Fossiliferous cherts
deutsche Version
Scolecopteris sporangia with hairs, unique specimen
Unlike the conspicuous slabs with large compressed fossils often seen in museums, fossiliferous cherts usually do not make large fossil exhibits but nevertheless impressive samples if cut and polished, with life-like three-dimensional preservation in partially transparent chalcedony, often beautifully coloured. The preservation of tiny detail even on the micrometer scale contributes to their scientific value.

Photograph: Evidence for preservation of tiny detail in transparent chalcedony. Sporangia of the tree fern Scolecopteris with clearly seen cells on the surface, quite uncommon variety with two hairs emerging near the pointed top of the capsules, (cut off at the lower sporangium, seen as white spots,) unknown species ?
Lower Permian, Döhlen basin, Saxony, Germany.

Fossiliferous cherts have not received due attention as they are most often found as pebbles in gravel pits and fluvial deposits and along rivers but rarely as outcrops, which makes them unsuitable for stratigraphy.
As an exception to this rule, most of the famous Rhynie chert recovered hitherto, now stored at museums and universities and not yet fully investigated, was dug up by trenching into subcrops of in-situ chert layers [1]. Smaller amounts have been found scattered in the area, with sizes of a few grams up to 65kg.

Big chert boulders are rare because the layers break into pieces while being washed out of the banks and carried along by the river. The width of the fragments is usually not much larger than the thickness of the chert layer.
The width can even be smaller if the brittle layer had been bent by deformation of the stratum where it is embedded.
The biggest chert boulder from Döhlen basin is a Lower Permian chert of 31kg with numerous fern frond fragments, found in a trench in a glazial fluviatile deposit at Hänichen near Dresden.

The lately increased interest in fossiliferous cherts in Saxony has been brought about by lucky incidences: (1) first new find of a "maggot stone" since 1893 by Gert Müller in 1985 after purposeful search, (2) essay on the old "maggot stone" finds by M. Barthel 1987 [2], which encouraged the present writer to look for fossiliferous cherts in a wider area, (3) earthwork and construction pits in fluviatile deposits with fossiliferous cherts, since 1991. This interest is held up mainly by the activity of non-professionals who are fascinated by the encounter with ancient life forms but don't care much about stratigraphy. It manifests itself in lay websites as and the present one.

It is strongly emphasized that the field work of collecting fossiliferous cherts differs much from searching for and recovering compression fossils in sediment rocks where a hammer is a suitable tool. Fossiliferous chert pebbles and boulders should never be hit with a hammer. Same as agate pebbles and silicified wood, they should be cut with special blades, after inspection from outside and choosing a suitable cutting plane. Luckily, stone cutting with narrow blades is a widespread trade nowadays.
Judging from experience, anyone who has carried with him or her a hammer for hours feels the urge to hit something with it, which can mean the destruction of a unique fossil by irrecoverably shattering it to pieces. Also, most often details are poorly visible on a 
fracture face but better on the naturally smoothed surface. The safest way to resist the temptation to shatter a valuable sample is to leave the hammer at home and take a pickaxe or big chisel along to poke into the ground or into the gravel banks.
In order to show that the danger of irreparable damage is real, a confession of the venerable A.G. Lyon is quoted here:
"Preliminary examination, by removing small chips with a hammer, ... . Unfortunately, during the preliminary examination, the block shattered, but two related pieces of moderate size were recovered ..." [3]. Hence, the title of the related publication reads "On the fragmentary remains ...
".  In fact, no chert piece equalling the shattered one has been found within half a century, hence the true size and shape of the fossil have remained obscure.
Incidentally, a fossil closely related to the shattered one serves as evidence that one can find tiny things
 by carefully inspecting the smooth natural surface of the chert piece: See Rhynie Chert News 29 , 51 , 71.

H.-J. Weiss   2010
        emended 2015

[1]  N.H. Trewin: History of research on the geology and palaeontology of the Rhynie area, Aberdeenshire, Scotland.
      Trans. Roy. Soc. Edinburgh 94 (2004 for 2003), 285-97.
[2]  M. Barthel: Der Madenstein aus dem Rotliegenden des Windberges.
      in: H. Prescher u.a.: Zeugnisse der Erdgeschichte Sachsens, Leipzig 1987, S.121.
[3]  A.G. Lyon: On the fragmentary remains of an organism referable to the nematophytes, ..., Nematoplexus rhyniensis gen. et sp. nov.
      Trans. Roy. Soc. Edinburgh LXV (1961-62), 79-87, 2 plates.

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Chert formation
Permian cherts
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