The Hoodoos of Yellowstone are not actually hoodoos. Hoodoos are typically sedimentary rocks that are formed in pinnacle shape. They are the result of harder, more erosion-resistant rock capping softer, less erosion-resistant rocks. The resistant cap keeps a soft shaft of underlying rock from eroding away (see Bryce Canyon National Park in Utah for an example). They were incorrectly named early in the park’s existence and the name has stuck.
These Hoodoos are large travertine boulders that broke away from Terrace Mountain just west of the Hoodoos. One look at Terrace Mountain verifies that the Hoodoos came from that location. At one time Terrace Mountain was a large hydrothermal system, similar to Mammoth Hot Springs, but has long since dried up. Over time, a landslide brought much of the dried rock formations tumbling down to where they currently reside in their heaped location.
The short but intriguing drive through the Hoodoos is a fun event, especially for children that seem to enjoy being told that they are driving through the place where the Flintstones use to live. A short one-way off-road loop takes you through the big boulders and is part of the historic old stagecoach trail of 1899. While in this area, look down the valley for evidence of the glaciers that covered this area during the last ice age. Elongate piles of gravel called moraines are the result of glaciers flowing down the mountains, bulldozing the earth as they go.
The Hoodoos was mentioned in our article titled: "Best Places to Go and Things to do with Kids in Each of Yellowstone's 14 Road Sections"