The Letná Formation was first formally defined in 1966 by Havlíček and Vaněk and crops out in the
Prague Basin, a northeast–southwest trending minibasin stretching over between
Rokycany in the southwest and
Úvaly in the northeast. The Prague Basin contains an Ordovician to Devonian succession in the larger
Bohemian Massif. The formation conformably overlies the
Libeň Formation and is overlain by the
Vinice Formation. The Letná Formation is the thickest unit in the sedimentary sequence with a thickness of more than between
Beroun and
Prague. The formation consists of a rhythmic alternation of
sandstones (
greywackes and
subgreywackes), A rather diverse fauna composed of brachiopods, trilobites, echinoderms, conularids, and non-trilobite arthropods has been found in outcrops located between
Zdice and
Chrustenice. This area probably represented a rather shallow environment in the northwestern part of the basin. The fossils are usually fragmentary and preserved as internal or external moulds associated with angular claystone fragments within quartzose sandstones to siltstones. Accumulations of these fossil remains are commonly found in thick-bedded quartzose sandstones of light-grey and yellow or grey-brown color, which might have been deposited in areas sheltered from wave activity by sand bars. Fossilized gut remains have been observed in four of the about twenty trilobite species present in the Letná Formation:
Birmanites ingens,
Dalmanitina socialis,
Deanaspis goldfussi and
Selenopeltis buchi. These four trilobites belong to the
Dalmanitina-
Deanaspis Association, characterizing rather proximal and shallow environments of the Letná Formation. Most of the non-trilobite arthropods discovered in this formation, including forms with non-mineralized exoskeletons, are also found associated with
D. goldfussi and/or
D. socialis. This suggests that the development of conditions conducive to soft-tissue fossilization was not uncommon in the environment represented by the
Dalmanitina-
Deanaspis Association. == Fossil content ==