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Tenaya Lake, Yosemite National Park

The Moon is Full


Re: What came first?

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avatar What came first?
October 30, 2013 07:27AM
The lichen or the rock?

Tent in der for scale.

Not sure. Was contemplating this... if it's possible... since you see rock out there that has been
sitting around for 100s of years that looks like it fell there yesterday...


Chick-on is looking at you!
Re: What came first?
October 30, 2013 09:05AM
I would say that the rock was first, then the lichen for a number of reasons. The lichen is entirely on the upper half of the rock and it appears that where there is no lichen, the rock is overhung, so little to no water making it unviable for it to grow there. So it seems very unlikely that the rock happened to come to rest in that particular orientation without other evidence of lichen have grown on other sections of the rock. Additionally you would think there would be evidence of rockfall in the area but there does not appear to be any. Plus, as the presence of a nearby clif is lacking, that's a long way for the boulder to roll, persumably the rock is an erratic, so the rock wins first place.
avatar Re: What came first?
October 30, 2013 04:23PM
It's an erratic. I don't think that precludes the lichen being there first though.

Chick-on is looking at you!
avatar Re: What came first?
October 30, 2013 04:25PM
Shall we name it Buster's Erratic? Seems fitting.smiling smiley
Re: What came first?
October 30, 2013 10:21PM
My take:

That rock was put there 10,000 years ago by the glacier that carved out the SW face of Conness. It's part of the terminal moraine damming Roosevelt Lake. The lichen grows on that side of the boulder because it's south facing and gets lots of sun exposure in addition to rainfall. You can see lichen facing the same way on the other rocks in the photo. The lichen started growing on the rock when it was uncovered by the melting glacier or when the glacial till surrounding the boulder was carried down canyon by Conness Creek. Certainly the lichen wasn't around when the block was still getting carried along by the glacier... the roundness of many glacial erratics tells you just how violent their journey in the belly of the glacier is. The only way the lichen would be older than the block's current position would be if the block was cleaved off during a rockfall or earthquake. Given it's location and position I'd say that's unlikely. Some lichens are more than 1,000 years old though so you never know.

Rockfall dating via lichen size measurements
avatar Re: What came first?
November 01, 2013 10:58AM

As you would say:
"here's a bonus picture" from the same trek:

Chick-on is looking at you!
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