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30 July 2015
Stromatolites of the Helena Formation, Grinnell Glacier Cirque, Montana
My favorite place to have lunch in Montana is at the Grinnell Glacier cirque in Glacier National Park. This is the dining room table: You’re looking at a bedding-plane-parallel exposure of Mesoproterozoic stromatolites here. Every few years, I’m lucky enough to hike up there with motivated students and share food atop this unparalleled view into the shallow seas of more than a billion years ago. Stromatolites are sedimentary structures that …
27 July 2015
Conglomeratic ne plus ultra
There’s something about conglomerates that just draws me in. Here’s a lovely example — you might even say it’s an exemplar — from Sandy Hollow in Montana: That’s the basal conglomerate of the Cretaceous Kootenai Formation, one of the mappable units in this mappable region. Feast your eyes on those well-rounded pebbles!
23 July 2015
Purcell Sill, layer-jumping
After reading the post last weekend about the discordant offshoot of the Purcell Sill, Rich Gashnig (post-doc at Georgia Tech) sent me a few photos he shot at Piegan Pass (at the head of a side-canyon adjacent to the one containing Grinnell Glacier). They show the Purcell Sill leaving one stratigraphic horizon and jumping to another, with the intermediate zone of Helena Formation limestone bending to accommodate the different positions: …
18 July 2015
Purcell Sill → Dike, Grinnell Glacier Cirque, Montana
The Neoproterozoic Purcell Sill is a stark, obvious black stripe in the strata of Glacier National Park. Here it is emerging from behind “The Salamander” glacier, above Grinnell Glacier Cirque: Zooming in, you can see the “baked” (bleached) zones above and below this concordant intrusion. But this time, during my visit to this special place, I noticed a discordant offshoot from the main sill: See it? Up there at the …
16 July 2015
Faulted sulfide vein, Golden Sunlight Mine, Montana
Here’s a really cool hand sample my student Xiuming found when were were exploring the waste rock piles at the Golden Sunlight Mine near Whitehall, Montana, the week before last: That’s a vein of pyrite and chalcopyrite, offset along a series of three small fault zones: Pocket faults! Three for the price of one.
9 January 2015
Friday fold: a surging rippled whale of Grinnell Formation
The Friday fold is asymmetric, overturned, and chock full of primary sedimentary features. Join us in Glacier National Park’s Mesoproterozoic Belt Supergroup.
17 April 2014
Pseudopictographs
I found this interesting looking slab of gray limestone last summer in the Bridger Range of Montana, in one of the talus slopes on the north side of Sacagawea Cirque. The high-contrast pattern reminded me of something, but I couldn’t say quite what. Then I realized: it looks like one of those indigenous pictographs, where the artist puts their hand up to the rock and spits paint all over it, …
26 December 2013
Mississippian invertebrates from the Lodgepole Limestone
On a cold winter’s day, Callan harks back to a summer’s afternoon fossilizing in the Rocky Mountains. A few choice images of Mississippian-aged marine invertebrates are shared.
11 October 2013
Friday fold: below the Dorado Thrust
Here’s a scene from last summer’s Regional Field Geology of the Northern Rockies course… students examining and sketching some tight folds in Cretaceous strata of the Western Interior Seaway, crumpled beneath the Dorado Thrust (a more southerly equivalent of the infamous Lewis Thrust to the north)… I’ve featured this site before, in a previous Friday fold. Photo courtesy of Tom Biggs, University of Virginia.
27 September 2013
Friday fold: crumpled red argillite
Red argillite (Grinnell Formation?) and white quartzite strata from Glacier National Park, Montana. Heavily adorned with lichens… With bedding traced out… Happy Friday to you!
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.