RELATIVE GEOLOGIC TIME. The time scale left is NOT to scale (compare the spans of the different periods). In reality, the Precambrian is 7 times longer than ALL the Phanerozoic Periods combined (see scale to the right for better proportions). A tour of geologic time is avaiable from UC-Berkeley's Geologic Time Machine. Events studied in these pages are Proterozoic and Paleozoic. In Wisconsin as a whole, the youngest rocks are Devonian (Cambrian in the Baraboo Hills area). The Quaternary Period is almost all "Ice Age".

EON ERA PERIOD SPAN  (m yrs) "AGE" Phanerozoic Cenozoic Quaternary 0-2  Mammals Tertiary 2-66  Mesozoic Cretaceous 66-144  Reptiles Jurassic 144-208  Triassic 208-245  Paleozoic Permian 245-286  Amphibians Pennsylvanian 286-320  Mississippian 320-360  Devonian 360-408  Fishes Silurian 408-438  Ordovician 438-505  Marine  Invertebrates Cambrian 505-570  Proterozoic Also known as  Precambrian  time 570-2,500 First  Organisms Archean 2,500-3,800  Hadean 3,800-4,600

EON   Phanerozoic         Proterozoic           Archean         Hadean