610 SEDIMENTARY PROCESSES/Post-Depositional Sedimentary Structures Evaporite Pseudomorphs Evaporation of bodies of saline water leads to saturated solutions and to the precipitation of minerals on the floor of the lake or lagoon If saturated conditions persist, large, euhedral crystals may grow at or just below the muddy sediment surface When a subsequent flood or storm introduces both fresher water and coarse sediment, the crystals may be dissolved and their forms buried and cast by the coarser material The bases of sandstone interbeds in playa lake sequences are often decorated by cubic pseudomorphs of halite, commonly with hopper faces (Figure 10), whilst in marine or lagoonal settings, gypsum pseudomorphs may occur Raindrop Impressions These occur as small circular or elliptical depressions on the upper surfaces of beds of fine sediment They record the impact of raindrops or hailstones and their preservation depends on the cohesive strength of the sediment They are quite often associated with desiccation cracks Periglacial Deformation Figure 10 Halite pseudomorphs on the base of a fine grained sandstone bed Triassic, Cheshire, England This results mainly from the expansion of water on freezing and the contraction of frozen sediment during extreme freezing This takes place on a seasonal basis in an active layer close to the surface and creates metre-scale patterns of polygonal cracks Open cracks are filled with wind-blown sediment and material collapsing from the crack sides during periods of thaw Expansion on freezing leads to the pushing up of ramparts on the edges of the cracks In vertical section, frost polygons appear as sharply tapering wedges with complex, poly-phase fills Convolute Figure 11 Large, discoid, calcite cemented concretions developed in mudstones Lower Jurassic, Yorkshire, England