Deposition of basal Pleistocene, warm-water clastics (Red Crag) on top of Paleocene-Eocene marine mudrocks (London Clay s.l.) triggered a phase of intense syndepositional mud tectonics. The Red Crag was deposited in subsiding mud-withdrawal basins, separated by rising mud diapirs and mud ridges. Mud deformation ranged from plastic flow to liquefied flow, with extrusion of allochthonous mud sheets. Tidal bedforms in the Red Crag allow us to estimate the duration of deposition and mud mobilization. As the mud dewatered, upward water flow though the Red Crag changed the color of the clastic sediments, revealing the geometry of the fluid-escape pathways, with narrow conduits feeding up into surface blow-out craters. The deformation has previously been interpreted as a later, postdepositional, process during the subsequent periods of glaciation (cryoturbation), but it is clearly syndepositional with the non-glacial Red Crag. This may have important consequences for civil engineering, because it indicates that this level of the London Clay s.l. has been prone to catastrophic failure and even liquefaction (akin to catastrophic quick clay failures of coastal Scandinavia) in conditions similar to the present day. However, large engineering projects on the same substrate (offshore windfarms, nuclear power stations, etc.) have been constructed on the assumption that the mud deformation is a product of glacial conditions.