Scientists can estimate the time at which rocks at Earth's surface became exposed (through glacial scour, faulting, sediment deposition, exhumation, etc.) in a given area using terrestrial cosmogenic nuclide (TCN) geochronology. The idea behind this technique is ingenious in its simplicity. Imagine, for example, a glacier advancing over the landscape and scouring the rocks underneath. As the glacier retreats, fresh rock surfaces become exposed to the atmosphere. Galactic cosmic rays then bombard the fresh minerals exposed at the Earth's surface, producing rare nuclides such as beryllium-10 (10Be) and aluminum-26 (26Al) in the process. Thus, measuring the concentration of TCNs in rocks at the Earth's surface allows scientists to estimate how long a surface has been exposed and/or the rate of surface denudation.