The Edwards limestone of Lower Cretaceous age is the principal water-bearing formation in a belt 5 to 25 miles wide that extends from Austin southwest to San Antonio and thence west through Uvalde and Del Rio to Comstock, a distance of about 250 miles (see Fig. 1). Throughout this belt it supplies water to wells for stock, industrial, irrigation, and municipal use and to a series of fault-springs, some of which are among the largest in the United States. These springs have played an important part in the economic and cultural development of Texas. In 1718 the Spaniards stationed a department of light infantry near the springs at San Antonio and soon after established missions and constructed elaborate systems of canals for irrigating considerable areas. After the winning of Texas' independence large numbers of settlers moved into the area. This influx was greatly accelerated following the admission of Texas to the Federal Union and towns were soon established near all of the major springs. However, it was not until about 1880 that the rapid development of the Edwards limestone aquifer by wells was begun.