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The Purcell Sill shown here at Grinnell Glacier stands out in stark contrast as the black stripe in the middle of the strata.

Sill intruding Dolomite of the Helena Formation, Glacier National Park, 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

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:

In addition to the main, obvious jump at left-center, there are two other offsets (one,perhaps, apparent offset) further to the right:

Zooming in, you can see the “baked” (bleached) zones above and below this concordant intrusion.

Zooming in further, you can see four people standing on the contact metamorphosed zone on the left side of this dike:

But this time, during my visit to this special place, I noticed a discordant offshoot from the main sill:

In addition to the main, obvious jump at left-center, there are two other offsets (one,perhaps, apparent offset) further to the right:

See it? Up there at the head of that valley: