Mt. Monadnock Geology Question

[hikingmaineac]

Does anyone know why the rock (granite?) on Monadnock is full of the interesting shapes I've attached below? I'm guessing they're either fossils (but don't think so because I don't think it's sedimentary rock) or something caused by the way the igneous rock cooled.

If anyone has the time and/or the understanding, I'd bet that the answer is in here: http://www.dartmouth.edu/~earthsci/MONADNOCK.pdf, but it's been too long since I took Sed-Strat (and I was in it mostly for the off-campus lab work and b/c it was a prereq for the mini-term to New Zealand).

I've uploaded some pictures:

View attachment 2340

View attachment 2341

Thanks!

 

[moonrock]

Those rocks were sedimentary but were deformed and metamorphosed.
"Granulite" refers to a high (temperature) degree of metamorphism.
When that happens, minerals within the the original rock (probably sandstone and shale) recrystallize and even exchange components.
Those white objects look like crystals that grew radially, possibly feldspar.
But my metamorphic petrology is pretty stale.

MR

 

[Jason Berard]

The second pic looks like a snow groomer just went over the rock.

I wondered the same thing when I was up there earlier this fall.....

 

[Fitz]

Crystals

FWIW: Hard to say definitively from the pictures, but they look like metamorphic rocks. The first one looks like it contains staurolite crystals. These can be twinned and are often called "fairy crosses" I believe.

The second is trickier. it could be a igneous rock (dike) with some crystals entrained (feldspar), or a metamorphic rock layer with more staurolite or similar metamorphic minerals.

 

[erugs]

I think that the pictures show Littleton Schist. I call them "chicken toes."

 

[MonadnockVol]

Chicken Toes

I have it on pretty good authority that the stuff in question is sillimanite.

 

[Hobbitling]

try this
http://www.dartmouth.edu/~earthsci/monadnock.html

 

[Steve-o]

I've seen this on many places on Monadnock.

It's called Diamond Plate.
Home Depot sponsors, and sends private contractors out there to etch the stone, in a diamond plate format, to provide more traction, in difficult places.

Actually, I know nothing about geololy, but, all the metamorphism and folding that goes on out there makes you want to learn.

 

[erugs]

The following is from a friend of mine who teaches geology classes here at UNH Manchester:

Hi Ellen:
As noted in the Thompson paper that the individual referred to the mineral is sillimanite, which is a metamorphic mineral. It looks like one of the previous respondents in the thread noted this too. For more info on this mineral check out this link. Buried in the pictures is one from Groton NH. This mineral is characteristically associated with the Littleton Formation and is found at several localities around the State. You will see a lot of the mineral hiking around Mt. Washington, where the Littleton Formation is exposed.
http://www.mindat.org/min-3662.html

 

[moonrock]

The growth habit in photo 1 looks like staurolite but I'm used to seeing that as black or dark green and more sharply prismatic (in micaceous schist in Ct). Almost like little Allen wrenches. But I took mineralogy back in 1980.

Sillimanite would be consistent with high temperature metamorphism.
The rock in photo 1 is quite weathered, which is why the crystals stand out in positive relief.

The exposure in Photo 2 looks banded, more like a gneiss. The near foreground looks like it has some pinkish feldspar, and the band behind it may be quartz. The midground has a darker groundmass and the background is lighter again, unless that's the lighting.
There is a fracture parallel to the compositional separation between the mid and background, plus a separate fracture pattern throughout. That thin, light vein on the left probably was injected along a fracture, but its wiggly appearance indicates it has since been slightly folded.

FYI, sillimanite's tough, high-PRESSURE polymorph, kyanite, is cited as one reason why NC has the tallest mountains east of the Mississippi.
But since treeline (theoretically) is at 8000 ft, the views generally aren't as good.

 

[HockeyPuck]

I have it on pretty good authority that the stuff in question is sillimanite.

I agree with the Monadnock expert.

The 'turkey tracks' are silimanite. It's a mineral in the rock that has a hardness greater than the other minerals it was once mixed in with it.

 

[HockeyPuck]

Rock #2 is a Glacial Striae formation.

As a glacier moves / melts it carries within its ice materials of all sizes. They range from huge boulders to small particles of clay sediment. Some of this material stuck in the bottom of the ice scrapes away soil and carves grooves into rock. The bottom of the glacier acts as a huge file.

You can observe a similar effect by dropping an ice cube on the ground. Clods of dirt and small pebbles get stuck on the ice and act as an abrasive on your counter tops.. Image this same kind of picture, but on a much grander scale.

The striae indicate which direction the glacier traveled as it passed over a particular location. If you hold a compass while standing on the striae you'll notice the lines point directly N/S.

On many of the South facing surfaces you'll also notice a black tarry substance that appears like paint splatter. This is a mineral known as Horneblend.

 

[brianW]

"The Monadnock Guide" by Baldwin has a section on Monadnock geology and mentions the "chicken tracks" Best book on Monadnock in my opinion. Only problem is it is out of print. The Fitzwillim Public Libary has a copy. The section on trails (past and present) and points of interst are worth it.

