'Petrified human activity'

I'm reading a really lovely book at the moment: 'On Looking: Eleven Walks with Expert Eyes' by Alexandra Horowitz. She takes a series of walks with experts on a diverse range of subjects, including a geologist, an artist, an urban sociologist, a doctor, a child and a dog, seeking to notice what they see and how they see it and to discover the reasons most of us don't see the same things. It's beautifully written and very fascinating. 

In it is this paragraph: 'Together we climbed up a few marble stairs out of the museum. Each step was irregularly concave, worn down by the footfalls of countless visitors ascending and rounded at their leading corners, from countless descents. This erosion is petrified human activity. Each of those steppers toed the marble and pushed seventeen (or so) of its molecules forward or to the side. After millions of steps, these gentle shovings changed the shape of the rock from tabletop flat to soft undulance.' 

It immediately put me in mind of the steps leading down from Victoria Road to Salts Mill, worn down by the wooden-soled clogs of millworkers and the rubber soles of modern-day visitors' trainers. They're solid Yorkshire gritstone rather than marble but worn and rounded in just the same way from countless comings and goings. 

I highly recommend the book - and I highly recommend slowing down to really look at the things you pass often and yet hardly notice in their familiarity.