To get to this cache you’ll need to sign up for a docent led hike or bike ride with The Irvine Ranch Conservancy (http://www.irvineranchconservancy.org or irlandmarks.org. or 714-508-4757) through the Fremont Canyon Wilderness. While the trips are free, the land is still private property and you’ll have to pay an entrance fee to park at Irvine Regional Park. These trips are strenuous and involve at least 1000 feet of elevation. You will also get to see more than just the coal seam.
The following information was compiled from the sources listed at the end of the cache.
This coal seam is located in the Paleocene Silverado Formation. The Silverado Formation is comprised of nonmarine and marine sandstones, siltstones, and conglomerates. The upper, more recent, portions of the Formation contain the coal you see. This coal is a low quality coal called lignite. Even so, thicker sections were mined for a short while in the late 1800s.
These mines, the Black Star Coal Mine (N33.774 W117.673) and Santiago Coal Mine (N33.782 W117.703) are located south and southeast of this location. Both of these locations are on private property. When they were active the town of Carbondale sprouted up, but when the mines shut down the entire town was dismantled. The only thing left is a California Historical Marker with a short history (N33.752 W117.6725). That plaque mentions a mine called Santa Clara, but I haven’t found it.
As mentioned, this coal is a low grade coal called lignite. Coal formation begins with the accumulation of organic matter (typically plants) in an area where they won’t decompose easily, such as a peat bog. Over a long period of time, a thick layer of undecomposed plant mater accumulates, along with any sediment deposited during that time. At some point, the entire area is buried and begins to be compressed deep in the earth.
Through the pressure of the rock above and heat from within the earth, the organic matter begins a transformation into coal. Initially it turns into lignite, a soft often brown coal. The resulting layer is about 20% of their original thickness.
As the lignite is compressed and heated to between 100 to 200 degrees Centigrade, bituminous coal, a soft black coal, is formed. During this process some of the water and volatile compounds (chemicals that evaporate easily) leave the coal.
Continued heat and pressure convert the bituminous coal to anthracite coal, a hard shinny coal. The continued heat and pressure drive more of the water and volatile chemicals out of the coal, and allow some chemical reactions to occur resulting in the anthracite. It is estimated that an anthracite coal seam is between 5 – 10 % of the thickness of the original peat layer.
In this case, the transformation process was cut short at the lignite stage when the processes that created the Santa Ana Mountains brought the coal seam back up to the surface.
As you can see here, the coal seam is not very thick. However, due to the historic presence of coal mines miles to the south and southeast, the seam appears to extends for a miles. This is relatively typical of coal seams. Coal seams are often relatively thin layers that extend for miles. Commercially viable mines mine coals layers that are a meter or more in thickness. Based on the above estimates for how thick the original peat later from which the coal is formed, the original peat layer must have been anywhere from 30 to 60 feet thick to create a 3 foot layer of coal and cover huge areas.
From where the coal is exposed in the road cut walk up to the top of the hill (if the docents will let you) to N33 48.357 W117 42.498 where you can see more of the coal seam exposed at the surface a little way down the hill.
Logging requirements:
Send me a note with :
- The text "GCPD2P Coal Seam - Fremont Canyon" on the first line
- The number of people in your group.
- Have a close look at the coal and send me an e-mail with some of the features that you see that likely make it not a viable commercial source of coal.
There are many detailed papers on the formation of coal. The following sources were used to generate this cache.
- http://www.athro.com/geo/trp/gub/coal.html I’d be happy to include any specific references on this location.
- Preliminary Digital Geological Map of the 30’ x 60’ Santa Ana Quadrangle, Southern California, Version 2.0, Compiled by D. M. Morton. Open File Report 99-172. USGS