The ‘‘Paleozoic era’’ literally means ‘‘ancient-life-time’’ and was dominated by invertebrates including many varieties of now-extinct shellfish. The ‘‘Mesozoic era,’’ or ‘‘middle-life-time,’’ was a time of dinosaurs and the earliest mammals. The ‘‘Cenozoic era,’’ or ‘‘recent-life-time,’’ is dominated by mammals.
Unconformities are very common and subdivide a rock column into ‘‘formations’’ of consistent geological age. ‘‘Formation’’ is not a time designation but a rock designation, and a single formation may incorporate a variety of rock layers. Formations for the most part are identified from fossils, which is very important in exploration drilling for oil.
Radioactive dating depends on constant rates of radioactive decay of certain isotopes, and indicates that the majority of geological time was without life except for single-celled plants, or algae. These rocks generally have been buried under younger rocks and constitute part of the ‘‘basement complex.’’ They may be lumped together in age as ‘‘pre-Cambrian,’’ which means that they originated prior to the earliest, Cambrian, period of the Paleozoic era. Granitic shield areas often are referred to as ‘‘pre-Cambrian shield.’’