RADIOMETRIC TIME SCALE
A technician of the U. Geological Survey uses a mass spectrometer to determine the proportions of neodymium isotopes contained in a sample of igneous rock. Cloth wrappings from a mummified bull Samples taken from a pyramid in Dashur, Egypt. This date agrees with the age of the pyramid as estimated from historical records.
Charcoal Sample, recovered from bed of ash near Crater Lake, Oregon, is from a tree burned in the violent eruption of Mount Mazama which created Crater Lake. This eruption blanketed several States with ash, providing geologists with an excellent time zone. Charcoal Sample collected from the "Marmes Man" site in southeastern Washington. This rock shelter is believed to be among the oldest known inhabited sites in North America. Spruce wood Sample from the Two Radiometric dating table forest bed near Milwaukee, Wisconsin, dates of the last advances of the continental ice sheet into the United States.
Bishop Radiometric dating table Samples collected from volcanic ash and pumice that overlie glacial debris in Owens Valley, California. This volcanic episode provides an important reference datum in the glacial history of North America. Volcanic ash Samples collected from strata in Olduvai Gorge, East Africa, which sandwich the fossil remains of Zinjanthropus and Homo habilis -- possible precursors of modern man.
Monzonite Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon. Quartz monzonite Samples collected from Half Dome, Yosemite National Park, California. Conway Granite Samples collected from Redstone Quarry in the White Mountains of New Hampshire. Rhyolite Samples collected from Mount Rogers, the highest point in Virginia. Pikes Peak Granite Samples collected on top of Pikes Peak, Colorado. Gneiss Samples from outcrops in the Karelian area of eastern Finland are believed to represent the oldest rocks in the Baltic region.
The Old Granite Samples from outcrops radiometric dating table the Transvaal, South Africa. These rocks intrude even older rocks that have not been dated. Morton Gneiss [see Editor's Note ] Samples from outcrops in southwestern Minnesota are believed to represent some of the oldest rocks in North America. Carbon samples are converted to acetylene gas by combustion in a vacuum line.
The acetylene gas is then analyzed in a mass spectrometer to determine its carbon isotopic composition.