0.541 to 4.4 Billion years ago
Covers nearly 90% of Earth's History
Early geologists referred to rocks below the lowest Cambrian strata as Precambrian. No fossils were initially known from these rocks—later geologists would discover Precambrian fossils—and therefore Precambrian rocks from one location could not be correlated with Precambrian rocks found elsewhere.
Today with radiometric dating, trace element mapping, and isotopic fingerprinting geologists can correlate these rocks and have divided the Precambrian into three eons: The Hadean, Achaean and Proterozoic.
Precambrian rocks are exposed at the surface in Northern Wisconsin, but are overlain by a thick sequence of Paleozoic rocks in Southern Wisconsin and Illinois.
Today with radiometric dating and trace element mapping and isotopic fingerprinting geologist can correlate these rocks and have divided the what was called the Precambrian into three eons: The Hadean, Achaean and Proterozoic. We will only brief examine these ancient rocks.