Sorry about taking an extra day to give you the answer. These days are busy, and sometimes things get slightly out of hand.
The real reason... I’m busy reshooting videos for the refresh of our “Power Searching with Google” MOOC (online class). If you missed it on the earlier passes, there will be another chance for you to take it beginning around mid-April. Stay tuned—I’ll be sure to let you know exactly when it is. (But in the meantime, that’s what’s consuming all of my time these days.)
On to our search challenge…
Recall that the questions were (1) When did the mine open? And, (2) What was mined here?
First, you have to realize that the GPS data wasn’t quite right. Once you’ve extracted the lat/long from the EXIF data, you’ll quickly see that there’s no labyrinth exactly at that location. The quarry hole and labyrinth are actually at: 37.853058, -122.190440 (I assume you figured that out and just panned around the location in Maps for a minute before finding the actual location).
By looking at the map I could easily tell that this location is part of the Sibley Volcanic Regional Preserve, which is a beautiful open-space preserve run by East Bay Regional Park District.
So I started with the obvious query:
[ Sibley Volcanic Regional Park history ]
I then did:
[ "round top" intext:quarry sibley ]
and found an article in BayNature (a local natural history magazine) that mentions the quarry was “Kaiser Sand and Gravel.” (I later watched a video that was posted on the Sibley Parks page where naturalist Steve Edwards says the same thing. See: http://science.kqed.org/quest/science-hike/sibley-volcanic-regional-preserve-exploration/ )
Now we have our first clue: The quarry was the “Kaiser Sand and Gravel” company.
I know that counties often track industrial sites and mines are often described (for historical reasons) as being in a particular county. So after looking at my map to see what county the Sibley Preserve was in (A: Contra Costa) my next query was:
[ Kaiser quarry Contra Costa ]
Which led me to a remarkable map at Mindat.org (a site that compiles information about mines—who knew there would be such a resource!) http://www.mindat.org/loc-3448.html#themap
A quick look at the map, however, tells me there are TWO mines/quarries in that immediate vincinity. The “Henry J Kaiser Company Mine” (aka “Kaiser Sand and Gravel) AND the “Tunnel Rock” mine. Let's check out these alternate lead.
A search for:
[ “Tunnel Rock” mine Contra Costa ]
leads me to http://mineral-resources.findthedata.org/l/112485/Tunnel-Rock, which gives me the lat/log (37.855202, -122.201851) for the Tunnel Rock min and a description of what they mined (“construction sand and gravel; broken stone”)
Well… THAT’s interesting! Is it really the same mine by two different names? Doing the same thing for:
[ “Kaiser * mine” Contra Costa gravel ]
show a few additional results. (Since there are SO many Kaiser mines, I had to add in “gravel” to limit my search to something managable).
Looking at the content, I see much of the data comes from the book: "Mines and mineral resources of Contra Costa County" (1958) – alas, it’s not available on the web. (I discover later that this book is an extract from “California Journal of Mines and Geology, volume 47 (1951)”… also not available on the web.)
BUT I can find a copy in the library! (And I’ll head over to Stanford later this week to see if I can find it in the stacks.)
I still haven’t figured out if this is a different mine, or the same mine. Searching for
[ “henry j Kaiser company mine” ]
(a variant phrase I picked up from reading), I find another resource http://www.mindat.org/loc-76633.html that gives me the lat/long for this mine: 37°51'36", -122°12'1"
Continuing my search, I find another good hit at http://minerals.livescience.com/l/28961/Henry-J-Kaiser-Company that repeats what we saw earlier—that the quarry was used for broken rock, sand, and gravel.
So… after much looking around, I discover that there are actually TWO mines near that location. There’s a great page on Mindat.org that shows the distance between the mines: http://www.mindat.org/usdistance.php?loc=76663 (shows that Tunnel Rock and Henry J Kaiser Company mines are within 1700 feet of each other). Their map shows it like this:
And the Kaiser mine:
Here’s my Google Maps version of the two mines.
The Kaiser mine is the red arrow, and the Tunnel Rock mine is in blue.
So now… when did they open?
Like other readers, I’ve only been able to find a few clues here and there. I found a couple of hiking guide books that claim the mines were opened in the 1930s… but they don’t say where THEY found that information, so I’m worried that the reliability might not be very high.
And when I look at this location with Google Earth (using the Time Slider), I see the following images—from 1939, 1993, and 2002.
So… I’m a little stuck here. I think we’ll have to find an additional source of information. Were these mines REALLY active between sometime between 1993 and 2002? OR… is the Google Earth image for 1993 just mislabeled??
More research is required! Ideally, we'd like to find a different aerial archive image with dates in the 1930s and '40s; or maybe someone's first-person account of mining in this area.
The labyrinth: By the way, the labyrinth is the work of Helena Mazzariello: see her page on the labyrinths at http://home.earthlink.net/~friendsofthelabyrinth/
Result: Still searching!