You can visit the Upper Cretaceous but never the Late Cretaceous
Geologists keep two parallel vocabularies. The Late Cretaceous is a stretch of time, so nobody can go there. The Upper Cretaceous is the rock laid down during it, so you can stand on it, as fossil hunters do at Hell Creek, where Tyrannosaurus was found. Geochronology is the science of putting dates on that rock.
Absolute ages come mainly from radioactive decay. Knowing an isotope's half-life, scientists measure how much has decayed, or more often how much of its daughter product has built up, and work back to when the clock started. Slow decayers suit long spans but give coarser dates, so methods are matched to the job and often combined. Radiocarbon handles organic material younger than about 60,000 years. Uranium–thorium covers cave formations, corals and fossil bones up to roughly 700,000 years. Uranium–lead, frequently applied to tiny zircon crystals and suited to samples older than about a million years, is one of the two workhorses of geological dating; the other, argon–argon, dates volcanic rock and ash layers at early human sites and reaches down to a few thousand years.
Other clocks read different signals. Exposure dating measures rare nuclides such as beryllium-10 that cosmic rays create in surface rock, revealing when a landform appeared, while burial dating tracks how those nuclides decay once the material is covered. Luminescence techniques detect light stored in quartz or feldspar and can date fired pottery or cooking stones. Amino acid dating is cheap but needs calibration from other methods, and works best on shells in ocean sediments.
Relative methods place rocks in sequence. Magnetostratigraphy matches the pattern of reversed and normal magnetic layers in a rock pile to a master polarity timescale, itself built from seafloor anomalies and dated lavas. Paleomagnetic poles can be compared with a continent's known wander path. Shifts in carbon-13 and strontium isotopes correlate strata worldwide, and volcanic ash can be fingerprinted chemically and tied to eruptions of known date, a method called tephrochronology that archaeologists also use.
Geochronology works alongside biostratigraphy, which assigns rocks to periods by their fossil assemblages but cannot give an absolute age on its own. Combining several independent indicators sharpens the answer, a principle known as consilience.
Source: Geochronology
