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Hello everyone! Today I ventured out to the Mazonia-Braidwood State Fish and Wildlife Area for my first time, and it was… something. This post is going to serve as both a journal of my day, and as a request for assistance as I try to figure out what exactly a concretion looks like.371CF9C7-1630-4AFD-9682-7DD32584581A.thumb.jpeg.f5ea015d0af4723353337b74305f825a.jpeg


Once I got my permit outside the office (which is just down Huston Rd a bit from the IL-53 and Huston Rd intersection; this is for people like myself who struggled to find an address or location of the office), I headed to Mazonia south unit, where I went to fossil hunting site 3. There were not really any exposed rocks here, so I went deeper into the brush. I spent two hours picking up random rocks that looked remotely orange and red, until I realized that I was probably looking for the wrong types of rock in general. I had seen a ton of pictures online of concretions from Mazon creek, but all of them were of the fossils people found on the inside, which left me befuddled about what the outside looked like. I walked around until I found a place with cell service and planted myself on rock so I could watch a couple YouTube videos of people who had visited the area before. I learned a little about what I should be looking for, but I was still very confused. I managed to find the rock below, which looks vaguely like a fossil and gave myself some false hope, but I believe it’s probably just the way minerals formed on the rock that are deceiving:



As morale dwindled, I made my way back toward the parking area and decided to walk a bit down the road. I found a neat little skull in a ditch as I was walking:



As I kept going, I started stopping at some small exposures within a few feet of the road, and that is actually where I saw what I now understand to be the nodules I should be looking for. It was one of the two below:


It made me feel a bit better that I found an actual concretion, as I went much of the afternoon without seeing any rock exposures. I have come to believe that fossil hunting site 3 may have a dearth of concretions, though I may be incorrect. Anyway, after finding the first concretion, I found a few similar rocks. However, I’m not entirely sure if they are potentially fossil containing material or if they’re just rock:


They all have the dark red sections that I associated with iron, so I took them with. That is about the extent of what I grabbed that I thought were  concretions, though I realize now that not all of those are even concretions.


As I was wrapping up a mostly forgettable day, in the last roadside stop before my car, I came across this:



I asked elsewhere on the interwebs and was informed that it might be a trackway, which I would think would be from an insect. However, if it is something else, please do let me know. Regardless, I was thrilled with this. I didn’t know what it was, but I did know it wasn’t something I had ever collected. In fact, today was the first day I have ever collected fossils from organisms that lived on land. It more than made up for the cold, gloomy weather and the disappointment I had experienced earlier. Just make sure to know generally what you should be looking for before you get there, unlike me.


Now to my big question. How do I differentiate concretions from any regular old weathered rock? Are they typically round to some extent, as I see on others’ posts, or do they sometimes occur as jagged shapes? Are spherical rocks more likely to be regular rocks, whereas concretions are generally flatter in a dimension? I understand their shapes and sizes are diverse, but are there any patterns that tend toward a concretion over any a generic rock?


As I finish up this topic, I do have one question about the two oval concretions I showed in my hand. Are the theoretical fossils inside the “egg” shape that lies inside the shell, or would they be on the outside of the egg shape? In other words, once I remove the rest of the shell from each of them, do I do the freeze-thaw cycles on the rock inside the shells?


Thank you for reading and helping, and I apologize for all the questions!

Edited by Ordivician19
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You should consider joining a fossil/mineral club like ESCONI to put you in touch with people with knowledge of the local fossil sites.

Clubs also get access to places to fossil hunt that individuals cannot always get into. Well worth any dues or annual fee.


Before any fossil trip I make, I try to make sure I know what I am looking for, What types of fossils can be found, and be aware of what the fossils/rocks of the area look like.  Even Googling images of fossils from a certain area is helpful.

When I Googled "Mazon Creek Concretions"  I was presented with a number of pictures of unopened nodules. Further refining of other search terms may yield more/better results.


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Sounds like my first 2 trips to Mazonia-Braidwood (only you came away with more rocks than I did). :P


I came with only a vague clue as to what I was doing and severely unbalanced ignorance to experience ratio. Took me to the third trip to realize that the concretions weren't going to just jump out at me from the trails. ;) The official map of the area will help when discussing the area:




For the most part you'll want to be focusing on the two sections south of the large Braidwood Lake. The top one is labeled Mazonia South Unit and the one south of South I'll call the "Lake Unit" for seeming lack of a more official name. It is the one containing Monster Lake, Eagle Lake and Pondo Lake. It really helps to look at this area from the satellite view as you get a better idea of the topology of the area:




If you zoom into the area just north of the Ponderosa Lake parking area you'll see that it is corrugated like a Ruffles potato chip. These are the traces of the century old "tipple piles" where they dumped the overburden material from the coal strip mines into long ridges side-by-side forming these elongated piles. They have much worn and eroded and are mostly covered with scrubby trees and underbrush (and possibly the highest density of ticks in the known universe). :Horrified: You'll also see there is a trail around the lakes that is poorly marked in the Google Map overlay (the trails don't align very well with where the overlay things that are). This is a very long walk and I've never completed an entire circuit around the lake (especially since I tend to be carrying a backpack of heavy concretions before too long). Many of the tipple piles are encompassed by the lake. People launch boats from the ramp and go fishing and I suppose if you were boat-enabled you might be able to get to some of the more remote tipple piles within the lake. Can't say if you'd find anything there as I've never tried. Just on the west edge of the lake complex you'll see the foot trail (no cars permitted) that runs south before it turns east to loop around the lakes. I've walked down this trail several times and ventured off into the bush heading west to walk up and down the tipple piles. I strongly recommend bringing a compass (not a compass app or a smart phone GPS). Cell coverage can be pretty poor and GPS (mine anyway) often struggles to lock into a signal deep in the brush. If you have a low-tech compass and head west from this trail you'll always be able to re-orient and head east to rejoin this trail after your searching up and down the tipple piles is complete. Way too easy to get disoriented and wander around in that section for far too long--the up and down of the tipple piles are all too similar and natural navigation by terrain is pretty much useless once deep in the underbrush.