 

[Puck]

Sillimanite

Ahh, to be imortalised by a rock

http://en.wikipedia.org/wiki/Benjamin_Silliman

 

[Dr. Dasypodidae]

In the first photo, the high-grade schists with the sillimanite pseudomorphs ("turkey tracks") of the Littleton Formation in N.H. are now generally mapped as the Rangeley Formation (after similar rocks mapped in Rangeley, Maine, by the U.S.Geological Survey). See Stop 5 in Tim Allen's guidebook chapter for a geology field trip in western N.H. with a reference to "turkey tracks," which are also nicely exhibited on outrcrops in Pinkham Notch.

http://kilburn.keene.edu/Research/WestCentralNH/

In the second photo, I believe that the left-to-right fabric is the foliation of the metamorphic rocks rather than glacial striations, although there well may striations or at least glacial grooves and polish preserved on the granite surfaces in the background, as there are across many parts of Mount Monadnock, especially along the trail (I forget the trail name) on the ridge from the north.

The bedrock geology in the Mount Monadnock area is extremely complex, with wonderful examples of igneous and metamorphic rocks, including migmatites, which are a mixture of both.

 

[moonrock]

Rock #2 is a Glacial Striae formation.

The striae indicate which direction the glacier traveled as it passed over a particular location. If you hold a compass while standing on the striae you'll notice the lines point directly N/S.
.

I respectfully disagree: If you mean the prominent right-left linear "texture" in the middle portion, I think that's metamorphic foliation and compositional banding. You can see lighter bands alternating with darker gray (they are near-vertical). And they run parallel to the thicker white band in the mid foreground.

The Laurentide continental glacier flowed semi-radially from its accretion zone, which arced through east-central Canada The Great Lakes and the large western Canadian lakes run sort of parallel to it. But locally, the internal ice would deflect partly around topo obstacles like mountains and into deeper valleys and lake basins, even when the former are covered.

I've seen striations in limestone outcrops along northeastern Lake Ontario shoreline. Along with the shape of the points and bays, it shows the ice flowed not south-southeast, but southwest, INTO the lake basin. That worked well because the "grit" under the glacer consisted mainly of pebbles of igneous and metamorphic rock from Canada - much harder than the limestone.

Topo maps through much of Mass and CT show egg-shaped hills of hard bedrock sculpted (and plastered) by the ice sheet(s). It might be harder to striate unweathered high-grade metamorphic rock on top of a mountain, where an overflowing continental glacier would be thinner and perhaps carrying less bedload. But ice wedging could quarry and "pluck" boulders off the down-flow side of the mountain.

 

[moonrock]

In the second photo, I believe that the left-to-right fabric is the foliation of the metamorphic rocks rather than glacial striations, although there well may striations or at least glacial grooves and polish preserved on the granite surfaces in the background, as there are across many parts of Mount Monadnock, especially along the trail (I forget the trail name) on the ridge from the north.

I guess I should actually GO there and look some time.

Do you think that granite would be easier to abrade/polish ?
BTW how high is the mountain ?

 

[Dr. Dasypodidae]

Do you think that granite would be easier to abrade/polish ?

In general, softer sedimentary and volcanic bedrock outcrops, which are not common in the Whites, better preserve glacial striations than harder metamorphic and plutonic bedrock outcrops, which are common in the Whites. Because granite bedrock is relatively harder than the stones being carried in the sole of glacier ice, the granite tends to preserve polish and grooves, as opposed to striations. But, I have found glacial polish best preserved on quartz blebs and veins in both metamorphic and plutonic rocks in Whites, as the quartz has not weathered as rapidly as the surrounding bedrock since deglaciation. Unfortunately, glacial polish is difficult for interpretation of ice-flow direction.

 

[smitty77]

I guess I should actually GO there and look some time.

Do you think that granite would be easier to abrade/polish ?
BTW how high is the mountain ?

3,165 feet above sea level, nearly 2000 feet (according to wikipedia) higher than the surrounding landscape. The area where the #2 photo was shot, IIRC, is located on the White Dot Trail, just beyond a shaded portion of the trail locally known as Paradise Valley. From this location the trail swings to the right for the final climb to the summit, about another 10 or 15 minutes of hiking.

Someone feel free to correct me if I'm wrong. It's been a while since I was last there.

 

[hikingmaineac]

The area where the #2 photo was shot, IIRC, is located on the White Dot Trail, just beyond a shaded portion of the trail locally known as Paradise Valley. From this location the trail swings to the right for the final climb to the summit, about another 10 or 15 minutes of hiking.

Someone feel free to correct me if I'm wrong. It's been a while since I was last there.

I don't remember exactly at which point on the White Dot trail I took this photo . It was on the descent (We went up Parker to Cliff Walk to White Arrow), and I remember thinking "Hey, if I'm going to post that first picture on VFTT, I might as well have this one as well."

Thank you all for your input. It would seem that next time I have a geology question, there are many capable and knowledgeable folks to be found here.

-Brandon