Most of the fossils occur in the green siltstone beds and lower sandstones, formed by the rivers and floodplains of the Jurassic period. (fossilmuseum.net)
Methane clathrates in continental rocks are trapped in beds of sandstone or siltstone at depths of less than 800 m. Sampling indicates they are formed from a mix of thermally and microbially derived gas from which the heavier hydrocarbons were later selectively removed. (en.wikipedia.org)
And just 1.2 meters above them, the grey siltstone holds trails of scratch marks, thought to have been made by animals with exoskeletons, walking on jointed legs — the earliest evidence of arthropods in Earth's history. (scientificamerican.com)