Rocks and Minerals

Rocks near San Antonio

Limestone makes hills and caverns when the weak acid, carbonic acid (rain absorbing carbon dioxide), dissolves limestone.  At road cuts, you see bands (strata) of harder and softer limestone.  Quarries in San Antonio and New Braunfels extract limestone which is heated in rotary furnaces to make lime, calcium oxide, which makes concrete.   Alamo Quarry Market is where the Alamo Cement Company made a lot of cement from limestone.  Though we consider limestone to be boring, people in other states think limestone is exotic.

Chert nodules, hard, heavy, and brown for landscaping, is a type of quartz.  It is from diatoms in the ocean falling to the ooze at the bottom.  Chert nodules are embedded in limestone, at Government Hills State Natural Area, since both limestone and chert form at the bottom of seas.  Chert was used by American Indians to make arrowheads, at Alibates National Monument north of Amarillo.

Granite is at Enchanted Rock and Big Bend National Park (Chisos Mountains), and in New Hampshire, the Granite State.  And the Texas Capitol.  Granite is lava (magma) that cooled slowly underground, forming crystals.     Big Bend also has volcanic ash (called tuff), a whole canyon of it.  

Sandstone is in east Texas and in Monument Valley, Utah.

There is metamorphic rock east of Lake Buchannan.  There used to be graphite production.