The Edwards Aquifer began forming approximately 100 million years ago in the Mesozoic Era. At that time, Texas was covered by a shallow sea that was habitat for prehistoric marine animals. When the sea creatures died, their remains—skeletons and shells made of calcium carbonate—were deposited on the sea floor. The calcium carbonate and other minerals from seawater formed layers, or strata, on the sea floor. After a while, these layered deposits compacted into limestone, known today as the Edwards and Associated Limestones. About 17 million years ago, a shift in the Earth’s crust caused central Texas to be lifted above the level of the sea. As the ground rose, the limestone broke, producing fractures and faults. The result was a stair-step landscape that is now referred to as the Balcones Fault Zone.
The Edwards Aquifer has been a source of water for people in south central Texas for more than 12,000 years. Today, it is the primary source of water for approximately 2 million people.