Coarse boulder diamicts capping the Johnson Mesa and Carcass Creek Terraces are not associated with the Bull Lake glaciation as previously thought, but were deposited 180±15 to 205±17 ka (minimum age) and are the result of debris flow deposition.Desert pavements on the Johnson Mesa surface give exposure ranging from 97±8 to 159±14 ka and are 34–96 kyears younger than the boulder exposure ages.
Cosmogenic nuclides such as these are produced by chains of spallation reactions.
The production rate for a particular nuclide is a function of geomagnetic latitude, the amount of sky that can be seen from the point that is sampled, elevation, sample depth, and density of the material in which the sample is embedded.
Decay rates are given by the decay constants of the nuclides.
These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age.
The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic field, solar winds, and atmospheric shielding due to air pressure variations.
Rates of nuclide production must be estimated in order to date a rock sample.By the time the cosmic ray cascade reaches the surface of Earth it is primarily composed of neutrons.When one of these particles strikes an atom it can dislodge one or more protons and/or neutrons from that atom, producing a different element or a different isotope of the original element.Chlorine-36 nuclides are also measured to date surface rocks.This isotope may be produced by cosmic ray spallation of calcium or potassium.Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface.