.

WOW. What causes that? Is it some sort of calcification?

The step-like terraces form as heated water moves along the Morris-Mammoth Fault.
The hot water carries dissolved calcium and bicarbonate to the surface of the terraces
where pressure lessens. Carbon dioxide then escapes as gas and the carbonate
combines with calcium to precipitate as travertine.

(From the Yellowstone NP website)

Ah yes, Mammoth Springs.

I'll be here next week!
Can't wait!