Forum Index > Trail Talk > Name that rock
Previous :: Next Topic  
Author Message
RichP
sin rumbo



Joined: 13 Jul 2006
Posts: 3914 | TRs
Location: Seattle
RichP
sin rumbo
PostThu Jan 04, 2018 7:11 am 
Reply to topic Reply with quote
Thanks Brushbuffalo. That's fascinating.
Back to top
View user's profile Search for posts by this user Send private message Send e-mail Reply to topic Reply with quote
gb
Member
Member


Joined: 01 Jul 2010
Posts: 4340 | TRs

gb
Member
PostThu Jan 04, 2018 7:26 am 
Reply to topic Reply with quote
Brushbuffalo wrote:
Nancyann wrote:
Brushbuffalo, could you explain what geologic events would cause this giant chasm on Vesper Peak?
Blue Mountain views 12/23/17
Blue Mountain views 12/23/17

And similar ones on Big Four?
Blue Mountain views 12/23/17
Blue Mountain views 12/23/17

Nancyann, when you see an arrow-straight feature of size, it is almost always either a fault, a joint (fracture), or a man-made feature. There is a  significant fault (Darrington- Devils Mountain fault zone) extending near the summit of Vesper Peak and there are contacts between three rock formations of contrasting resistance to erosion: granitic rock ( tonalite), metavolcanic rock, and sandstone. Faults and large joints extending  up/down slope often channel surface runoff, and the prominent gash is most likely formed by that process....water running along a zone of weakness (fault, joint, or contact),  thus deepening it.

Big Four is composed mainly of metavolcanic rocks. The very steep but not particularly deep valleys on the south aspect of Big Four are due to stream flow but not along any particular zone of weakness. Note how they are rather evenly spaced with ridges intervening.
You can confidently distinguish between stream-eroded valleys (V-shape cross sectional profile) from glacial valleys (U-shaped).

For those who don't know Vesper, this south aspect is readily visible from many areas around the Seattle area. And the north side offers some of the finest granite rock for climbing in the state. I am enjoying these interesting conversations as well.

Brushbuffalo, could you discuss a little about the geology of adjacent Sperry Peak? The climbing rock on parts of Sperry is very hard and has almost a quartzite like nature, with embedded round pebbles.
Back to top
View user's profile Search for posts by this user Send private message Reply to topic Reply with quote
Brushbuffalo
Member
Member


Joined: 17 Sep 2015
Posts: 856 | TRs
Location: there earlier, here now, somewhere later... Bellingham in between
Brushbuffalo
Member
PostThu Jan 04, 2018 8:55 am 
Reply to topic Reply with quote
gb wrote:
Brushbuffalo, could you discuss a little about the geology of adjacent Sperry Peak? The climbing rock on parts of Sperry is very hard and has almost a quartzite like nature, with embedded round pebbles.

Sure, gb. Thanks for the question. Although I have been up Vesper I haven't ventured over to Sperry yet. So let's turn to the authority!  Geologic Map of the Sauk River 30- by 60- Minute Quadrangle, Washington, by Tabor et al ,
Sperry Peak, just northeast of Vesper, is composed of sedimentary rock, mapped as formation Tbs. Most of this formation is sandstone but Tbs also contains conglomerate.  Come to think of it, I remember seeing conglomerate boulders poking above snowpack on the trail to Vesper that must have been shed from Sperry.

Here is a detail to look for on your next Sperry trip.  Quoting from the map's explanatory notes: "Within the Darrington-Devils Mountain Fault Zone, conglomerate clasts are highly stretched, roughly horizontal, and parallel to the faults,  suggesting strike-slip movement along the fault zone."  cool.gif

[An active  modern-day strike-slip fault ( horizontal movement along the fault trace) is the famous San Andreas Fault Zone in California.]

