18 July 2015
Purcell Sill → Dike, Grinnell Glacier Cirque, Montana
Posted by Callan Bentley
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 head of that valley:
Zooming in further, you can see four people standing on the contact metamorphosed zone on the left side of this dike:
I went and searched for it in Google Maps, and found it quite easily emerging from that icy valley;
How this escaped my notice during my previous trips to this site, I’ll never know. I was glad to find something new there, though.
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.
Great pics. McGimsey 1984 USGS Open-File Report 85-0543 traces this dike nw across Granite Park to Fifty Mountain. One can stand on it at the Grinnell overlook, and with a bit of hunting, find it in the woods cutting the Snowslip and Lava just off the Loop trail 1/2 mile down west from the chalet, still on the Granite Park plateau. Pretty cool to have a 750my delta between igneous events, with the sediments still flat and monolithic. Elsewhere in the region there are 1060my (may have that wrong) intrusives as well. Would be fun to have a geolocated map of contacts, I snagged one or two in strobospot but got timed out by evening. And in the park, ideally no one will damage them forever.
Query whether in late late summer, with global warming (grinnell glacier compared to 1974 is scary) one can now see the contact between the Helena Sill and this dike, as they are geochemically the same, but not physically inked together…