Kathryn Shulz at The New Yorker:
Consider that sandstone, which began, some two billion years ago, as quartz crystals buried deep inside mountains towering over what is now the Upper Midwest and southern Canada. Time took apart the mountains, and rain dissolved most of the minerals in them, but the quartz remained. It was later washed into Precambrian rivers and eventually carried to a beach, where its grains were worn smooth and spherical by the waves. That beach was tropical, partly because the contemporaneous climate was extremely warm, but also because Wisconsin, at the time, was near the equator. As the sea retreated and other rocks and minerals were deposited on top of the former strand, the grains of quartz hardened into sandstone, which was gradually sculpted by wind, water, and glacier until, aboveground, it formed the topography of Wisconsin as we know it today. Belowground, it formed an excellent aquifer, thanks to those spherical grains, which—“like marbles in a jar,” as Bjornerud puts it—leave plenty of room for storing water in between them.
more here.
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