A geologic map or report typically is only a summary of investigations that frequently involve the collecting and processing of hundreds of rock samples, followed by the evaluation and interpretation of data from a variety of analytical techniques.
Relatively young deposits can be sometimes dated using tree rings, varved-lake sediments, coral growth patterns, and other methods.
Paleontology is the study of life in past geologic periods (fossil plants and animals), incorporating knowledge of an organism's phylogeny, relationships to existing organisms, and correlation to an established chronology of Earth History.
Paleontology is limited to the study of sedimentary deposits where fossils are preserved, but can be used in establish relative ages of nearby igneous intrusion, faults, and other geologic features.
With the cumulative experience of centuries of paleontological research, the chronology of many fossil species are well established in context of both geologic time and distribution.
Biostratigraphy is the science of correlation of sedimentary units base on the identifiable fossils they contain.
Paleontologists examine fossils of all kinds, but micropaleontology (the study of microscopic organisms) is perhaps the most useful method of dating because the remains of tiny organisms tend to be better preserved, more widely distributed, and may provide more precise age determinations than larger shells or bone material.
Palynologists separate pollen from sediments for correlation and paleoenvironmental reconstructions.
Geochronology is the science of dating and determining the time sequence of events in the history of the Earth.
This web page provides an overview of selected geochronology methods used by USGS scientists.