 The shale is made of alternating fine-grained layers of siliceous mudstone (compressed, hardened mud originally made of ground-up silicate rock) and calcisiltite originally animal shells. The calcisiltite layers contain relatively unremarkable shells and occasional non-biomineralized fossils (such as priapulid tubes). The soft-bodied organisms for which the Burgess Shale is famous are fossilized in the mudstone layers, which are between 2 and 170 millimetres (0.079 and 6.693 in) thick, averaging 30 millimetres (1.2 in), and have well-defined bases. Opinions vary about how the mudstone layers were produced: perhaps by mudslides from the top of the "Cathedral" limestone platform, when its edge collapsed occasionally; or possibly by storms that created back-currents that abruptly washed large volumes of mud off the platform. Each mudstone layer is the result of one such catastrophe. The Greater Phyllopod Bed, a 7 metres (23 ft) thick sequence consisting of Walcott's famous "Phyllopod Bed" plus the 5 metres (16 ft) below that, contains at least 36 layers, deposited over 10 to 100 thousand years, during which the environment was essentially stable.
