a1 Department of Geology, University of Reading, Reading RG6 2AB, U.K.
Sectioning Cruziana shows that an intrastratal mode of formation is common, as postulated by Seilacher. Beds with Cruziana are typically composite. Burrows were sometimes washed out and then recast by sand with primary structures. Formation as open surface furrows on an eroded and firmed mud, as postulated by others, is sedimentologically plausible in some instances but is unproven and seems biologically unlikely.
(Received March 16 1984)
(Accepted June 11 1984)