As part of the run up to ESCONI's 70th Anniversary, here is Flashback Friday post #18. If you have pictures or stories to contribute, please send them over to [email protected]. Thanks!
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We have our Fall Braceville Field Trip coming up next weekend - September 14th and 15th, 2019. Here is an article about Braceville collecting and concretions from the December 2007 issue of "The Earth Science News". The article is by Andy Jansen. Currently, Andy serves as Treasurer and Librarian of ESCONI. He's a valuable member of the ESCONI Board, a great friend, and an all around awesome guy! His wry sense of humor will catch you by surprise! I don't know what we would do without him! Say hello when you see him at the next ESCONI event.
So, Where’s The Shrimp?
By Andy Jansen
I’ve been picking and digging up concretions at Braceville’s tailings pile for over six years now and still no shrimp. The shrimp fever hit me when fellow ESCONI member, Don Cronauer, described his first fossil hunt at Braceville in which he found a perfectly formed shrimp. Wow, I had to find one of those too!
Fossil hunting has always fascinated me since childhood, but unfortunately, it was very hard to find fossils in the granites of north-central Wisconsin – no matter how hard I looked. When Don
began describing ESCONI and the many field trips to nearby fossil sites, I was more than intrigued and soon after that I joined ESCONI. Since the Braceville site is private property is private property, I knew that I had to join ESCONI for collecting trips.
My first ESCONI-organized trip to Braceville was with my daughter. It was thrilling, especially once I figured out which rusty rocks had potential. Within a few minutes of searching the ground hunched over, I came across an opened concretion, which turned out to be a pectinoid (a bivalve with a straight-hinge line; scallop). Naturally, the feeling came to me that this must
be a common fossil at this location and that I would be finding plenty more before the day was done. It took me another five years and literally thousands of concretions before I found another. For that matter, I believe Don still hasn’t found another shrimp. There is a lot to be said for beginner’s luck!
One observation I made early on was that the concretions seemed to run in veins, like gold strikes. During other ESCONI trips, I mined one strike for a whole summer with my cousin and dad using shovels and a mattock until we had dug a trench that was almost four feet down and over twelve feet long. It was one of the best veins we ever mined as it yielded at least eight five gallon pails of rusty “gold”. I’m sure there was still more to find, but the whole operation started to resemble a major public works project. It was time to stake a new claim somewhere else.
My dad and daughter, Laura, working the fossil vein
Another observation I made was that round rocks roll downhill. There were probably two major times when the concretions rolled. The first was during the mining operation (in the 1890’s)
when the concretions were being dumped out of a large bucket or off of a conveyor belt. Try pouring a pail of sand mixed with river rocks to make a cone pile and you’ll see that the rocks will tend to separate from the sand and roll to the base. The other event that would dislodge the concretions from their shale matrix is weathering, particularly erosion and frost heaving. Both mechanisms occur on a seasonal basis (for over a century now) and result in the concretions rising to the surface where wind, rain, and other fossil hunter’s footsteps start the rock on a roll. But of course, the urge to climb to the top of the Braceville mound is too much for the child in us to resist. (I wouldn’t recommend climbing it lately though, because its one side is nearly vertical and could collapse.)
Now how to open these treasures? Smashing them with a hammer is tempting, but on the few times I did it I had mixed results. Quite often the concretion would split on plane with the hammer’s strike point and not on the fossil’s plane. Unless you have the eyes and skill of a diamond cutter, I wouldn’t recommend it as a first choice. Tapping two concretions together long their edges yielded better results with less damage to the fossil image. The best method to start with is the freeze-thaw method (in water) that nearly all of the experienced ESCONI members advised me to try. There are limits to one’s patience with this method. Some concretions I have freeze-thawed over a hundred times in my freezer and have found them to be very stubborn. Most successful openings happened between five and twenty thermal cycles for me. But as another ESCONI member told me, “these fossils have been sealed for 300 hundred million years – they’ll open when they want to”. At some point the hammer will come out, but for now I’m just letting them enjoy the winter weather in old milk jugs.
My Braceville collection is constantly growing – there’s always some cycling through the freezer. Many of the concretions open to reveal nothing inside, which means the conditions were not right for the plant or animal to leave an impression. Generally, I get one keeper for every dozen duds. Here’s a rough tally of the fossils I’ve found at the Braceville site over the last six years:
92 jellyfish
20 plant stems/stalks
5 leaves
2 gastropods
86 bivalves
29 “plant mash”
18 worms
11 fanworms
3 bark pieces
3 shark egg cases
1 mass of fish larvae
1 sea cucumber
In addition to these, I have well over 40 specimens that are best described as “rocks with character”, many of which are probably coprolites and poorly preserved jellyfish and worms. This tally will definitely change as I am still learning how to properly identify all of the fossils I have, but it does give the beginner a rough idea of what to expect. After a while you no longer get excited about finding another jellyfish or clam. You’ll notice one absence in the list – no shrimp!
