Brown microbial lime boundstone containing diverse skeletal elements (upper right) adjacent to poorly sorted lime sand with calcite cement (lower left). Mississippian mound, North Alabame, 2.5 mm field of view. Pale colors of calcite crystals result from cross-polarized light. Sandstone contains crinoid fragments, chunks of boundstone, and many other kinds of particles, and fills a small void that was left open as the mound grew.
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