I realized that the first photo in Dana's post fortuitously catches a bit of the contact between the basalt and the older, underlying altered rock- here's a direct link to that picture.
Also, you might want to check out the comments and my responses on Dana's post. In particular, this: "Yes, the flow *is* thicker in the middle. You’re getting awfully close,
but I have no reason to think this was affected/influenced by ice at the
time of eruption, and I’ve not seen any pillow basalt in this area.
However, I think you’re on the right track. There is a key place name
I’ve avoided mentioning in the clues so far, but it’s related to why I
brought up Boulder Creek for clue 7."
And for the sake of completeness, I'll add the age of the volcanics and volcaniclastics in the area is perhaps 30 +/-5 Ma, the age of alteration is about 18 Ma, and the age of this volcanic cone and flow is >2 Ma, but I don't know an absolute age.
So once again, what will we see when we cross the road and look down toward Canal Creek?
Earth Day, the Geological Perspective
1 day ago