The recognition of materials and structures which are unrelated to the original floodplain processes of terrace formation is essential to the proper understanding of terrace morphology and stratigraphy. Two groups of processes have been active in modifying the Lower and Middle Pleistocene terraces of the River Thames since their formation: non‐fluvial deposition; and structural rearrangement by subsidence associated with solution of the underlying Chalk. Non‐fluvial deposits comprise solifluction gravels which are variable in character and may incorporate a range of pre‐existing deposits; and brickearths which appear in most cases to be redistributed loess mixed with non‐loessic components. The distinction between fluvial and non‐fluvial deposits is made using particle size, composition and fabric analysis. Structural rearrangement of terrace sediments has occurred due to subsidence into deep, narrow, steep‐sided pipes. Within such pipes, disturbance of primary structures and fabrics is severe. Over a wider area. primary bedding may be inclined towards pipes, and sediments may show evidence of shearing, faulting and brecciation. Interpretative problems arising from post‐depositional modification ar