Past Glacial Evidence

Glacial erosion has significantly reshaped Earth's surface with several distinct types of erosional and depositional features over geologic time. When climatologists and other scientists study the landscape and find these features, they are able to unravel clues about the area's past climate and determine when glaciers existed in the area and what the climate was like at that time.

HOW iCE MELTS

For years, scientists believed that when ice began to melt, the actual process was fairly simple: When a solid begins to heat up, the molecules within the ice simply acquire more energy and start jiggling around. The more they jiggle, the faster they transform from their solid state to a liquid. Based on a report from LiveScience.com, scientists now know that while this is basically true, it is not the entire explanation; there is more to it. Before this process begins, another, far subtler, occurrence happens. The melting actually begins as the atomic structure begins to "crack." This process is so minuscule that scientists have not been able to actually see the process.

In order to solve this issue, Arjun G. Yodh, professor of physics and astronomy at the University of Pennsylvania, created a way to conduct the experiment on a larger scale using transparent crystals resembling small beads. This allowed him to view the process in an optical microscope. The crystals behaved like a huge version of the atoms. Yodh and colleagues were able to determine that a "premelting" process occurs in the area where the atoms within solid crystals are not perfectly aligned, and they begin moving. Acting as imperfections, they begin moving first, then spread to the ordered portions of the crystals. This discovery illustrated that when ice melts, there is a "premelting" process that occurs before the actual melting temperature is reached.

One very interesting study was conducted by Dr. Julian Dowdeswell, director of the Scott Polar Research Institute at the University of Cambridge. Dowdeswell, along with a research team from Cambridge, was the first to use images taken of sediments found beneath the ocean floor to locate and identify glacial landforms from ancient glacial activity. Their methodology has offered an entirely new approach to paleocli-matology. In their study, seismic data and detailed images provided a three-dimensional view of the ocean floor in the area around the mid-Norwegian continental shelf. The research team was able to "see" deposits left approximately 2.5 million years ago, marking the first attempt at seeing 3-D ancient glacial landforms.

What they were able to find were elongated and streamlined ridges of sediment deposited by fast-moving ice streams. There were also plow marks carved out by the bottoms of icebergs being dragged along the seafloor. Ridges and moraines were also deposited transverse to the ice flow as the ice retreated (in a similar fashion as to how they are on land today).

As Dowdeswell has observed, "Submarine landforms like these are found at the marine margins of almost all modern ice masses, so it seems reasonable to assume they were also features of ancient ice sheets." He also adds, "What no one has done until now, however, is search for signs of these glacial landforms from millions of years ago. New developments in geophysics are giving us the ability to produce detailed images from hundreds of metres beneath the modern seafloor, and seismic data that enable us to produce 3-dimensional pictures of what the surface of Earth was like when it was being shaped by glaciers. That opens up new, exciting possibilities in identifying past ice ages—which we intend to take further."

Glaciers erode the landscape by the processes of abrasion and plucking with the sheer mass of the glacier traveling over the ground. The ice is able literally to lift the weaker blocks of material up and drag them along with the ice flow. This happens when water at the base of the glacier flows into fractures in the ground's surface. Once water enters, it freezes. When water freezes, it expands, which widens the fractures, eroding and weakening them further.

This process weakens the rock so that it can be plucked out of the ground and carried in the bottom of the glacier as it flows downhill. This "bedload" that the glacier constantly acquires as it flows abrades the ground as it moves, grinding rock into a flourlike substance, and leaving deep scratches in the ground's surface that geologists can later use to identify the existence of a glacier and the direction it was moving. When glaciers flow down mountain valleys, they leave a characteristic U-shaped valley.

As a glacier moves, it can carry an enormous load of broken pieces of rock from the mountain further up canyon. These pieces of broken rock can vary in size from small stones and dirt up to rocks the size of cars and be deposited on hillsides. These deposits are called glacial

One of the most recognizable landforms left after the existence of a glacier is the U-shaped valley formed from the abrasive grinding of the ice as it plows its way downhill. (Nature's Images)

erratics. Once the ice thins and melts, it can no longer carry all this heavy load of material, so it dumps it where it is. Moraines are another common landform feature. These are formed from the deposition of material from the glacier that is exposed once the glacier has melted and retreated. They are a mixture of all the eroded material the glacier collected and carried along with it as it traveled. Moraine deposits are common along the edges of where the glacier once flowed. They are also common at the terminus, or furthest reach, of the glacier.

Glaciers also deposit snakelike ridges called eskers that are formed by streambeds underneath the glaciers, as well as drumlins, which are streamlined hills. Another common glacial deposit is loess (pronounced "luss"). Loess is a very fine sediment, often referred to as "rock flour," and formed from the extreme abrasive action of glaciers. It is easily picked up by wind and blown over long distances to be deposited elsewhere. The loess deposits can be very deep; some in China are hun-

Erratics are a common glacial deposit. These granite boulders deposited on the hillside vary in size. The large one on the upper left is larger than a car. The granite pieces were carried about 10 miles (16 km) downhill during the last ice age about 12,000 years ago.

(Nature's Images)

dreds of feet deep. They also occur in the Midwest of the United States, principally the Mississippi Valley and Great Plains regions. There are also smaller deposits in Idaho and Washington and more significant ones in Alaska.

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