Yellowstone’s geologic history is probably as rich and diverse as anywhere in the world. You can drive by some of the oldest exposed rock in the United States or some of the youngest. Almost anywhere you go in the Yellowstone Park area, there will be some evidence of glaciation around you --- large U-shaped valleys (Lamar Valley), smoothly polished rock outcrops (Hellroaring Mountain) and dried lake beds (Hayden and Lamar Valley) left by melting ice dams created by the glaciers themselves. Sometime the ice dams gave way all at once and released spectacular amounts of water causing gorges to be cut quickly ( Small creek bed/gorge near Phantom Lake). At other places, bolder fields were created when rushing waters slowed and dropped their load as river gorges opened up to valleys (where Yankee Jim Canyon and Paradise Valley meet 20 miles north of the Park).
Before the glaciers came to Yellowstone, volcanism (volcanoes, lava, and large exploding things) ruled the day. And there are very few places, if any, that you can go in Yellowstone and not find evidence of volcanism.
If you know where to go in Yellowstone, you can even find dinosaur eggshells or petrified pine cones lying on the ground. Please leave anything that you find so that other people may experience the same feeling that you had when finding something geologically special. Also it is illegal to take rocks and many other things from the park.
The following map highlights only a few of the many geologic formations and related interesting features. There are two good books that I highly recommend for delving more deeply into the fabulous geology of Yellowstone National Park and surrounding areas. They are East of Yellowstone by Robert J. Carson and Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Park Robert B. Smith and Lee J. Siegel.
Heart Mountain Slide is an enormous landslide that happened about 48 million years ago. It is the largest landslide that has been identified on Earth's surface. A chunk of land near Silver Gate (Northeast Entrance) detached and slid as far as 45 miles to the east. Heart Mountain, near Cody, Wyoming, is caped with the remains of the slide. Research indicates that one of the contributing factors for the slide was volcanic gases penetrating between layers of rock.
Fossil Forest covers much of the northern and eastern parts of Yellowstone Park. Starting about 49 million years ago and continuing until approximately 35 million years ago, large volcanoes (super volcanoes) erupted and formed what is known as the Absaroka Mountains. During this time, the forests were periodically buried by volcanic ash, debris and mud flows. This cycle created most of the rock layers/mountains visible along the road from Silver Gate (Northeast Entrance) to Tower Junction. If you look at the rock cliff faces in Lamar Valley or Barnett Peak with a spotting scope or binoculars, you may see the light red or brown petrified logs or stumps. At one area, just two miles west of Tower Junction, there is a side road that leads to a petrified redwood tree standing upright, a tree 12 ft high by 4 1/2 ft. in diameter. In other areas of the park, petrified breadfruit trees, a type of tree found in tropical environments, have been found. When you enter the East Gate, you are surrounded by rock layers containing petrified forest. Look for petrified trees visible in some of the road cuts along this road section.
About ¾ of a mile west of Dunraven Pass on Washburn Mountain is a pull-off with an interpretive panel about the Yellowstone Caldera (mouth of the volcano). From this pull-off, one can look to the south, 45 miles away, and see one end of the volcanic caldera that created or (or destroyed) much of Yellowstone’s landscape. At this spot, you are standing on the other side of the caldera. The caldera is 45 by 35 miles wide. The Yellowstone Volcano has erupted 3 times in the last 2 million years with an average eruption cycle of 600,000 years. The last eruption took place around 635,000 years ago. But don’t worry. If you are in Yellowstone when the next eruption happens, you will be in what I refer to as the vaporize zone and your death will be quick; the rest of the world will suffer a long, slow death. Ash from the last Yellowstone eruption has been found in states as far away as Louisiana and Arkansas.
Canyon Village Education Visitor Center
This visitor center is a "must see" for those interested in the history of volcanism and glaciation in the Yellowstone area. The center is really useful for newcomers to the study of geology. A good overview of Yellowstone’s rock types, the Yellowstone hotspot and glaciation can all be had at the visitors' center with an investment of just 30 minutes of time.
Glacier Boulder Turnout
Here one can see evidence of the massive glaciers that once covered all but the highest peaks of Yellowstone. In some areas, the ice reached depths of over 3,000 feet. As glaciers flowed over the terrain, they picked up rocks, boulders and whatever else got in the way. When and wherever the glacier retreated or melted, the non-ice debris would remain. The boulders in the valley around this pull-off are evidence of the glacier's past presence. The technical term for these boulders is "glacier erratics." I call them "glacier poop." The last of Yellowstone’s glaciers melted around 14,000 years ago.
Lamar Canyon Rock Outcrops
The exposed pink rock at the mouth of the upstream end of the Lamar River Canyon is approximately 3 billion years young. This granite and gneiss rock was originally liquid magma that solidified deep underground. Around 55 million years ago, this rock was pushed up toward the surface and is now part of what is called the Beartooth Uplift. Later glaciers and other erosional forces removed the top layers of rock exposing the pink outcrops for all of us to see.