Tree-ring dating is how archaeologist determine the age of wood. However, it does not work for organic material. Scientist have a long history of examining pack rat middens in Chaco Canyon for archaeological evidence. In Chaco they have found seeds and pollen dating back between 5,000 and 10,000 years old. They know this by using a method called radiocarbon dating. Carbon-14 isotopes are found in all organic material. It begins to lose its radioactivity over a period of 5,568 years. Scientists determine the age of the material by measuring the radioactivity that remains. Another method called Achaeomagnetism is used to date soil that has been heated, such as the adobe used in Chaco Canyon fire hearths.