Life in the Canadian Shield...

Means rocks everywhere.
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You don't need GPR (Ground Penetrating Radar) to find rock at or just below the surface of the lawn.  For one thing, grass burns quickly in the shallow soil.
Here, we are literally breaking rock to gentrify the lawn and make it easier to cut.
Google Canadian Shield to find out about this massive geological landform covering much of Central and Eastern Canada and small parts of the US and Greenland.
Canadian Shield link
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See a larger rock get plucked by winch from it's socket where it's lain since the last ice age. 
( probably ;) )
Video link