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Posts Tagged ‘columnar jointing’

This final post of my Columns “Week” comes from an outcrop on I-84 along the Columbia River in northeastern Oregon. Technically this outcrop is in Washington, but since I took this picture from the Oregon side, and I am now a resident of Oregon, I am going to claim it as ours. This stretch of interstate is full of excellent examples of columnar jointing from the basalt flows that cover most of the state, and is a beautiful drive.

Becoming a bad habit for me is that this picture was taken from a moving car, and suffers from the slightest bit of blurriness and lack of good scale.

Even with the photographic flaws, I think it is apparent that there is some interesting distortion going on in these columns. Below, I highlighted the edges of most of the columns.

What stands out most to me about this outcrop is the change in direction of the narrow columns. Admittedly there is a high amount of fracturing going on, but the most apparent columns seem to diverge and converge in places.

Another interesting aspect of this place is the presence of what could be more, larger columns above the distorted, narrow ones. Here they are highlighted in yellow.

This gives me the sense that there are two separate flows existing here. A quick amount of research leads me to believe the lower flow could be the Middle Miocene Wanapum basalt (15.0 Ma), and the upper flow could be the  slightly younger Saddle Mountains basalt (13.5 – 6.0 Ma). Of course, to get a good grasp on the difference I would need to get my nose to the outcrop and take a better look.

If these are two separate flows, here is another annotated picture showing the contact between the two in blue.

Maybe I will find time to head back and get a closer look.

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I have recently transplanted myself in the Pacific Northwest, specifically the city of Eugene, OR. It has only been a couple of weeks, but I am already making myself right at home with the food, people and most importantly the geology. While I still have a lot to learn in terms of the local geology, what I have seen so far has prompted me to declare my own personal “Columns Week” on this blog. I have set aside three Oregon outcrops to discuss throughout the week, starting off with one suggested to me by Lockwood DeWitt at Skinner’s Butte.

Located on the northern side of town west of the campus, and just a short bike ride from my place, this outcrop shows some of the most spectacular basalt columns I have come across. I have been to Giant’s Causeway in Northern Ireland, and I think these ones belong in the same category.

The columns to the right are so well preserved!

Photo by V. Malinay

Photo by V. Malinay

If you click on the image for the larger version, you can see the arrest lines.

Special thanks to Lockwood DeWitt for bringing these to my attention, and also the guy climbing for the sense of scale.

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After spending an evening around a campfire spinning some good ole’ Ben Roethlisberger yarns, and good night sleep was needed. Unfortunately, we were in for a night of hurricane force winds. Not really, but it was pretty windy and the tent kept slapping me in the face all night. It must not be a fan of my low-brow satirical sense of humor.

Today we hopped back in the van and headed out towards Skyline Drive; which if you don’t live in Virginia, is where city folk bring traffic to rural folk. It is also a great place for some pretty spectacular views of the Commonwealth and West Virginia, and some pretty decent hiking trails.

After making a few stops along the road at some beautiful, if not incredible windy, overlooks, we parked and set out down the Limberlost Trail (Google Earth file). What the Limberlost trail lacks in strenuous hiking it makes up in a two geologically exciting outcrops of columnar jointing in basalt flows. This basalt is part of the Catoctin Formation which flowed to the surface during the rifting of Rodinia. The Catoctin is younger slightly as the Swift Run in found beneath it. As the basalt cooled, it contracted and cracked, leaving (for the most part) hexagonal columns with arrest lines running perpendicular to the joint surface, for every period of jointing.

The columns would have sat there looking like the Giant’s Causeway of Ireland for a few hundred million years until the Alleghenian Orogeny (AO) knocked them around. Taking the strike and dip of the columns, we were able to determine another instance of East over West deformation; further evidence of the AO coming from the east.

The typical case of columnar jointing exhibits six sides with 120° angles between. After deformation the “tops” (since we were unable to determine paleo-up) of the columns were sheared and stretched to create larger angles or in some cases shortened (100° and 90°). The arrest lines which were originally perpendicular to the edges are now at angles of ~106°.

After getting our gentile sufficiency of columnar jointing we headed back down the fire road to our vehicles, and were off to our campsite and then one final stop for the day (possibly my favorite of the trip).

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