2014-04-26 01.07.12

 Geology and Rock Exhibition

 Some Hill Country rock formations date back for more than one billion years. This exhibit features one specimen from each geological formation in the Hill Country and includes igneous, metamorphic and sedimentary rock types. In the past one billion years this area has featured mountain building periods as well as having been covered by oceans or seas more than once. Please do not deface our rock specimens.

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During much of the Cretaceous Period, c.135 to 65 million years ago, a warm show ocean covered the Texas Hill Country. Before the ocean covered all of the Llano Uplift area, located just north of Fredericksburg, fluvial systems were carrying the decomposed granitic sand and gravel to the encroaching shoreline. About 112 million years ago, the sandstone in the outcrop across the creek was deposited in a setting similar to the depositional system working along Live Oak Creek today. Geologists use an old axiom, “the present is the key to the past.” Sands and gravel were deposited in the same environment, except there were likely dinosaurs roaming the area.

The Texas Hill Country was formed by erosion rather than uplift. After the retreat of the Cretaceous seas that deposited thick limestone formations, the area was subjected to erosion. Drainage systems set-up by the depression of the Gulf of Mexico slowly worked their way into the limestone layers. More resistant limestone layers formed the “hill tops” as the less resistant clays and marls eroded away around them leaving a topography of mesas and canyons.

 

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