On USGS quadrangle maps from 1947-1992, oil wells are marked by hollow black circles.
Credit
Historical Topographic Map Collection/USGS
Usage Restrictions
N/A
License
Original content
Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.