Rare structures of biofilm stromatolites in algal-bound sheets of sand.


Photo by Claire Milloy.

This display rock, like specimen 11 to the left, was rescued from a median outcrop on Highway 417 in Kanata, about 30 km to the east of Almonte.  The full story is available at http://millstonenews.com/2012/11/metcalfe-geoheritage-park-receives-rescue-rocks.html

Stromatolites, from the Greek words ‘stroma’ (mattress, bed) and ‘lithos’ (rock) are layered structures.  The examples we can see in this display rock were formed by the trapping, binding and cementation of sand grains by thin biofilms of microorganisms, especially blue-green algae.  The sand was deposited in near-shore coastal environments as the ocean level gradually rose to cover much of the continent.

At this unique location was the first-reported occurrences in Canada of quartz arenite containing stromatolites and biofims.  Don’t worry, there won’t be a test later.  Think of the landscape in our area as a shallow marine environment with microbial organisms thriving on a sand bottom.  This would be about 488 million years ago, at the end of the Cambrian Period and beginning of the Ordovician.

During the Ordovician Period, the landmass that would become North America was close to the equator.  This ancient geological core, Laurentia, also included parts of present day Greenland and Scotland.


Image from Chapter 7 of the book “Four Billion Years and Counting” published by the Canadian Federation of Earth Science and Nimbus (available at the Almonte branch of the Mississippi Mills Library. 557.1 FOU). Image downloaded from http://www.fbycbook.com/chapter-7. Original by Ron Blakey ( http://jan.ucc.nau.edu/rcb7/globaltext2.html), with additions by Asia Reid.

During the Ordovician Period, much of the continent that would later become North America was under water.  Land is shown in green and brown on this these paleographic maps.  The lighter blue areas represent coastal or nearshore areas, the darker blue indicates deeper ocean waters.  Aspects of modern geography have been added for orientation.

Look for very thin alternating light and dark layers in this display rock.  During the Ordovician Period, the algal-bound sheets of sand were disturbed in shallow conditions by storms.  Along the side of the rock you can see how they were dislocated.


Photo by Claire Milloy.

A look-for section on the top of display rock 12 has been highlighted with red lines in this photo.



Photo by Claire Milloy.

Look-for sections along the side of display rock 12 have been highlighted with red lines in this image.

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