is used in conjunction with stream load measurements and sediment analysis. This technique measures chemical weathering intensity by calculating chemical alteration in molecular proportions. Preliminary research into using cosmogenic isotopes to measure weathering was done by studying the weathering of feldspar and volcanic glass, which contain most of the material found in the Earth's upper crust. The most common isotopes used are 26Al and 10Be; however, 10Be is used more often in these analyses. 10Be is used due to its abundance and, while it is not stable, its half-life of 1.39 million years is relatively stable compared to the thousand or million-year scale in which denudation is measured. 26Al is used because of the low presence of Al in quartz, making it easy to separate, and because there is no risk of contamination of atmospheric 10Be. This technique was developed because previous denudation-rate studies assumed steady rates of erosion even though such uniformity is difficult to verify in the field and may be invalid for many landscapes; its use to help measure denudation and geologically date events was important. On average, the concentration of undisturbed cosmogenic isotopes in sediment leaving a particular basin is inversely related to the rate at which that basin is eroding. In a rapidly-eroding basin, most rock will be exposed to only a small number of cosmic rays before erosion and transport out of the basin; as a result, isotope concentration will be low. In a slowly-eroding basin, integrated cosmic ray exposure is much greater and isotope concentration will be much higher. Measuring isotopic reservoirs in most areas is difficult with this technique so uniform erosion is assumed. There is also variation in year-to-year measurements, which can be as high as a factor of three. Problems in measuring denudation include both the technology used and the environment. Landslides can interfere with denudation measurements in mountainous regions, especially the Himalayas. The tow main problems with dating methods are uncertainties in the measurements, both with equipment used and with assumptions made during measurement; and the relationship between the measured ages and histories of the markers. This relates to the problem of making assumptions based on the measurements being made and the area being measured. Environmental factors such as temperature, atmospheric pressure, humidity, elevation, wind, the speed of light at higher elevations if using lasers or time of flight measurements, instrument drift, chemical erosion, and for cosmogenic isotopes, climate