Stop 4: Lava Flow Succession

This weekend we got a blessing of fall weather! Yesterday got into the mid-40’s, and today was 50°F! So, that ice finally melted off the ledges and I could get all the way out. To follow along with the stop description, check out the book share in the previous post  Stop 4 (p. 135). Instead of the direction to get to the starting place, I like to make my way there from the parking place. I think the starting place is also what’s referred to as “the southwest end of the wider part of the ledges.” In my amateur geology learning adventure, this is my best guess at interpreting this stop.

“This 80-m thick, porphyritic flow…continues for a considerable distance along the cliffs to the southwest, interrupted by a basalt dike.”

I can see what looks like a basalt dike, but wading out there will have to wait until summer 2020. The next sentence is unclear as to whether the sandstone lenses are there or “here” where the overlying basalt flow contacts the rhyolite flow.

“…rapidly becomes massive and subophitic in its central portion.”

I’m assuming this is the massive portion without amygdules.

“Farther up in the central part of the flow (to the northeast), vesicle cylinders are prominent…”

Given the diagram (Fig. 11 A, p. 131) I think this is a vesicle cylinder.

“At the top are several very thin flow units showing well-preserved ropy surfaces and lobes…This flow is overlain by several others…”

This is the part I feel the most confident in interpreting, and it’s one of my favorite spots! Above the pahoehoe top of the flow is the contact with another flow. This is the best example of pipe vesicles I’ve ever found, with what I believe are vesicle cylinders above in the massive portion!

“At the next ‘pocket beach’ at a small stream gully, climb up to the highway.”

This is again where I prefer to climb down to get to all these spots.

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