Wear long pants, long sleeves and decent shoes/boots. I suggest picking up some of the Velcro pant leg straps as they are useful for keeping ticks from gaining entry up your pant legs. Wear a shirt you can tuck in if possible and spray any entry points with some strong DEET spray to try to keep your tick to concretion ratio well below 1. I've found that hats are often quite useless while wandering around under the brush as the branches are quite adept at stripping hats from heads. My backpack has a loop at the top to make it easy to move around when it is not on your back. I cannot count the number of times that a tree branch has deftly slid within this loop to stop my forward progress while scuttling around under the shrubby undergrowth.


Things that will help you find concretions: 1) Get off the main roads and into areas where people haven't been hunting lately. Concretions weather out slowly and an area can be cleaned out of fresh concretions fairly quickly. 2) Hunt in areas where there is little or no vegetation on the ground. It can help to go early in the season before the ground re-vegetates after winter. Areas under the tree canopy get less sun and so there tends to be less ground cover under tree cover. 3) Steeply sloping surfaces accelerate erosion which uncovers fresh concretions. Hunting on flat ground is generally not nearly as productive as scrambling around (safely) on steeper slopes of the tipple piles. Sometimes the valley at the bottom of a steep slope accumulates concretions that have eroded out and slid down slope.


For further information on hunting this area, do some searches here on the forum. You'll turn up some wonderful topics:











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Mark Kmiecik
Posted (edited)
11 hours ago, Ordivician19 said:

As I finish up this topic, I do have one question about the two oval concretions I showed in my hand. Are the theoretical fossils inside the “egg” shape that lies inside the shell, or would they be on the outside of the egg shape? In other words, once I remove the rest of the shell from each of them, do I do the freeze-thaw cycles on the rock inside the shells?


Yes. The two in your hand in the photo are what you're looking for. They vary in size from the size of a pea to the size of a huge cucumber, 17 to 18 inches long, and everything in between. They are round or elongated and flat-ish. The "shell" part is exactly that and will probably fall off by itself after a few freeze/thaw cycles. Those two look quite promising. Remember to let them soak for at least a week to ten days before you start the freeze/thaw. Check for signs of opening every three or four cycles. Good luck.


You actually did fairly well for a first trip on your own. You found two more than I found on my first two trips. The grey rock with the "trackway" is the Francis Creek Shale itself, the member in which the nodules are found.

Edited by Mark Kmiecik
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Thank you all very kindly for your responses.

@Fossildude19 I got a bit overzealous when deciding to drive out there without proper preparation, so I’ll certainly try to do the necessary work before my next trip as far as knowing rock types and what to look for. I luckily did find a local fossil group a few weeks ago, but they actually happen to be from elsewhere in the Midwest as I took a bit of a drive to get out to Mazonia.

@digit I’m glad to hear that I wasn’t alone in my confusion, especially at first. It seems the only thing I was ready for was my anti-tick armor :heartylaugh:. A compass is something I’ll definitely have to get, as even a tiny venture into the brush turned me around a bit. And that’s on more or less flat land without any exposures. This seems like great advice for any outing.

@Mark Kmiecik I appreciate the confirmation on the two concretions, and the general description. I’d seen one concretion in my life prior to this weekend, and it was an opened and polished one gifted to me. Knowing what I need to be looking for is critical. I’m excited to start soaking what I do have, and the tips you provided are great.


Thank you all again! Your responses have been incredibly informative, and I will be using your tips on future excursions.

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Just to add some more local encouragement and perspective- your finds are about equivalent to my first trip to the park about 7 years ago. In the years since I have gone on an average of one trip per year, and still only find 1-2 gallons total on each trip. Additionally, out of all of those concretions, only a small handful will produce anything of note- for instance, from my 2020 trips I only ended up keeping 1 worm, 1 partial shrimp, 1 fern, and a couple of nice Essexella "blobs". Everything else was blank, poorly preserved, or an unexceptional Essexella (no knock on this fascinating fossil, I just have limited storage space!).


It's a tough spot to collect in, honestly probably the most physically difficult that I go to regularly, and the rewards can be slim as well. But the potential is there to find something incredible and if you are able to go enough to establish a regular spot you can get more consistent results. It also has the advantage of being open to the public without need to get special permission or be a part of a club. 


Speaking of clubs, I will second @Fossildude19's suggestion of joining ESCONI- they have twice yearly trips to a Mazon-area spoil pile which is much easier to collect at than the park, but produces very similar fossils.  



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Yeah, the first time anywhere can produce absolutely nothing and so can the 25th time.  You can say you collected "Mazon Creek" now and that's something.  I've only driven through Illinois - would have been great if I had had the time to check it out like you did.

Edited by siteseer
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