Sunday, May 17, 2015

Hash slabs

It is common for fossils in the wild to be found in beds of uncountable fragments, some identifiable to species, others only barely recognizable as fossils. These assemblages are often called "fossil hash", for obvious reasons. Although you can have conceivably make a hash out of just about any kind of body fossil and some kinds of more discrete trace fossils (it takes a bit of creativity here), the term is usually applied to invertebrate fossils. This leads to the phenomenon of "hash slabs" or "hash plates", a more portable portion of a bed of fossil hash. Hash slabs can make great display or educational pieces, and a good slab rewards continued study; you can never find everything the first time, and as you learn more, you are able to find more.

The Paleozoic is great for hash slabs, because of the great diversity of small organisms with hard parts. Depending on when you are in the Paleozoic, contributors to a sea-floor hash may include various sponge groups, two groups of extinct corals, bryozoans, brachiopods, bivalves, nautiloids, ammonites, snails, trilobites, crinoids, various extinct echinoderms, and fusulinids (large foraminifera), with the odd fish tooth or spine thrown in. Compare that to the Cenozoic, where we're down to essentially corals, bivalves, snails, echinoids (sea urchins), and pieces of marine vertebrates. Cenozoic marine sediments are also limited to coastal areas, and as the word "sediments" implies, are often not as well-lithified as the Paleozoic rocks.

A fairly typical Decorah Shale hash slab, with brachiopods, bryozoans, snails, crinoid columnals, and trilobite fragments.

A close-up on the lower left corner of the above slab.

In the Twin Cities, the Decorah is favorable for hash slabs because not only does it feature the requisite diversity, but also because the thin limestone beds naturally weather into hand-sized pieces with fossils exposed in relief. Very little preparation is necessary for a nice piece: all you really need to do is to clean off excess Decorah mud, an operation that requires only water and a toothbrush, preferably one for which you have no other plans. You can certainly do more preparation if you want, but that requires a bit more experience. Ask your dentist about dental tools and choose a less-promising piece for practice, not your favorite. You can pick a surprising amount of shale out of nooks and crannies, but the limestone is sterner stuff, and the fossils tend to be less stern. When collecting, you should keep economy and quality in mind. You don't want to strip an outcrop for rocks that will linger in a box, to be disposed of by uninterested heirs decades from now, when they could have been someone's first slab.

The patches of linear marks are probably part of some kind of trace fossil.

A close-up on a slab featured earlier. Clockwise from top: ramose bryozoan, snail, frondose bryozoan, and fenestrate bryozoan.

A few bits of practical information:
The photography was done with a regular old digital camera on "idiot" setting, with a dark background supplied by the underside of a lap desk. Other suitable substitutes for the traditional black cloth include dark shirts without other printing, and books with dark covers; the idea is just to get something with an even color that is also not shiny. The scale bar is a cut-out of a ruler run through a copier-scanner, with the spaces between alternate ticks colored in. It's not publication-quality, but it's fine for a venue like this. Regular rulers can also work, although they are often larger than the objects you want to photograph, many have shiny surfaces, and you have to worry about depth-of-field issues once the scale is too large to plop down on the slab without obscuring important parts. Coins are more for when you run across something interesting and are without a formal scale; aside from the issue of their being specific to a country, their shininess is also a problem at many light levels.

That honeycomb thing a bit below and right of center is a tabulate coral, probably Lichenaria. It's encrusting a brachiopod.

Four strophomenid brachiopods and an inarticulate form, along with small shell fragments and bryozoan bits.

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