The geology of Southern CA can get very complicated very quickly. Most of Los Angeles lies in a large deep ocean basin. For the last few million years this basin has been rising due to compression between the Pacific and North American plates (we are right on the plate boundary a.k.a the San Andreas Fault). So most of the rocks around us are Cenozoic ocean sediments (such as mudstones/shales) full of fossil sea creatures and oil! The San Gabriel Mountains (and all the mountains bordering our basin) are a different story, they contains blocks of land that are much older. They are bits of islands or continent fragments that crashed into the western edge of North America as the Farallon plate was subducting underneath. Long story short, some of the rocks in the mountains formed over 1-2 billion years ago during times when the continents were in completely different places than they are now. Instead of the layers of young ocean sediments of the basin, they contain a wide variety of igneous and metaphorphic rocks that are not necessarily related to each other than that they have been smashed together!
One old rock of particular interest to mineral collectors is a gneiss near Ontario Peak. This metamorphic block also contains beds of marble with pyrite and lazurite (Lapis Lazuli - very rare and difficult to find). The gneiss is most likely a metamorphosed ocean sediment as it contains a good amount of aluminum (Al) and carbon. The aluminum comes from clay minerals, so "muds" often have aluminum, not enough to mine Al metal, but enough to make Al rich minerals like corundum (Al2O3). Corundum won't grow until a certain high pressure has been reached. At lower pressures, Al rich minerals like micas will grow first.
Corundum is a popular mineral! When it is vivid red we call it ruby, any other color and it is sapphire. The Cascade canyon corundum is a pretty pinkish-red, maybe a ruby, maybe not quite. It is darkened by inclusions of graphite (C) and rutile (TiO2). Most of the crystals are tiny but can reach up to an inch! Dravite tourmaline can also be found in tiny gemmy yellow-green crystals. Both of these minerals fluoresce vividly in UV light.
Some time ago a landslide moved much of the rock down into the canyon where is can still be collected along San Antonio creek. Much of the collecting area is within the borders of the Angeles National Forest and collecting is illegal without a permit. Collecting permit applications are available from the Forest Service.
The Gem and Mineral council recently had a field trip to collect the corundum. Getting there is not easy! It was almost 100 degrees and field trippers must hike down into the canyon and then fight through bushes and poison oak, climb over fallen logs and boulders! Once at the creek, in the shade, collecting is very pleasant.
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Click to read more about the field trip and Tony's helicopter search and rescue!
One old rock of particular interest to mineral collectors is a gneiss near Ontario Peak. This metamorphic block also contains beds of marble with pyrite and lazurite (Lapis Lazuli - very rare and difficult to find). The gneiss is most likely a metamorphosed ocean sediment as it contains a good amount of aluminum (Al) and carbon. The aluminum comes from clay minerals, so "muds" often have aluminum, not enough to mine Al metal, but enough to make Al rich minerals like corundum (Al2O3). Corundum won't grow until a certain high pressure has been reached. At lower pressures, Al rich minerals like micas will grow first.
Corundum is a popular mineral! When it is vivid red we call it ruby, any other color and it is sapphire. The Cascade canyon corundum is a pretty pinkish-red, maybe a ruby, maybe not quite. It is darkened by inclusions of graphite (C) and rutile (TiO2). Most of the crystals are tiny but can reach up to an inch! Dravite tourmaline can also be found in tiny gemmy yellow-green crystals. Both of these minerals fluoresce vividly in UV light.
Some time ago a landslide moved much of the rock down into the canyon where is can still be collected along San Antonio creek. Much of the collecting area is within the borders of the Angeles National Forest and collecting is illegal without a permit. Collecting permit applications are available from the Forest Service.
The Gem and Mineral council recently had a field trip to collect the corundum. Getting there is not easy! It was almost 100 degrees and field trippers must hike down into the canyon and then fight through bushes and poison oak, climb over fallen logs and boulders! Once at the creek, in the shade, collecting is very pleasant.
.JPG)
Click to read more about the field trip and Tony's helicopter search and rescue!