--------------
Passing rocks and trees like they were standing still
Back to top
View user's profile Search for posts by this user Visit poster's website Send private message Send e-mail Reply to topic Reply with quote
NorthBen
Member
Member


Joined: 29 Feb 2012
Posts: 31 | TRs

NorthBen
Member
PostWed Jan 10, 2018 12:20 pm 
Reply to topic Reply with quote
Here's one that caught my eye this past summer, taken near the summit of Gardner Mountain:

Thoughts?
Back to top
View user's profile Search for posts by this user Send private message Reply to topic Reply with quote
Brushbuffalo
Member
Member


Joined: 17 Sep 2015
Posts: 856 | TRs
Location: there earlier, here now, somewhere later... Bellingham in between
Brushbuffalo
Member
PostWed Jan 10, 2018 1:50 pm 
Reply to topic Reply with quote
NorthBen, thanks for the picture of that very interesting boulder. I have some  comments based on location of Gardner Mountain and using the Geologic Map of the North Cascade Range by Ralph Haugerud and Rowland Tabor. To give you a more definite answer I would return to that fun choss pile again and carefully observe the bedrock outcrops over a wide area. I camped on top of Gardner many years ago, but was more interested in the goats and the rare "green flash" I saw at sunset so didn't pay enough attention to the rock.

Part of the boulder is a course-grained conglomerate fragment consisting of subangular to subrounded clasts, including at least one granitic clast. The boulder is likely a bit of  the bedding contact between a conglomerate and a very well-sorted sandstone. There are other thoughts I have about this rock that involve sketches, arm-waving, and other things not possible in this reply to your question.  Fascinating, but I really need to be there to look around!  Sometimes it's hard to identify a single "tree" outside of the context of the " forest."

There are several different formations that occur on and around Gardner Mountain, which together are known as "rocks in the Methow block."  You may have an example of the formation abbreviated Kpv, and this is part of the Pasayten Group,  which also has a companion formation Kps ( sedimentary) found along the lower western base of Gardner over toward Abernathy Peak.

Kpv is "predominantly andesitic breccia and tuff; locally fluviatile [river] maroon siltstone, sandstone, and conglomerate. About 90 million years ago, volcanoes erupted on the  floodplains of rivers that flowed over sediments of the former Methow Ocean, burying both the river deposits ( Kps) and the underlying Methow Ocean sediments under volcanic rocks." However, the individual particles (clasts) in your boulder don't look to be volcanic.
The maroon color, which is much more distinctive in some locations in the Methow region, is due to oxidation of stream-deposited sediment.

There is another formation outcropping near the top of Gardner,  Ktf. I recall seeing spectacular conglomerate boulders of Ktf on the PCT on the east slope of Powder Mountain between Rock Pass and Woody Pass, but I digress. Ktf is the Three Fools sequence,  which is " predominantly thick-bedded sandstone, minor thin-bedded sandstone, argillite, and conglomerate deposited in deep-water submarine fans.  Between 105 and 110 million years ago, ......volcanic islands formed in the western part of the Methow Ocean. The weight of these islands caused the ocean to deepen [isostatic adjustment]. Occasional flows of sediment-rich water ( turbidity currents), perhaps created by submarine landslides or storm waves, cascaded from the east and west into the depths to create these turbidite deposits."   Problem with this is that the boulder is quite different than a turbidite.

Please pardon my vagueness in this response.  "it could be this, or that, or something else".....bottom line is I'm not very confident in a definitive ID as far as which formation this boulder was broken off from.. . dizzy.gif  I often am challenged trying to identify a loose rock from a single picture without... or even with....scale, and without seeing it in field relationships. But I enjoy the challenge. It's fun to be conjectural, and probably makes for amusing (and maybe even occasionally informative) reading.   winksmile.gif The North Cascades have some of the most complex geology anywhere. It has taken field geologists many decades to try to figure it out.....and they are still trying!

--------------
Passing rocks and trees like they were standing still
Back to top
View user's profile Search for posts by this user Visit poster's website Send private message Send e-mail Reply to topic Reply with quote
  Display:     All times are GMT - 8 Hours
Forum Index > Trail Talk > Name that rock
  Happy Birthday payton, arcticlights69!
Jump to:   
Search this topic:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
   Use Disclaimer Powered by phpBB Privacy Policy