Tuesday, October 27, 2015

Medicine Lake Obsidian

Medicine Lake, California
Obsidian Galore!

Earlier this year I landed a contract to update the old book "Gem Trails of Northern California" by James Mitchell. It's time for a make-over; the current book has few color photos, no GPS coordinates, and has been overtaken by events in more than one locale. But for the purposes of this blog post, I recently visited Medicine Lake, California, and found quite a bit of obsidian there. This will be a new locale for the updated book, which won't hit the stores until 2017.

The area in question isn't really near any town - it's about 30 miles south of the Oregon-California border, a long ways from CA 139, CA 89, and US 97.
Figure 1. Medicine Lake.

Several geologists have studied the area, and I got some good help from Howard A. Powers, "The Lavas of the Modoc Lava-Bed quadrangle, California," published in July, 1932 in the American Mineralogist. Powers mapped much of the obsidian, and this gem helped guide me:

Figure 2. Geologist Howard Powers mapped much of the obsidian here in his 1932 paper.

We drove in from the north, hitting two well-known tourist spots along the way: Petroglyph Point, and Captain Jack's Stronghold.

Here are two GPS coordinates to get started:

41.84539, -121.39129   Petroglyph Point
41.82477, -121.50008   Captain Jack's Stronghold

Petroglyph Point was very interesting - I didn't see many animals or humans depicted, which was odd. Lots of cool symbols, tho, and very much worth the stop. The turn from CA 139 is easy to find.

Figure 3. Symbols at Petroglyph Point.

If you're not familiar with the history of this area, a band of about 60 Modoc Indians held off a far superior force of US Cavalry for many months. As negotiations to end the stand-off faltered, some of the warriors convinced Captain Jack (real name: Kintpuash) to slay General Canby, for which Jack was subsequently hanged. Here's a link: https://en.wikipedia.org/wiki/Kintpuash.

It was fun to bring along fellow Recon patrol member Dirk Williams, who is a full-blooded Modoc Indian himself. Dirk seemed to easily slip into some of the foxholes and firing positions.

Figure 4. Dirk Williams explores one of the foxholes at Captain Jack's Stronghold.

From Captain Jack's we went further south and passed numerous lava tubes and caves. The basalt lava here is ropy - called pahoehoe (pa-hoy-hoy) in Hawaii, and the name stuck. Learn more at http://www.decadevolcano.net/photos/keywords/pahoehoe_lava.htm. We passed on the caves and went almost due south to the first big flow, but the lava here isn't attractive obsidian. So we moved further south and camped for the night at Medicine Lake Campgrounds. We chose A.H. Hogue Campground, selecting a nice spot right on the lake. The water was very low.
Figure 5. Low water at Medicine Lake in October, 2015.

Note that there are trails to the lava flow from the campgrounds, and there's decent obsidian on the south end of that flow.

The next morning we drove almost due east on 43 N 77 from the lake to Little Glass Mountain. There was decent obsidian at two spots:
41.57541, -121.69026   Little Glass E
41.57153, -121.69909   Little Glass W

There's good access from either spot, although the E site seemed to have better glass.

Figure 6. Good obsidian easily collected at Little Glass Mountain, west of Medicine Lake.

From here we headed east toward Glass Mountain. It was disappointing to see "No Collecting" signs on the southern edge of the flow. There are many more access points to the north, however, and several archaeology sources list aboriginal quarries on that side of the flow.

41.62092, -121.48708   North side of Glass Mountain

Further to the east, near Cougar Butte, there's a final addition to your obsidian collection: spherulitic obsidian. This type of rock has inclusions of gray cristobalite:

Figure 7. Tiny cristobalite spheres in bands of obsidian near Cougar Butte. 

Mindat discusses this further at http://www.mindat.org/loc-215047.htmlhttp://www.mindat.org/loc-215047.html.

Here are the GPS coordinates for this spherulitic obsidian locale:

41.65874, -121.45459   Spherulitic obsidian in bands

There are probably many more spherulitic occurrences out here. Just a couple more notes - I first came into this area from the west side, via Macdoel in late spring, but there was snow blocking me on Davis Road just a couple miles from Little Glass Mountain, and when I tried different routes, it was so early in the year that there were still many blown down trees blocking the back roads. Best wait until closer to Memorial Day...

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