The farther you go back into time, the more difficult
it is to find reliable clues. Fossils are important in the
reconstruction, but there are few fossils with hard parts in rocks older
than 630 million years (Precambrian).
Paleomagnetisms, which helped confirm plate
tectonics in Cenozoic rocks (65 million years and younger) is not very
reliable in the older the rock becomes. As new techniques become
available the configuration of the Earth’s crust will surely change.
Cambrian 570 million years
Pennsylvania, 380 million years
Silurian 430 million years