The Cretaceous-Tertiary (K-T) boundary is a distinct layer in the rock strata laid down about 65 million years ago. Its creation coincides with the mass extinction of the dinosaurs and it contains high levels of iridium, an element found in asteroids.

On top of the food chain, they had few concerns.
It was bathwater warm, there were plenty of ferns.
A Cretaceous Isaiah could claim he’d divined
that God in his wisdom had clearly designed
a world without end for the dinosaur kind.

Then out of the blue it all changed for their lot—
one day they were kings, and then they were not.
All over the world the great dinosaurs died.
They starved, or they froze, or were suddenly fried.
They found out the hard way the prophet had lied.

Yet, from the beginning the end had been known,
foretold in the orbit of one giant stone.
It smashed into earth. Ash covered the land—
what happened to T-Rex explained by the band—
a line of iridium drawn in the sand.

It’s circles in circles in this grand design,
a ballet of masses, mostly benign.
But sometimes the circles go bump in the night.
So don’t say I’m silly and switch off the light
when I wake up feeling that something’s not right.

—Robert W. Crawford