limestone - oncolitic limestone - Lower Cambrian Chambless Limestone, Marble Mountains, Calif. - display specimen

The Chambless Limestone contains algal oncolites, rounded colonial growths of a cyanobacteria assigned to the catchall genus Girvanella. The dark gray rounded spots are colonies of Girvanella.

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 - with 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).

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