Upon disembarking at Fort Jefferson, in the Dry Tortugas, the last thing I expected to photograph was a stalagmite. Yet here it was: on the floor of the interior arched hallway, wet with the dripping of calcium-rich water from above. The drops fell from seam in the ceiling masonry, with several shallow stalactites (mostly an inch or less tall) dropping water onto this mound, which formed in the same way as stalagmites in caves.When the fort was built, the roof above was covered by several feet of sand, held by grasses and other plants, to capture and filter rainwater for the occupants’ uses. Unfortunately, the water was not potable, and only could be used for bathing and cooking, thanks to abundant mineral salts leaching from the sand. One of those compounds, calcium carbonate (same as in limestone, seashells and coral skeletons), formed this mound on the floor.
Garden Key FL (20 Nov 15) Looking E
24.6294, -82.8737