Carp Ridge Geology with Dave Forsyth – 15 Oct 2017

Written by Janet Mason

We journeyed back in time with geophysicist Dave Forsyth by viewing six sites along a transect (cross-section) of the Carp Ridge, from 1 billion year old Pre-Cambrian Grenville Basement granitic gneiss and syenite to 480 million year old early Ordovician bio-sediments.


First stop was on the northwest edge of the ridge at an outcrop that Dave called a “large mud puddle” – layers of bio-sediments (cyanobacteria) from a tropical, CO2 filled atmosphere that formed calcium carbonate (CaCO2) rocks about 460 to 470 million years ago.  These rocks constitute a large store of unreleased carbon.







The second stop was just up Woodkilton Road where we viewed karst features – “clints, grykes, and pits” – eroded and fractured calcium carbonate.  This time we “looked down” on the top of the bio-sediments.

We then visited an outcrop of Potsdam Sandstone that had a granitic origin, but over 90% of the granite’s minerals had been washed away leaving mostly silicate.




Our fourth and fifth stops took us back 1 billion years on the north and south sides of Thomas Dolan Parkway where we saw the “pluton” that sits in the middle of the Carp Barrens.  A pluton is a body of intrusive igneous rock, in this case syenite, which is similar to granite but without quartz.  The pluton sits in Grenville basement Canadian Shield granitic gneiss that forms most of the Carp Ridge.  Both formations have intrusions of darker igneous dykes.






We visited a mica (biotite) excavation pit likely dating from the 1870’s.  We also found magnetite, which demonstrated its properties by its attraction to a magnet.








Stop 6 was on Thomas Dolan Parkway near the Carp Road intersection.  Carp Road lies over and along side the Hazeldean Fault.  Here we saw the juxtaposition of Hazeldean Fault brittle deformation with more ductile Grenville deformation.  When the road was constructed, the ductile side of this outcrop was covered in shards of mica, but these have eroded away with time.  Also visible were pink rocks, secondary deposits of calcite.




Our final stop was an outcrop of Sharbot Lake Domain marble on Carp Road north of Thomas Dolan Parkway.  The marble, a metamorphosed limestone, is high in silicates and over 1 billion years old.  We tested for marble by squirting an acid solution on the rock and watching it fizz as the acid reacted with the alkaline calcium carbonate.

Our tour ended with Dave remarking that “we were breathing the carbon dioxide from earth a billion years ago.”   Now that’s a journey back in time!