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I wonder if Mike Keedwell (letters, Ground Engineering, September) sees evidence of 'viscous behaviour' in the ground in this photo taken in a flight along the Grand Canyon 18 months ago? Below the canyon rim are rocks up to 1,000 million years old, formed when palaegic western North America was often inundated with warm shallow equatorial seas. Deep among these rocks there are ancient desert sands, which show fossil tracks of reptiles earlier than the dinosaurs.