On the same October morning that we visited Stony Mountain, we also drove a few kilometres northwestward to Stonewall Quarry Park. There, we examined strata of the Stonewall Formation, deposited in a warm tropical sea some 445 million years ago. The rocks at Stonewall contain a variety of marine fossils, most notably corals and stromatoporoid sponges.
Their pleasant greenhouse world was “messed up big time” by the Late Ordovician mass extinction, the first of the big five in the Phanerozoic history of life. The extinction was associated with and may have resulted from a glaciation on the southern paleocontinent of Gondwana. It was far distant from our local rocks, which were deposited in tropical Laurentia, but its effects on sea level and water chemistry were global.
The horizon represented by the top surface of Stonewall Quarry is also roughly the horizon after which the world went bad. As my friend Bob Elias is fond of pointing out, that surface at the quarry is covered with glacial features, striations and chatter marks! Are these at all related to the end-Ordovician glaciation? Of course not, as these striations are just a few thousand years old and the result of the last of the Pleistocene continental glaciers. But it is another excellent example of adjacent analogues.
And on this lovely morning, with the fog lifting and the sun still relatively low, the striations showed wonderfully across the damp dolostone.
© Graham Young, 2012