Fossiliferous Rocks for Dummies

Fossil in Burgess Shale

We regularly subject the staff geologist to some rather irksome copy from the keyboards of freelancers, especially those who plied their trade at eHow. Some, in fact, still ply their trade there, but that's another problem. Whatever. Whether it's trying to say that oil and natural gas form in pockets, pools, or layers; or misinforming readers that the tectonic plates float on liquid magma; he's seen just about everything. Well, he thought he had before he came upon Cathryn Whitehead and her Sciencing.com post, "The Rock Most Likely to Contain Fossils."

Simply put, the answer Whitehead needed to give was "Sedimentary rocks." To her credit, Cathryn labored mightily to say just that. Her problem was that she was required by the site to pad her answer out to at least 300 words. She did that, actually expanding the world's volume of knowledge with a whopping 570 words... except that not all of them are correct.

Whitehead's opening statement covers it all:

"Fossils, the preserved remains of animal and plant life, are mostly found embedded in sedimentary rocks. Of the sedimentary rocks, most fossils occur in shale, limestone and sandstone."

Truth be told, that's all she needed to say... except there was still the pesky minimum word count. Because of that little detail, Cathryn visited upon the internet such factoids as,

"Mud forms when larger rocks erode into tiny, usually microscopic, particles. These particles settle in the calm waters of lakes, swamps and the ocean, covering creatures that live there. Mud and clay combine with minerals and other particles over time to harden into shale." – Wait: where did that clay come from? How does it differ from those "minerals"? What are those "other particles"? [HINT: "clay" refers to a group of minerals...] "Fossils inside shale often include brachiopods, fossilized plants, algae, crustaceans and arthropods trapped in the hardened mud." – What about other animals, like clams, snails, fish, even vertebrates? Oh, and by the way? Crustaceans are arthropods. "Limestone forms when calcite from the water crystallizes or when fragments from coral and shells cement together. " – Ugh. Apparently, Cathryn's unaware that micrite, also known as lime mud, is mostly the remains of calcareous algae. Calcite is quite unlikely to spontaneously crystallize from sea water.

4. "Conglomerate rocks form from combinations of large and small rounded pebbles, often containing quartz, cemented together over time... Conglomerate... rocks do provide fossils periodically, however, in the pebbles that make up the rocks."¹ – Saying that conglomerates contain fossils because some of the clasts may be fossiliferous is quite a stretch!

As so often happened in the eHow family, Whitehead got the right answer. Unfortunately for Cathryn (and her readers), she was stuck needing another 300 or so words. Since she lacked the knowledge to reword more authoritative references without introducing errors, she created the post we hereby award the Dumbass of the Day.

¹ It's interesting that this section has been rewritten to remove the claim that conglomerates "form faster than shale, limestone and sandstone and are under more pressure from fast-moving waves..." Wonder when that happened...

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