 The shale is made of alternating fine @-@ grained layers of siliceous mudstone ( compressed , hardened mud originally made of ground @-@ up silicate rock ) and <unk> originally animal shells . The <unk> layers contain relatively unremarkable shells and occasional non @-@ biomineralized fossils ( such as <unk> 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 . 
