Calcite raft

Rafts and folia form at/near the water table. Calcite rafts precipitate directly at the water surface, supported by surface tension. Folia, which resemble bracket fungi in shape, form on vertical to overhung surfaces and accumulate within the intertidal range of the water table. 

Figure 9. Scanning electron micrograph of the underside of a calcite raft that precipitated at the water-air interface in Browns Room. Perimeter has long crystals, whereas the central part of the raft is characterized by mostly equant crystals. Four small rafts coalesced to form this large one. Scale bar is 1 mm.

Jones, B., 1989, Calcite rafts, peloids, and micrite in cave deposits from Cayman Brae, British West Indies, Can. Jour. Earth Scl, 26 654-664. 

Figure 3. Map of Devils Hole. BR is Browns Room, and M is Millers Chamber. Mammillary calcite is the only speleothem morph below the water table. Rafts currently form on the surface on the main chamber and in Browns Room. Folia only form in Browns Room, and popcorn forms in both Millers Chamber and Browns Room.

Calcite precipitated at/on the water table or in the intertidal range forms as rafts and as... 

Calcite that precipitates directly at the water-air interface forms rafts of calcite crystals. The rafts float on the surface, supported by surface tension (Hill and Forti, 1997). When the water surface is disturbed by wind, seismicity, tidal fluctuations, flood inflow, or divers air bubbles, the rafts sink and often become cemented to the bottom (Fig. 8) (Riggs et al., 1994). Rafts start out as roughly circular aggregations of crystals that, if undisturbed, coalesce into large (up to 10 cm across) aggregations that are variable in general outline. Because the water surface in Browns Room is protected from disturbance by wind or flood inflow, rafts in Browns Room consistently are much larger than those that form in the open part of Devils... 

Figure 8. Accumulation of rafts (arrows) at a depth of about 28 meters below Browns Room in Devils Hole. Rafts have been cemented in place by continued precipitation of mammillary calcite. Thickness of bedrock block left of the arrow is about 15 cm.

Flowstone precipitates from carbonate-saturated, externally derived water as it flows down the above-water-table walls of Browns Room. Its crystal structure is most similar to the speleothems described by Kendall and Broughton, (1978) and is petrographically distinct from folia, rafts, and mammillary calcite. By identifying flowstone deposits in Devils Hole, we have a second proxy record for the elevation of the water table... 

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