An unusual, laminated, spherulitic limestone with cherty layers forms part of a volcanogenic sequence in the Midland Valley of Scotland, 27 km west of Edinburgh. It preserves a cross section of the early Carboniferous terrestrial community. Microcrystalline silica laminae containing inclusions of calcite or dolomite may be primary, and support a hot-spring origin for the deposit. Oxygen isotope analyses of the silica and carbonate are consistent with the precipitation of silica from hot, possibly boiling, hydrothermal solution. Such hot spring waters were presumably heated by hypabyssal intrusives associated with the West Lothian volcanic center, only 5 km to the northwest. Silica was probably precipitated by acidification when such waters entered a very local, fresh-water lake. At the same time, calcium carbonate was precipitated from surface water, due to an increase in temperature and reduction in acidity. Such precipitates covered wide areas of the lake floor and the fossils lying on it. Algal or bacteriogenic precipitation may have been responsible for the formation of accretionary growths around nuclei of wood, other fossils, and rock clasts.

The preserved terrestrial biota includes almost-complete individuals of a reptile and of four amphibian groups: temnospondyls, anthracosaurs, loxommatids, and aistopods. These are the oldest known, fully land-going tetrapods. Truly aquatic forms are absent. Invertebrates include: large eurypterids, which may have been partially terrestrial; the oldest known proven terrestrial scorpion (Gigantoscorpio); the earliest harvestman “spider”; and millipedes. Several land-plant assemblages are found in the sequence. Permineralized plants show exceptional anatomical preservation when enclosed within accretionary nodules. Fusainized (charred) gymnosperm wood and pleridosperm leaves indicate the presence of wildfires.

Fully articulated amphibian skeletons occur within the laminated limestones, whereas only disarticulated skeletons have been collected from the lapilli tuffs. Such taphonomic evidence supports an epiclastic origin for the volcaniclastic rocks, deduced from study of clasts and bedform analysis. Fragments mass-flowed or were rain-washed from the flanks of a small basaltic volcano onto or into an area of sinter deposition.