After the structural information has been extracted, the next step is to work out as much as possible of the geological history of the area. Fundamental in this connection is the determination of the ages of the different horizons, preferably according to the geological time scale but at least relative to one another. Often seismic lines pass close enough to wells to permit correlating the seismic horizons with geological horizons in the wells. Refraction velocities (if available) may help identify certain horizons. Occasionally a particular reflection has a distinctive character that persists over large areas, permitting not only it to be identified but also other events by their relation to it. Notable examples of persistent identifiable reflections are the low-frequency reflections sometimes associated with massive basement and the prominent reflection from the top of the Ellen-burger, a limestone encountered in Northern Texas.