limestone - display specimen showing typical dissolved surface found in arid climates - Lower Cambrian Chambless Limestone, Marble Mountains, Calif.
In arid areas, calcium carbonate limestone and calcium-magnesium carbonate dolomite develop a rough dissolved surface from the action of weak carbonic acid that forms as atmospheric water picks up carbon dioxide. This specimen is an excellent example of that solution surface. Ultimately a karst topography can develop, with solution furrows deep enough to stand in without being able to see over the edge, and with solution of the limestone along underground joints forming caves and sinkholes. The Karst Plateau in Slovenia and Italy exhibits this topography, was studied early, and gave its name to this landform.
Limestones are cliff-formers in arid regions, forming vista points chosen by countless geology students as a spot to have their lunch. The Madison Limestone in Wyoming, one such rock unit, has been named "The-seat-of-your-pants-formation" from the effect of a leisurely lunch consumed with feet dangling over the edge of the cliff - almost instant tail-end air conditioning. In humid regions, limestones weather to clay and form valleys instead of cliffs. A good question for your students would be to ask, "Under what type of climate did this form?" (arid) and then "How do you know?" (the scratchy surface).
The Chambless Limestone also contains algal oncolites, rounded colonial growths of a cyanobacteria assigned to the catchall genus Girvanella. These specimens were selected for the dramatic surface. The small rounded dark patches are Girvanella.
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