Columbus limestone is a common Devonian formation in central Ohio. Exposures are numerous, ranging from many quarries in central Ohio to the banks of the Olentangy and Scioto Rivers. It is quite fossiliferous, bearing many brachiopods, along with a scattering or pockets of gastropods, blastoids, crinoids, cephalopods, fish parts, and rarely, trilobites.
The most "common" trilobite is Coronura, which is only found as a pygydium. Intact Coronura are extremely rare. Other trilobites (typically fragmented) found in the Columbus Limestone are Basidechenella, Trypaulites, Anchiopsis, Odontocephalus, and Phacops.
Fish parts are typically plates, spines, and teeth. The plates may be very large, dimpled with various sized pustules. On a boulder at first glance, a plate appears to be a coral colony. When prepped out, the fishplates turn to a beautiful chocolate brown and black color.
Blastoids are sometimes found, 2 species predominate from the Columbus Limestone. Nucleocrinus verneuilli is big and round with no ornamentation, Codaster pyramidatus is an oval shape with 5 points on the top forming a star shape. Several genuses of crinoids are usually found in fragments, but occasionally a complete one is found. The Columbus Limestone contains many types of mollusk, brachiopods, coral, bryozoa, etc.
The Columbus limestone is fabulously fossiliferous, however it is also notoriously hard. I received three specimens to prepare, each solidly encased within the limestone; I had my doubts as to what I could accomplish. Surprisingly, the dolomite air abrasive dusted off the exposed fossil with relative ease, but as expected did not cut the limestone. My ARO pneumatic scribe barely made a scratch on the hard rock. I pulled out the big gun, the Chicago airscribe, and carefully and slowly worked around the rock surface away from the fossil, finding I could controllably remove the matrix.
The fossil I chose to prepare first was a large blastoid. The only exposed section was a small portion of the calyx. As I slowly approached the blasoid with the Chicago, the limestone separated neatly and perfectly from the fossil! The blasoid, a chalky mound when I received it, began to look great after the initial dusting and scribe work. I worked the matrix around the blastoid, making sure not to work into a hole or ditch. Amazingly the fossil cleaned up beautifully in just none night.
The next fossil was a large spiny gastropod. Unfortunately most of the spines appeared to be broken off. I hoped to expose intact spines from below the limestone surface. This fossil required patience and a light touch to not damage any unexposed spines. I worked back and forth between the Chicago and the air abrasive. I decided to mix in about 20% AlOx #17 into the dolomite powder. It did not cut the limestone any better, however it did remove loose pieces of matrix and exposed the fossil’s pits and crevasses much better than plain dolomite.
Although the gastropod was big, fat, and sassy, every spine had been sheared off. I chose to expose only the top third since the spines were all missing and money was an issue with this prep job.
The last piece was a double blastoid. Much more confident now, I worked back and forth with the Chicago and air abrasive. This piece turned out great, with one perfect blastoid and the other slightly crushed but intact.
I recently worked on a few trilobite pygidia. The Coronura had dense, stable exoskeleton and cleaned off nicely. However the Phacops and Trypaulites pygidia were very flaky and powdery from extensive weathering. There was not much I could do to dress them up without major restoration efforts.
Columbus Limestone is a difficult rock to work with, but the specimens are usually well preserved and offer the preparator a good opportunity to clean up some unique and beautiful specimens. Patience and care while working a strong hammer (such as the Chicago) directly up to the fossil will provide fabulous end results. I am told my client who collected the specimens and figured they were unpreparable, is still smiling!
What's New? -*- Home -*- Qualifications -*- Equipment
Clients -*- Shows -*- Publications -*- Guestbook -*- Links
Email Marc
Behrendt at FossilPrep@aol.com
This site is
maintained by Basic Page
You are our visitor!
Best experienced with
Click here to start.