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Today, sea level is at a relative "high stand" because the majority of the glaciers had melted by about 10,000 BP and minor glacial melting has slowly continued (with occasional reversals) throughout recorded human history. The ancient shoreline of the last ice age is now under approximately 390 feet (120 meters) of water. For this reason, most early civilization seaport cities are currently under water (this may be the historic origin of the biblical [[Noah]] story).
In the distant past, sea level has been significantly higher than today. During the Cretaceous (labeled K on the graph), sea level was so high that a seaway extended across the center of North America. Brightly colored coastal and marine sedimentary deposits from this age contribute to the spectacular scenery of Utah, northern Arizona, and Colorado.
These alternating high and low sea level stands repeat at several time scales. The smallest of these cycles is approximately 20,000 years, and corresponds to the rate of precession of the [[Earth]]'s rotational axis (see [[Milankovitch cycles]]) and are commonly referred to as '5th order' cycles. The next larger cycle ('4th order') is about 40,000 years and approximately matches the rate at which the Earth's inclination to the [[Sun]] varies (again explained by Milankovitch). The next larger cycle ('3rd order') is about 110,000 years and corresponds to the rate at which the Earth's orbit oscillates from elliptical to circular. Lower order cycles are recognized, Hundreds of similar glacial cycles have occurred throughout the Earth's history. The earth scientists who study the positions of coastal sediment deposits through time ("sequence stratigraphers") have noted dozens of similar basinward shifts of shorelines associated with a later recovery. The largest of these sedimentary cycles can in some cases be correlated around the world with great confidence.
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