Friday, November 16, 2012

More On The Work of Groundwater

Jewell Cave, South Dakota
For us groundwater folks there is nothing more important than...groundwater.  After all, in sheer volume, our under-the-earth resource dwarfs the amount of fresh water in all the lakes, rivers, streams, swales, bayous, creeks and whatever else holds fresh water on top of the land.  It also does a yeoman's job of supplying fresh water for humanity - from drinking water to irrigation to industrial and every other use there is.

So what other work does groundwater do?  The extra job I'm going to talk about today is forming caves - not all of them, but groundwater plays an integral part in the formation and/or existence of many caves around the world.  One of the more interesting jobs in this line is the work groundwater did in the Jewell and Wind Caves in the Black Hills region of Southwest South Dakota around the town of Custer.

To make a very long story short (pardon the pun) the Black Hills uplift some 70-40 million years ago thrust the largely igneous basement rocks up through the overlying sedimentary rocks - including the two major regional formations known as the Madison Limestone (300-450 feet thick) and the Minnelusa Sandstone lying on top of the Madison.  The central dome has since been eroded away exposing the igneous and metamorphic rock which is completely ringed by the sedimentary formations that dip gently away in all directions.  It is in the Madison Limestone where these vast caves exist.  Jewel Cave is the second longest series of cave passages in the world - some 160 miles of currently mapped passageways.  There are undoubtedly more, but mapping this extremely complex labyrinth has not been completed. 

The real story of the role groundwater has played is also still undetermined for sure.  There are 4 theories as to how these were formed, and all of them involve groundwater, but no one knows for sure - yet.  They are:  (1) rising thermal waters through time; (2) confined groundwater moving down-dip to springs; (3) infiltration through the porous sandstone; and (4) relics of the 300-million-year-old paleokarst.

The fact that both caves exist in the middle area of the Madison is perplexing.  The passages don't extend down dip very far, they rarely reach up and connect with the overlying Minnelusa Sandstone, and they don't reach down to the base of the Madison limestone either.  In fact, in Jewell Cave, they don't even intersect the current groundwater table.  However, while the precise role groundwater has played is not known, few doubt that groundwater somehow did this over time.  For a much more technical rendering of these caves, click here.

I'd like to help map the rest of these caves, but I'm afraid my girth alone would prevent it in all but the largest passageways.  Besides, I don't like dinky dark dank dens - be they associated with groundwater or not.

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