6 February 2019

Dalradian schist at Glen Roy

Posted by Callan Bentley

We saw last week how glaciation carved out a valley in Scotland called Glen Roy. As the glacier ground into the landscape, it liberated tremendous numbers of sedimentary particles from the bedrock, which is composed of Dalradian metamorphic rocks (mainly porphyroblastic schist in my observation). Then once that now-U-shaped valley had been deglaciated, a new glacier dammed it, making a lake that rose and filled the declivity of Glen Roy, notching lakeshore terraces into the surrounding hill sides. As the valley filled with lake water, waves took the cobbles and gravel along the shores and turned them over and over, burnishing off the rough edges. Over years and years, this not only notched the valley walls at constant elevations, but it turned the sediment into accumulations of fairly well rounded gravels.

Here’s a piece of fresh schist, with prominent porphyroblasts of garnet and staurolite:

You’ll note that this second specimen has nice crenulations (small folds) in its foliation, and that its edges look a little less angular -rounded no doubt by the sloshing waves of the glacially-impounded lake that used to fill Glen Roy:

I collected only one specimen from the site, and this is it, both front and back, as two GIGAmacro images. You’ll note that it is an even more well-rounded cobble:

Link 0.8 Gpx GIGAmacro by Callan Bentley

Link 0.8 Gpx GIGAmacro by Callan Bentley

[If these embedded GigaPans show an error message, just refresh the page so they load properly.]

I particularly like the second view here because it shows a pronounced band of garnets crossing the cobble – a compositionally distinct layer within the metamorphic rock which is best explained by a compositional layer within its protolith. That is: this was a layer in the sedimentary rock that this schist used to be with the right elemental ingredients in it, so that when the rock encountered high temperatures and pressures, it reacted just so, and made a layer of garnets. You’re looking at an utterly recrystallized sedimentary bed in that zone of big red dots.

These metasedimentary rocks of the Dalradian Supergroup are a big player in northern Scotland. I also saw them at a bunch of other locations, as these blog posts show.

They’re everywhere! But this cobble was one of my favorites.