We use the German word "Lagerstätte" (meaning "repository") to refer to rock formations which contain a large concentration of fossils, or which contain extraordinaraly well preserved fossils. The Burgess Shale, and the Solnhofen Limestone depicted in our line of Pocket Lockets are two of the most famous, and most scientifically valuable of these Lagerstätte.
Due to the conditions at the time when these rock formations were laid down (such as low oxygen levels, high salinity, or rapid sedimentation), bacteria and other agents of biodegradation were not able to decompose the soft-parts of organisms before these organisms were encased in sediment, and the process of fossilization had begun.
Conditions during the formation of the Solnhofen limestone were so perfect for fossilization, that even delicate feathers were preserved. It was the discovery of these feather impressions on the wings of Archaeopteryx, an animal with many reptilian features, that helped establish the close link between birds and dinosaurs.
The Burgess Shale is a far older formation, which gives a valuable glimpse into the first flourishing and diversification of multicellular life.