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Downtown Geology Lesson

A tour can yield a wealth of information to the attentive

Posted: Sunday, October 09, 2005

Source: Christopher DeWitt, supervisor of Juneau's John Rishel Mineral Information Center

One doesn't need to travel to the Juneau Icefield or Kuiu Island to learn about the geology of Southeast Alaska. A walk around downtown Juneau offers plenty of information to the attentive.

On a recent Saturday morning, Christopher DeWitt, supervisor of Juneau's John Rishel Mineral Information Center, led a geology walk downtown.

DeWitt started his presentation by showing pictures of plate movement dating back 200 million years. Alaska didn't begin to take its current form until then, when the Yukon-Tanana Terrane collided with and was attached to the North American continent. Several terranes, including the Alexander, Taku and Yukon-Tanana, were sandwiched side by side, colliding northeast toward the North American Plate.

Southeast Alaska's high mountains and rich gold deposits resulted from millions of years of thrusts and metamorphism.

Standing at a parking lot in front of Wells Fargo bank on Second Street, DeWitt pointed to outcrops of rocks on Mount Juneau trending northeast. These represent the east-to-west thrusting of rock caused by the collision of the Pacific Plate with the North American Plate.

Gastineau Channel is located on the Fanshaw Fault, where the Taku Terrane and Gravina Belt meet. Mountains on both sides of the channel head toward the mainland.

"On the Juneau side, the (alignment) goes toward the mountains and the slope tends to be steeper. You have more a tendency of rock fall and avalanche," DeWitt said.

"On the Douglas side, the dip goes towards the channel," he said. "You are more susceptible to have small landslides. On a clear day, you can see the scars that are spotty and scattered around. They are the places where the landslides occur."

Lance Miller, a geologist who had worked in the minerals industry throughout Alaska, Mexico and Asia, said the glaciation didn't start shaping Juneau's landscape until about 20,000 years ago and ended about 10,000 years ago when ice covered most of Canada, the Upper Midwest and New England.

"You look at the peaks of Douglas Island. They are all rounded because of the ice," Miller said. "Mountains that are higher than 3,500 feet have sharp peaks."

According to "Roadside Geology of Alaska," by Cathy Connor and Daniel O'Haire, the Taku Terrane is a hodgepodge of strata that have been deformed, intruded and metamorphosed several times.

"Juneau sits on the boundary of Gravina Belt and Taku Terrane, so rocks on the Juneau side have a higher level of metamorphism than rocks on the Douglas side," DeWitt said.

Outcrops of greenschist metamorphic rocks on Telephone Hill show that zones of marine shaley rocks have been cooked and squeezed under high temperatures.

Sparkling gold flakes people find on the riverbeds or on the beach come from a 100-mile-long zone of gold-bearing quartz vein that runs from Windham Bay to Berners Bay. It is called the Juneau Gold Belt.

Because of Juneau's rich deposit of gold, mining historian David Stone said, some parts of Juneau's landscape resulted from mining activities.

For example, people can find Danish flint pebbles on the beach of Thane Road and Treadwell. In the early 20th century, mining companies shipped the pebbles from Greenland to grind the ore in mills, Stone said.

People can also find jasper, a reddish and brownish kind of stone, on Thane Beach. During World War I, mining companies used jasper from Wyoming for grinding instead of Danish pebbles, because of an interruption in transportation.

The human influence on geology can be as small as the pebbles one sees on the beach to as big as the expansion of downtown Juneau.

"Thirty percent of downtown Juneau was built on crushed rock from the A-J mine," Stone said. "A-J mine was the biggest gold mine in Alaska. It provided 105 tons of gold and 100 million tons of crushed rock."

Stone said since the 1930s the city started filling tidelands with dump rock from the mine. The Juneau City Hall, Centennial Hall, Prospector Hotel and anything beyond Front Street, which used to be the tide line, are sitting on land created by waste rock.

DeWitt said one can learn more about Juneau's geology and minerals by breaking open some quartz rock with iron stains.

"You can break it open and get a fresh surface," DeWitt said. "You can often see a different combination of minerals such as galena, pyrrhotite and sthalerite. If you are lucky, you might even find a speck of gold."

• I-Chun Che can be reached at ichun.che@juneauempire.com.



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