When hiking recently in my neighborhood, I saw this gleaming apparition appear in an eroded gully in a dirt road: Those multicolored stripes are varying compositions in a zone of ultramylonite: ductilely-sheared-out rock that formed in the deep equivalent of a “fault” in the Blue Ridge basement complex. We call it a “shear zone” most … Read more
Reader Christian Gronau writes with this Friday fold contribution: Greetings from Cortes Island, BC – at the opposite end of the Strait vis-a-vis Lopez Island. Your Mountain Beltway blog is always of interest, and I have been following it for several years by now. Thank you for putting the effort into this worthwhile website. Quite … Read more
A quest to visit the “first shear zones” described in the scientific literature leads to an alternate location, and some GIGAmacro images of samples from the real, original spot.
At the birthplace of the term “mylonite,” we can find Friday folds hidden in the foliation.
A final post from my visit to the Laramie Range with Bob Bauer and Fred McLaughlin earlier in the summer -Recall that there is schist and granite at this site, and much of the granite shows cross-cutting or inclusive relationships indicating it is younger than the schist, while still younger mafic dikes cross-cut than the … Read more
Two summers ago, my wife (then my very new wife) Lily and I participated in a two-week workshop for teachers on the energy resources of Wyoming. We also indulged in a visit to a mineral resource site: South Pass City, a site in the southern Wind River Uplift that was the second oldest incorporated town … Read more
A week ago, I featured six new GigaPans here, of the extraordinary rocks in Klingle Valley, DC. I hadn’t been able to get the next phase of imaging that site ready in time for the post, but here it is: an annotated view of the outcrop. Annotation color code: PINK = Granite contact BLUE = … Read more
Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty … Read more
Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) … Read more
The Friday fold combines a moment of insight on a field trip in the Archean Superior Province and a new paper published this month in the journal GEOLOGY.
The Friday before last, I was in DC for a fun geology/botany field trip, and I got the opportunity to stroll around the barn at historic Peirce Mill, a historical grain mill along Rock Creek in Rock Creek Park, Washington, DC. The barn is immediately south of Tilden Street NW. It appears to have been … Read more
I went into work on the weekend recently, and found two lovely new specimens of mylonite in the lab. Not sure whose they are, or where they came from, but I knew I had to photograph them so I could share them here with you:
Today, I’ve got a batch of additional photos of the Walker Top Granite to share, variably mylonitized (sheared out under ductile conditions)… Nice garnets in some spots: Delta porphyroclast: Lovely sheared-out rocks…
Check out these lovely images (mouth-watering, even) of the protomylonitic and mylonitic Walker Top Granite of North Carolina’s Inner Piedmont. There’s also some views of the field trip participants and scenery.
Here’s one of the cool rocks I saw on my pre-GSA-Charlotte field trip to the Neoacadian Inner Piedmont field trip: the protomylonitic Walker Top Granite (~366 Ma crystallization), which was deformed as the terrane it was intruded into got shoved up and to the west (and perhaps the southwest!). Here’s an animated GIF I made … Read more
Callan attends the Geological Society of Washington’s fall field trip, examining the relationship between grape-growing and the underlying geology of two provinces in northern Virginia: the Blue Ridge and the Valley & Ridge. With GSW compatriots, Callan visits Hume Vineyards in the Blue Ridge basement complex and North Mountain Winery in the Shenandoah Valley. This is part I.
Christie Rowe sent me these two images. They were taken by Ben Melosh and Louis Smit. The folded layers are mylonites of the Pofadder Shear Zone in South Africa. I love it when folds are expressed not only in profile, but also in three dimensions. Nothing in life could possibly be better. We’ve featured the … Read more
It seems like a good morning to return to Ontario, and to the Archean shear zones exposed at the Quetico/Wabigoon subprovince boundary of the Superior Craton. Readers will hopefully recall that I spent several days absorbing structural goodness from these rocks on a field trip before the Minneapolis GSA meeting last fall. The trip was … Read more
The Friday fold “guest fold” series continues with a folded ultramylonite from South Africa.
Callan shares images of extraordinary boudinage outcrops in strained Archean meta-basalts on the Quetico-Wabigoon subprovince boundary within the southern Superior Craton. Ogle these gorgeous structures in awe and wonder.