Counting the seasonal variations in sedimentary layers deposited underwater ("varves") is another technique for determining the age of the earth. So, a physical count is used to corroborate a theoretically derived date.
The two requirements for varves to be useful in dating are
1) that sediments vary in character through the seasons to produce a visible yearly pattern, and
2) that the lake bottom not be disturbed after the layers are deposited.
Varves can be harvested by coring drills, somewhat similar to the harvesting of ice cores discussed above. Overall, many hundreds of lakes have been studied for their varve patterns. Each yearly varve layer consists of
a) mineral matter brought in by swollen streams in the spring.
b) This gradually gives way to organic particulate matter such as plant fibers, algae, and pollen with fine-grained mineral matter, consistent with summer and fall deposition.
c) With winter ice covering the lake, fine-grained organic matter provides the final part of the yearly layer.
Regular sequences of varves have been measured going back to about 35,000 years. The thicknesses of the layers and the types of material in them tells a lot about the climate of the time when the layers were deposited. For example, pollens entrained in the layers can tell what types of plants were growing nearby at a particular time.
Because both varve formation and radioactive decay are well understood, they can be used for accurate, consistent dating. For example, P.E. Olsen (Science 234:842-848, 1986) studied sediments deposited over a period of 40 million years in the Newark Basin of New Jersey, and found excellent agreement between isotope dates and dates calibrated by varve layer counts. Remarkably, he was also able to relate regular variations in sediment layer thickness to the motions of the earth that define our calendar and clock. He found variations with periodicities of 25,000, 44,000, 100,000, 125,000, and 400,000 years. These match the Milankovich cycles of the irregularities in the earths motion around the sun (presently 21,000, 41,000, 95,000, 123,000, and 413,000 years, but subject to small variation caused by passing stars, etc.). The Milankovich cycles influence the amount of solar radiation reaching the earth. The change in solar radiation apparently produces corresponding changes in rainfall and sediment deposition.
The Green River shales of western Wyoming consist of 6 million varves (alternating layers of marlstone and kerogen).
The particles that form these layers are microscopic, and take many days to settle in perfectly still water. Similar varved sediments are forming today, and each cycle is known to represent 1 year. To form this one deposit in the global flood of Genesis would require the formation of about 1 layer per second! The whole 6,000 years of ICR earth history would require a rate of about 3 varves per day. And this deposit is but a thin layer in the total geology of the earth.