31. The Upper Cretaceous rock units confine the Edwards Aquifer within the Edwards Group and Georgetown Formation. 32. The rock unit in which the falls are formed is referred to as the Jeffersonville limestone. 33. All these rock units together form a super sequence of rock known as the Grand Staircase. 34. This rock unit represents a forested setting of rivers and lakes that formed between episodes of volcanism. 35. Geologists employ biostratigraphy, the use of index fossils, for dating sedimentary rock units like the Coon Creek. 36. This can be used to qualitatively identify thinning or thickening of a rock unit in different directions. 37. Locally it immediately overlies other rock units . 38. Rock units are typically represented by colors.39. The Upper Cretaceous rock units confine the Edwards Aquifer within the Edwards Group and the Georgetown Formation. 40. The majority of the rock units shown on the 1976 map were mapped in 1973 by reconnaissance methods.
