From 1816 to 1950 it is estimated that between 1,100 to 1,500 manufactured gas plants existed in the United States. These plants produced approximately 15 trillion cubic feet of gas resulting in the production of 11 billion gallons of by-product tars (Eng, 1985). These plant sites have largely been forgotten due to the passage of time and many are still unlocated. Due to the large number former gas plants sites and the amount of waste they represent, these plants sites are one of the largest generic sources of wastes in the United States (Eng, 1985).
Stratigraphic, sedimentologic and petrographic studies of the Lower and Upper Cretaceous in northwest Sonora show that deposition of the Bisbee Group occurred at the northern margin of a back-arc marine basin, and of the El Chanate Group and El Charro volcanic complex in a closed continental foreland basin. This study also finds that the Proterozoic-Paleozoic formations in northwest Sonora (Caborca terrane) were not part of the Cretaceous landscape, thus raising doubts about the existence of the Mojave-Sonora megashear.