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As the ice sheet slowly spread south over the North the first theory of glaciation. In 1841, his theory was American continent, it pushed the sub-Arctic and temperate published as Essai sur les glaciers ("Test on the glaciers"); it zones further south. Adjacent to the ice was a land of tundra, was the first detailed, scientific case for glaciation. followed by a zone of shrub tundra, and then there were scrub Louis Agassiz, who was also converted to the glacier birch forests, boreal forests (coniferous trees such as spruce, explanation for these geologic curiosities, forged ahead and fir and pine) and finally, further south, deciduous forests. integrated all these geologic facts to formulate a theory that a Eighteen thousand years ago, when the last glaciers were at Great Ice Age had once gripped the Earth. It was published in their maximum southern extent in North America, the Gulf his 1840 book Etudes sur les glaciers ("Studies on the Coast was also colder and drier. Annual rainfall in southern glaciers"). In Systéme glaciaire ("Glacial system"), Louisiana and Mississippi was as published in 1847, he presented much as 40 inches lower than it is further evidence gathered from all today. Boreal forests extended as over Europe that supported his far south as northern Louisiana, James Croll concluded that the theory. In 1846, Agassiz travelled Mississippi and Alabama. Sparse naned to America, where he discovered forests of northern pine likely most plausible driving force even more evidence of glaciation. covered the north central Gulf In 1848, he accepted a position at Coast. Oak and hickory forests behind climate change were Harvard. grew in the river bottoms. Florida variations in solar radiation By 1870, the theory that there was drier and its average annual “Ls were ancient periods of extensive temperature was as much as 10°F striking the Earth. ice was generally accepted by the cooler and was covered with sparse, scientific community. scrubby vegetation, sand dunes and open grasslands. Tall grass prairie, The Earth's Orbit and Wobble with pine and aspen growing in the With a scientific consensus that river bottoms, adorned central Texas. West Texas was likely the Great Ice Age existed, the quest was on to find what a prairie covered with short grass. caused it. The first theory, introduced by Joseph Adhémar, It is a geologic fact that ice has had a great impact on North was based on the Earth's axis tilting back and forth over a America, besides the geological evidence of moraines 22,000-year period, commonly referred to as the precession of (mounds of glacial debris), kettle lakes, gouged bedrock and the equinoxes (25,800 years is considered the most accurate erratically placed boulders. number today). As time passes, Greenland and Antarctic ice core the constellations slowly change samples have demonstrated that on a specific date (typically levels of carbon dioxide (CO 2) measured at the vernal equinox), have fluctuated over millions of moving backwards through the years. Lower levels of CO: depict zodiac. Today, the Sun rises in cooler periods in the Earth's the constellation of Pisces at the history, but it is unclear whether spring equinox. For 2,000 years these lower levels were the cause before that, it rose in Aries. For or the effect. a ‘ the next two thousand years, A great amount of scientific North American beginning around 2070, it will rise research has been applied to the Glaciations in Aquarius. study of ice and ice ages and This tilt of the Earth's axis is much knowledge has been gained, called the plane of obliquity and it but why the ice ages occurred is F extends outward to form a great just as much a mystery now as it circle in the celestial plane, was when Joseph Adhémar known as the ecliptic. This angle published the first detailed ice age is called the obliquity of the theory in his 1842 book ecliptic and the present inclination Révolutions de la mer: déluges is at 23.5 degrees to the vertical, périodiques ("Revolutions of the [once Advance but it varies from 24.5 to 22.1 sea: periodic floods"). Present Wart degrees. As we know, this angle [| stacier netreat Period of the Earth's axis defines the seasons in temperate climates. According to Adhémar's theory, whichever hemisphere had a Discovering the Great Ice Age As early as 1787, Bernard Kuhn believed that erratically positioned boulders in the Swiss Jura were the result of longer winter would experience an ice age. Thus, every ancient glaciation. After Scottish geologist James Hutton 11,000 years an ice age would occur alternately in one visited the Jura seven years later, he arrived at the same hemisphere and then in the other. conclusion. However, until the first half of the 19th century, James Croll, a self-taught scholar and one-time janitor at the prevailing model to explain the observable geologic the Andersonian College and Museum in Scotland, objected evidence was that it was a result of the biblical Great Flood. to Adhémar's theory. He concluded that the most plausible German-born geologist Jean de Charpentier was captivated by driving force behind climate change were variations in solar these boulders and moraines, and during the 1830s he formed radiation striking the Earth, called insolation, as a result of most plausible driving force behind climate change were variations in solar radiation Aftonian Yarmouth Glaciations Sangamon [i cise ctvance [| tacier metres 42 = NEXUS JUNE — JULY 2007 striking the Earth. www.nexusmagazine.com