This chapter focuses on two methods by which we can give an exact date to buildings or tools, which tell us a good deal more about men than lists of kings and battles. One method is by tree rings. Where human tools are embedded in the mud they can be dated. This method of dating by annual mud layers works best in the neighbourhood of retreating ice sheets. The method of annual mud layers can be used to date sections of the past. Sometimes evidence for a yearly cycle is overwhelming. For example fossils of adult insects are found in one part of each layer, and of their larvae in another. Bradley counted enough varves in the Eocene formations of Colorado, Utah, and Wyoming, to be able to say that a particular epoch lasted between five and eight million years. During this time a thickness of about two thousand feet of sandstone and oil shales were laid down in two lake beds.