Building Boulders  

The unique, sculpted granitic boulder piles seen throughout the park are the result of jointing, weathering, and erosion. 

1. Magma cooled and solidified miles beneath the surface to form various types of granitic rocks including the monzogranite that is common in the park. 

2. Cracks or fractures— called joints— formed while these rocks were still underground due to stress from geologic forces. 

3. Groundwater entered the joints and began rounding out the edges and corner, and weathering the rock into smaller pieces. 

4. Erosion carried away small particles of broken-down rock, exposing rounded boulders. 

Notches mark the height and give us insight into past soil levels. The notches form because water in the soils is slightly acidic, slowly dissolving the rock away. Tafoni are concave pits of varying size that form on the rock’s surface from wind erosion or from soil contact (similar to how notches form). 

Exploration Area 

Scramble among the roadside boulders. See their crystalline structure. Feel their surfaces. Discover unique contours. 

Glossary 

Magma: Pressure and heat below the earth’s surface melts rock into a molten or semi-molten fluid called magma. When magma cools, igneous rocks are formed.  

Erosion: A geological process in which natural forces such as wind or water move natural materials like rocks, dirt, and sand.  

Weathering: natural process when water, wind, heat, plants and other forces, slowly break or change the rock. 

Monzogranite: a specific kind of biotite-rich granite rock. The monzogranite here is light-colored and is commonly seen in the northern part of Joshua Tree National Park.  

Tafoni: Rhymes with macaroni. Tafoni is a natural process that results in the formation of rock cavities. Tafoni occurs when long exposure to moisture eats away at a rock face. Stop by Skull Rock and look at the eye sockets and nose to see large examples of tafoni.