Meteor crater, Arizona

This crater, near the town of Flagstaff in the Arizona desert (below), is 1,300 metres (4,200 ft) across and over 180 metres (600 ft) deep, with a raised rim 40 metres (130 ft) high. Rocks in and around the crater show signs of intense heat. The impact that caused this crater occurred about 10,000 years ago.

Osborne and Tarling 1996:20

The Cordilleran Orogeny and the Rocky Mountains

The formation of western North America took place in a series of disturbances, the Cordilleran Orogeny, which began in the Late Jurassic period (150 million years ago) and lasted until about 40 million years ago. . .

30 milion years ago

As the Pacific plate was subducted at a shallower angle, the disturbances spread eastwards. This brought it near enough to the surface to push masses of continental crust upwards and eastwards--uplifting the blocks of sedimentary rock that had accumulated during the previous 80 million years or so. This series of cataclysmic thrusts formed a chain of mountains stretching from Mexico to northern Canada--the Rocky Mountains

During the Eocene period, about 30 million years ago, volcanic activity and uplifting of very ancient strata took place across a wide area of the Rocky Mountains. Since then the focus of geological activity has shifted west agin--to the coastal ranges and the San Andreas Fault, the Rocky Mountains now being a stable part of the North American continental craton.

Osborne and Tarling 1996:128-129

The Milankovitch cycle

In 1920 a Yugoslavian meteorologist, Milutin Milankovitch made measurements of the changes in the levels of the Sun's radiation reaching the Earth. He was able to show that the heat from the Sun, and therefore the temperature of the Earth, is affected by three factors which occur in cycles: variations in the Earth's orbit around the Sun; variations in the Earth's orbit around the Sun; variations in the tilt of the Earth's axis; and wobbles of the Earth's axis caused by graviatitional interations between the Sun, the Moon and the Earth. The cycles are 100,000, 41,000 and 22,000 years long respectively. These periods tie in well with the duration of recent ice ages.

Since glacial episodes are very uncommon in the Earth's history, it is generally agreed that the Milankovitch cycle has to be combined with other factors for a glacial episode to occur.

Osborne and Tarling 1996:150

We know the reasons for the regular 10,000-year fluctuations in temperature that have brought about the advance and retreat of the ice throughout the last 2 million years. It is less easy to explain why the Earth is in its present long cold phase. Many scientists now believe the answer partly lies with the geography of the Earth, and in particular the positions of the continents in the polar regions.

Osborne and Tarling 1996:150