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Alcatraz 'the rock' Formation Earthcache EarthCache

Hidden : 02/15/2010
2 out of 5
3 out of 5

Size: Size:   other (other)

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Geocache Description:

As an earthcache, there is no “box” or “container” to discover. Rather, with this cache, you discover something about the geology of the area. For more info, consult

There is a fee to visit the island, as it is part of the National Park System (

Alcatraz Island is one of several islands in the San Francisco Bay that show us interesting variations of the Franciscan Complex rocks of several different terraines.

Originally, during the Ice Age eras, these islands were the tops of hills. As the ice melted at the end of the last Ice Age, the valleys were flooded, forming these islands.

Unlike several of the other islands (Angel Island, Yerba Buena Island, Red Rock, Brooks Island, and the Marin islands), the graywacke rock on Alcatraz is part of what is called the ALCATRAZ TERRAINE, an unaltered version of graywacke (that can also be seen on Telegraph Hill in S.F.). The Alcatraz formation covers most of the northern and eastern parts of San Francisco.

Graywacke in this area was formed by subduction action as the North American Plate collided with the Farallon Plate, and slid under it. The sediment that was scraped from the North American landed in the trench in the bottom of the subduction zone. There it compressed into sandstone. The turbidity flows that were responsible for the deposit of Graywacke is basically an underwater landslide (often started by earthquakes). During most turbidity flows, the large sediment settles out of the water first then lighter to the finest sandstone and shale layers. The next turbidity flow then deposits a new course to fine layer above the previous one. If a turbidity flow settles the layers quickly, they are more jumbled, which means that the whole formation is approximately the same composition.

Graywacke is gray when it is fresh, but ages to a brown (as you can see). Graywacke differs from Sandstone in that the sediments vary dramatically in size and composition, making the rock look "dirty", especially in comparison to the relatively even texturing and look of sandstone. The Bay Area has at least 11 different types of Graywacke that have been brought together (after their formation) by slip-fault action, and they vary dramatically in their composition, reflecting their varied origins.
Graywacke is deposted in one of two ways -- either as one massive turbidity flow or by several small ones. The texture of the rocks and coloring (based on sediments present) help us tell how many flows (and thus deposit originations) were involved in creating the formation of graywacke that we view. Sometimes, fracturing makes it LOOK like there are a lot of layers of graywacke. How can you tell the difference? The best way is chemical sampling! Otherwise, visual identifiers include DISTINCT coloring/texture denote several flows, while similar texture and simply seeing "dividing lines" tends to denote fracturing.

Folding is created by layers of sediment (first stronger, then weaker layers of rock) that were laid down in the ocean (as part of the sedimentation of graywacke. There are different types of "ribbon chert" folds though it is beyond the scope of this earthcache to describe in detail these different types. (see (visit link) for a great description with pictures of different folds)

Logging Requirements:

PLEASE SEND YOUR ANSWERS SOON after you log you find or I may need to delete it!

1. Based on the rocks you can view from this location (as well as your walk UP the hill from the dock), do you believe that this island is created from ONE or SEVERAL turbidity flows?
2. Ahead of you is a "fold" of rock, composed of chert. Measure the WIDTH of this fold, and describe the COLORS and TEXTURES of the rocks WITHIN this fold.
3. Post a picture of yourself and/or your GPS at the listed cords. If you don't wish to post a picture answer this question: what is the WALKWAY at your feet composed of? (pavement, cobbles, bricks, etc.)

Great USA weather temp/map: (visit link)

FTF: StinkyMiller

Additional Hints (No hints available.)