Boulder Canyon EarthCache

Boulder Canyon

© Brad Hubley, 2025

In front of you is an example of an alluvial fan; these triangular formations are created when a stream or river flows out of a steep, narrow channel and deposits sediment onto a flat, wide plain. The narrow point of the fan is known as an apex, while the wide triangle is known as the fan's apron. The velocity of the water flow decreases as it reaches the bottom of the canyon and the sediment that has been carried by this water is deposited outwards in a fan shape. This sediment, known as alluvium, can consist of cobble (fist-size rocks), gravel, sand, and finer pieces known as silt.

Types of Alluvial Fans:

Bajadas:

A bajada is the result of the blending of several alluvial fans and are common in dry climates such as deserts where flash floods wash alluvium down from nearby hills. When such a fan occurs underwater, they are known as subaqueous fans.

_{By Dicklyon - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=74858579}

Colluvial Fans:

Colluvial fans are formed without water and are created by a process known as mass wasting which is the downward movement of rock, soil, or other material. Landslides are an example of a water-free mass wasting.

_{https://www.geological-digressions.com/the-lithofacies-of-colluvium/}

Debris Cone:

A debris cone is a steep-slope alluvial fan which is closer to the shape of a half-cone that a flat fan. These types of alluvial fans are created by the slow accumulation of alluvium over many centuries.

_{https://www.researchgate.net/figure/Debris-flow-fan-along-the-Santa-Rosa-Mountains-near-Borrego-Springs-San-Diego-County_fig2_278395630}

Logging Requirements:

To log this Earthcache, send me the answers to the following questions (please do not include them in your log):

1. Based on the text above, what kind of alluvial fan do you think has been created here?
2. Walk a short distance (no more than 10m) past the large boulders you see in front of you and approximate a 1m x 1m square on the ground. Is this area an equal mixture of the types of sediment described above, or does the surface consist of primarily one type of sediment?
3. To the right of the fan, you will see an anthropomorphic structure. What is it called and what purpose do you think it serves?
4. Optional: a photo of you, and/or the fan is always appreciated.