Tecopa Opal Beds and Emigrant Pass Trilobites, Sat-Sun, Oct 29-30, 2022 (updated 10/26)

UPDATE 10/26:

Hi everybody,

Three days left until we meet in the Tecopa area — the forecast is good, and the fieldtrip is a go!

I’ve got questions from people who weren’t ready to camp with us – yes, it is fine to join us only for a day, your choice.

For people who would like to join us on Sunday for trilobite collecting only, here are the coordinates of the Emigrant Pass parking area where we will be: 35°53’04.2″N 116°03’52.9″W (35.884511, -116.064700). We should arrive there by 9 AM.

Hope to see you all there!

UPDATE 10/9:

The current heatwave’s long tail end does not want us in the desert next weekend – the forecast shows temperature going up to 96F for Friday and down to 91F for Sat, but there is a pretty good chance that Saturday will be even hotter than that. I cannot risk it, so I’m moving the new date by 2 weeks to the Oct 29/30 weekend. I’m sorry if it negatively affects anybody’s plans (again).

The heatwave season lasting 6 months by now (since the end of April) is quite detrimental to collecting and not a good sign for next year either, but it looks like we might need to get used to it step by step…

UPDATE 10/2:

A bit of bad weather news is forcing my hand – it looks like another secondary heatwave is forming in the desert. In the last week the forecast was shifting upward one degree per day pretty much. It is showing 94F for Sat 8th currently. These are not safe conditions for enjoyable collecting there. At this point I have to postpone by one week. An early forecast shows max 88F for both Sat 15th and Sun 16th. This is a bit more bearable, but not for everybody. I do not advise attending to people with health problems, as 88F is quite taxing on the body.

All the dates are shifting by 7 days to the Oct 15/16 weekend. There is still some chance that the heatwave will not let go that easily, in that case I’ll have to postpone again to November timeframe. I’ll send another update in a week.

UPDATE 9/25:

The trip is still on schedule for the Oct 8/9 weekend. The current heatwave is supposed to subside in a week — the forecast for Tecopa shows 86F on Sat 10/8, it should be all right for collecting most of the day. Might get a bit toasty on Sun with 88F after lunch, but most people should find some trilobites by then. Hope to see you all there!


Hi, fieldtrip friends!

We will start the fall season with a 2-day trip to classic California locations! The locations are easily accessible with regular low-clearance cars.

On Sat, Oct 29th, we will visit the famous Pleistocene Lake Tecopa Beds in search of the nodules containing precious opal. Some of the opals resemble white porcelain, frequently with pale signs of colorful fire. However, the best fire reflection is exhibited by translucent grains, several mm in size, sometimes up to a centimeter. Most of the grains are smaller, though. Some cavities are quite angular, which inspired a recent article in Mineral News’ May 2022 issue by Bob Housley et al. The article posits that the angular cavities were formed by the dissolution of gaylussite, soluble sodium & calcium carbonate monoclinic mineral found in alkali lakes in the desert, including Searles Lake. The opal filling such cavity becomes an opal pseudomorph after gaylussite. The cavities in the nodules can host other minerals, too, like gemmy scalenohedral (dogtooth) calcite crystals or chabazite. The walls of some cavities are covered with microscopic pearly-opalescent rectangular platelets of searlesite, a sodium borosilicate mineral. It was discovered at Searles Lake but is more known from the Boron pit occurrence.

Gully Entrance
Dark Nodule Layers in Lacustrine Bed

We will meet on Saturday at 8:00 AM at the side of SR-127 at these coordinates: 35°53’09.8″N 116°15’26.3″W (35.886041, -116.257310). The meet place is 217 miles away, an approx. 3-hour drive from Pasadena, a few miles south of Shoshone, just off SR-127 road. If you get to the Zabriskie site, you are 1 mile too far north. The mouth of the gully is easily seen from the road, so even if you get there late, you’ll be able to find the group easily. My plan is to get there Friday afternoon and spend the night on site. Camping is quite rough, with lots of rocks in the washes, last time there were some flat spots, but big areas of desert got rearranged by the recent monsoonal storms.

Here are the Google directions: https://goo.gl/maps/V6ETcDK3U8nwteRt6

On Sunday morning, we will leave the opal area for a short 16-mile ride up the Old Spanish Trail Hwy to the Emigrant pass. There are many outcrops of trilobite fossil-bearing shale near a parking pad near the pass, creating many opportunities to collect between the road and the wilderness boundary. After the morning fossil collecting, our trip will conclude.

The essential tool for the opal beds is a regular pick hammer to dig out nodules and break them up to check for opals. The trilobite hunting might require something heavier to break up or lift out bigger shale blocks, like a sledgehammer and gad or longer pick, but nice headpieces up to 8cm were found with a regular pick as well.

Hope to see you all there,