Protecting Gratiot Lake

EAGLE HARBOR – Members of the Gratiot Lake Conservancy learned about both the geological forces that created the lake and ongoing efforts to protect it at the conservancy’s annual meeting Monday night.

The Gratiot Lake Conservancy held its annual meeting at the Eagle Harbor Community Center Monday, capped by a speech from retired Michigan Technological University professor Bill Rose on the formation of the Keweenaw Fault.

The Gratiot Lake Conservancy is partnering with the Keweenaw Invasive Species Management Area to prevent invasive species. The groups were at a recent fishing tournament in Lac La Belle to promote awareness of the importance of washing boats before bringing them into a new body of water; not doing so has led to the spread of many invasives.

“We have an aquatic species called Eurasian water milfoil, which looks very pretty, but it’s a real problem,” said Vice President Bonnie Hay. “It’s already invaded some areas of the Portage … we don’t want that in Gratiot Lake. It would be nice to keep it out of all the lakes of Keweenaw County if we could.”

Upcoming programs include an aquatic plant workshop with Janet Marr on Aug. 24-25 on identifying invasive species. Jane Herbert, senior water resource educator with Michigan State University Extension, will host a workshop on the importance of preserving or restoring native vegetation along shorelines from 9 a.m. to 4 p.m. Aug. 26; the session will start at Gratiot Lake and move to the Keweenaw Land Trust’s Marsin Center for the afternoon.

Before a standing-room-only crowd, Rose gave a presentation on the Keweenaw Fault, explaining the origins and working his way landmark by landmark up the coast.

The break is responsible for many of the notable sites of the Keweenaw, such as Douglass Houghton Falls and Hungarian Falls. Gratiot Lake itself is fault controlled, and was formed by a glacial melt into low-lying ground about 10,000 years ago.

“It’s such a strong influence on landscapes, it helps us understand the landscape,” Rose said. “These places wouldn’t exist without the fault.”

The Mid-Continent Rift ripped apart a large chunk of the Midwest, stretching from Kansas to Lake Superior. About a billion years ago, a collision with the Grenville Tectonic Zone slammed the fracture back together.

That created a thrust fault, a geological rarity where one side moves up relative to the other.

That fault both created the conditions allowing copper deposits to form and allowed them to get closer to the surface, Rose said.

“If it wasn’t for the fault, they’d be too far from the surface, and we never would have known that they were there,” he said.

The fault bisects the Keweenaw into geologically and topographically distinct parts, the west side of the Keweenaw Peninsula has higher ground and contains basalt; the other is flatter and has a softer Jacobsville sandstone.

In Keweenaw County, the fault curves so that the break occurs just off-shore. The standing-room-only crowd oohed at an overhead photo of the fault east of Bear Bluff, where the start of the sandstone could clearly be seen in shallow waters off the shore.

Despite its scale, the fault hasn’t been seismically active for hundreds of millions of years, Rose said.

While the rift zone can be compared to locations such as the Red Sea or the Rio Grande Valley in New Mexico, copper in the Keweenaw doesn’t have an easy comparison geologically.

“We just don’t have an example of this kind of mineralization,” Rose said. “It’s an oddity.”

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