Deformed pillow basalts from southern Ontario

This is the same roadside outcrop that I showed you yesterday, with all that great vertical boudinage, but now you’re looking down, on a “pavement” (horizontal) outcrop. The rocks that were so dramatically stretched out in the vertical dimension, actually show some discernible primary volcanic structures on the horizontal plane. These are cross-sections through Archean pillow basalts: underwater extrusions of mafic lava in an ancient sea. The same sorts of structures form today in places like Hawaii, and along the mid-Atlantic Ridge.

Crazy to think about these structures forming, billions of years ago, when hot mafic lava got squirted into the primordial sea, and then later squeezed and sheared, effectively promoting vertical extrusion of these rocks with a vertical foliation and a vertical lineation. And to top it all off? A nice ~40 rounds of glaciation over the course of the Pleistocene, followed by the indignity of the Canadian road-construction establishment blasting through them (and turning a pavement exposure that’s fairly pedestrian into an outcrop with vertical faces that reveal the depths of its deformational history.