Posted by: Alan Pitts | September 19, 2011

Pillow Basalt, Bencorragh

From the peak of Bencorragh.

Over the previous two posts I discussed a few members of Western Ireland’s Neoproterozic age Dalradian Super Group, the Lakes Marble and the Bennabeola quartzite, all of which I encountered during my 6 week field course in Ireland.

Today I’m moving onward and upward in the geologic column, to the base of the Ordovician system and looking at some great pillow basalts in the Bencorragh formation of the Louch Nafooey group.

Sure, I have seen pillows before. However, I think this is the first time I’ve ever seen such a clean cross-section of piled pillows. Notice how the tops are all convex up, shaped like muffin tops, and the bottoms taper to a thin point conforming to the tops below.  Here is a quick annotation to emphasize the point.

Pillow basalts are a primary igneous structure, formed as lava erupts into water which cools the outside of the pillow at a much faster rate than the inside.  They are also a useful “up” structure, providing the up direction at the time of cooling.

These pillows have (apparently) no interstitial sedimentary material in between them, it’s just all basalt.

Here is one that looked good enough for a rest, a picture every geologist’s mother should have.

Here are some more pillows with a huge bowl-shaped Cirque in the background, evidence of Ireland’s glacial history.

One last one to illustrate my boundless love for Ireland….or well, lets say, my sensibly bounded love for Ireland.

Here is a link to a video of pillow basalts in action I found on you tube.  There are lots of these.

And some good reading material on the Lough Nafooey Group.

-Ryan P.D, Floyd P.A. & Archer J.B. (1980). The stratigraphy and petrochemistry of the Lough Nafooey Group (Tremodaocian), western Ireland. Journal of the Geologic Society, London. Vol 137 pp443-458



  1. interesting article

  2. Eh, I studied this mountain during the 1994 summer… The Bencorragh area is very interresting : pillow lavas, turbidite (the Rosroe formation, south side of the Lough Nafooey), and sedimentary rocks on the south side of the mountain… And Dalradian rocks (sillimanite bearing migmatite) at the very south of the area, near the mouth of the river

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