Igneous Rocks and the Alamo Site
During the Miocene Epoch, between 5 and 23 million years ago, volcanic rock erupted over the faulted and tilted Paleozoic Era carbonate rocks containing the Alamo Impact deposits. Presently, the volcanic rock sequence consists of over 3000 feet of widespread ignimbrites, a deposit of volcanic ash and small rock fragments that flow rapidly from volcanoes. Capping the ignimbrites is a deposit of basalt.
In Lincoln County, Nevada, this sequence is made up of four formations: 1) the Hells Bells Canyon Formation, 2) the Hiko Tuff, 3) the Alamo Range Formation, and 4) the Badger Valley Basalts. Fortunately, not all of the Alamo Impact deposits were covered by volcanic rock, leaving plenty of exposures for geologists to study the Alamo Impact Event!