ZenGeology

And potholes! Let me show you how the stream
Made vortices against the side of this
Formation. Alcoves here were cut so fast ---
An inch a year --- that you could almost watch
Them form if you were patient. Fingernails
Grow just as rapidly. A big pothole
Develops in a human life. That's quick!

If you come back here in 10,000 years
You'll see something quite different. The land
Is lowering about a millimeter
Every century. This boulder that
We're standing on came down the river in
A flood about a quarter million years
Ago; it moved some fifty miles. And see
The patch of lichen here? It grows so slow
That by its size you know it's centuries
Old now. So we can tell this rock has not
Been much disturbed; beryllium that's formed
By cosmic rays which hit the surfaces
Provide us with a measure of its age.

And note this mica schist: it must have been
Pressed under 10 kilometers at most
Of overburden, 900 Centigrade,
To partly melt the crystals. That suggests
A geothermal gradient far more
Than anything that's normal nowadays
In Appalachian land. We're looking at
A half a billion years of history.