Santa Barbara is built upon a landform known as an alluvial fan, a fan-shaped deposit consisting of stream gravels and debris flow deposits with a high point near the Santa Barbara Museum of Natural History.
Rocky Nook is known for the beautiful, large boulders scattered throughout the park. Where did they come from and how did they get here?
The sandstone is the same composition as that found in the nearby Santa Ynez mountains, revealing their source. However, the boulders were not transported to their current location by man or machine, but by the forces of nature. Sometime in the recent geologic past, meaning within the past two or three thousand years, a large landside blocked Rattlesnake Creek about a mile from Rocky Nook Park, at what is today known as Skofield Park. The estimated volume of the slide was 10 million cubic yards, which resulted in a dam that was 60 to 100 feet high.
The dam caused water to back up into Rattlesnake Canyon and form a lake. Back then, the creeks ran year round, and eventually the pressure from all that water caused the dam to burst in a cataclysmic fashion and surge down the canyon. The debris flow went through Rocky Nook Park and the Museum of Natural History property, eventually stopping near the intersection of State and Alamar streets. The sound of 10 million cubic yards of earth and stone giving way all at once was likely heard as far away as the Channel Islands.
Boulders the size of cars were easily transported by the flow because the high density mixture of water, fine sediment, and boulders allows large boulders to "float" like corks on the top of the flow. The finer particles that were initially between the boulders drained away with the water after the debris flow, leaving behind the pile of open framework boulders that we see to this day.
Logging requirements:
1. Please send us a note with "Ancient Debris Flows: GC5ZWNY" in the first line.
2. How many members were in your party?
3. Describe the size of the rocks you see in the park. Are they flat or stacked? Is sediment present?
4. In your own words, describe how the debris flow must have appeared while it was occurring.