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Sunday, March 27, 2011

Gold in Glacial Outwash Plains

When a glacier melts there are large quantities of water that flows off its edge carrying large amounts of glacial debris that is deposited in front of the glacier.  The action of the rushing waters is characterized by being flat layers of sand and gravel or in some cases finer materials. This is called a "glacial outwash plain."

Glacial meltwater flowing from beneath a glacier.  Note the milky appearance of the water.
Photo by Siri Spjelkavik 

Another characteristic of outwash plains is the abundance of braided stream channels that are found in areas of a high concentration of sediments and running water.  This is a perfect environment for gold deposits to form wherever these is a sudden change in water velocity from fast to slow.

This glacial meltwater eventually flows off into the sea. If the land slopes up in front of the glacier it can form large glacial lakes of glacial melt water at the margin of the glacier.

Water flowing into these glacial lakes from gullies across the ice often form upraised deltas in the lake consisting of mostly sand and gravel. The bottom of the glacial Lake away from the delta deposits is covered with a layer of clay that forms a flat surface like a tabletop.

A glacial outwash plain, and how it is formed
After msu.edu 

The glacial melt water flowing across the glacier is so charged with suspended particles like silt and clay that it appears milky. This is called glacial milk that can be seen wherever glacial melt water is flowing off, or under a glacier.

Glacial outwash plains are recognized by their special kinds of agriculture that favors plants that grow best in a well drained soil. The most common crops found in one of these areas are potatoes.

Because these areas are usually well drained the glacial melt water has long since vanished, and it is difficult to recognize the drainage patterns of a braided stream channel as they now exist. One of the best methods of finding one of these ancient stream channels is by observing the amount of gravel that is present, and how it is deposited.

A stream channel can be recognized as an area containing abundant gravel. These ancient stream channels can often be seen when excavation exposes them to view. This is the most likely place to find gold in the glacial outwash plain.

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