Liberty Cap is the name of this formation and we found it in Yellowstone National Park at the north end of the park (Mammoth Hot Springs). I found the following information on vacationidea.com.
Liberty Cap is a 37-foot hot spring cone situated in the northern portion of Mammoth Hot Springs, at the bottom of Lower Terraces.
Liberty Cap was created by a hot spring which remained in one location for a long time, depositing minerals to build the cone. Today the cone is no longer active.
Liberty Cap was thought to resemble caps worn during the French Revolution and that is how it got its name in 1871.