The next post in my Columns “Week” series is an outcrop I saw along I-5 just north of Eugene back in March, and came away with an immediate impression. I was sick at the time, but still knew exactly what it was I was looking at as we drove past at 65 mph. Since coming back, this outcrop has become a bit more confusing to me. Not because I disagree with my initial impression, but because it has been explained to me as something else and I still agree with my first thought. Hopefully today I can get some input that will shine light on the situation to me.
Enough talk! Here is the outcrop in question:
This outcrop is located on the eastern side of I-5 with no safe place to park and walk up to, so these pictures are taken from across the interstate just off of Coburn Road near the McKenzie River.
Clearly there are some distorted basalt columns here, that is not in question. But, my initial impression was that these columns were distorted due to a lobate cooling front, and it has been explained to me (by very reputable sources) as a dike rolling into a sill. To help visualize my thought process, here is an annotated picture of the outcrop with the progressive cooling front in orange, and the direction of cooling shown with a red arrow. Also, a second picture with a passing tractor trailer to give a relative sense of scale.
Here is another angle of the outcrop where in my hypothesis the cooling front would be moving from the upper left of the picture towards the lower right.
Another interesting aspect of this outcrop is that the basalt can be seen overlying Oligocene aged marine sediments of siltstone and sandstone. Here is a picture of the contact between the two with another less than desirable “passing truck” for sense of scale.
Does anyone have a counter hypothesis? I would love to have a discussion on what I may not be seeing at this outcrop.
For good measure, here is a picture of the entire outcrop taken from a northern angle (again with cars for scale).