jack hammer.
One of the much larger pot holes in the river bed. This one comes just a few feet away from the previous picture with the scour marks.
Now the point of this post isn’t to show off what I know or how much I’ve learned in the past year. In fact it’s quite the opposite. Regardless of the classes I take, the books I read, and the research done, there will always be something for me to learn. Doing geology, and science in particular, means becoming comfortable with the unknown and getting cozy with your ignorance. This doesn’t mean one should become complacent in their knowledge though. It just means for every piece of information you learn, there are at least a dozen other pieces of the puzzle that still need to be discovered. People don’t get involved in science because they know it all, but because they don’t know it all. And it’s for that reason I will continue beating rocks and playing in the river bed when we go to Glide.