Monday, October 11, 1999

How is Central Texas like Swiss cheese?

When you look around the Texas Hill Country, you probably don't think of Swiss cheese. Austin has a flavor all its own and Swiss cheese tastes, well, cheesey, but they do have one thing in common: they're both full of holes! Caves and caverns in Central Texas make the landscape look like a great, big piece of Swiss cheese.

To understand where all the caves and caverns came from, we need to take a step back. Way back.

About 100 million years ago, Central Texas was at the bottom of a warm, shallow, tropical sea, not unlike the Bahamas today. The seas were filled with interesting creatures such as huge fish, sea turtles, plesiosaurs, or 20-foot mosasaurs with sharp, pointy teeth. (You can see a mosasaur on display at the Texas Memorial Museum.) But tiny, microscopic plankton were, in the long run, more important to Central Texas geology than these huge animals were.

The remains of dead plankton mixed with clay and sand washing off the land, as well as limy sediment precipitating from seawater. This mixture, known as marine sediments, buried the ocean floor. Tens of millions of years later, this region of North America rose in elevation, and the sea drained away. The marine sediments were left high and dry. Over time, these sediments hardened into limestone and shale and were eroded by streams, forming the Hill Country terrain we all know and love.

We normally think of rock as something hard, like a Christmas fruitcake. Many rocks are soft, however, and they erode easily. Limestone is this kind of rock. After a storm, rain water slowly seeps into the ground, adding to our groundwater. As the rain falls through the atmosphere, it mixes with carbon dioxide and becomes slightly acid. Because it is naturally acid, rainwater dissolves some of the limestone as it seeps through the ground. The groundwater has dissolved huge chambers and passages in the limestone beneath our feet, giving us the many beautiful caves and caverns here in Central Texas.

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