Geol102 lecture 20 geological time/folds and faults. Era: structure of the time scale. Bounded by major changes in/on the earth and with life on earth. Divided into 3 eons: hadean, archean, proterozoic. Represents about 88% of earths history: why we lack details of the precambrian. Abundant fossil evidence doesn"t appear until the beginning of the cambrian. Rocks are very old and subjected to tons of weathering and erosion. Many fossils and structures have been destroyed. Difficulties in dating the time scale: not all rocks can be dated with radiometric methods. Some have very low uranium contents: grains comprising detrital sedimentary rocks are not the same age as the rock in which they occur. Sedimentary rocks give ages all over the place: age of particular mineral in a metamorphic rock may not necessarily represent the time when rock formed. Multiple ages through process of going from grains to rock, through diagenesis then metamorphism.