With new geology map, Idaho rocks

Published: October 17, 2012

The new geologic map of Idaho.

Get the map

The map costs $20, plus shipping and handling.

To order, visit the IGS website at www.idahogeology.org.

By WILLIAM L. SPENCE — Lewiston Tribune

Geological history will be on vivid display Friday, when the Idaho Geological Survey unveils "a piece of art," Director Roy Breckenridge said.

The big map's colors are used to identify different types of rocks, as well as rocks of different ages. The colors are draped over a shaded relief map that highlights mountain slopes and other surface features in sharp detail.

"You don't have to be a geologist to be drawn to it. It encourages inquiry and discovery of the land we live in," Breckenridge said.

The map illustrates Idaho's multibillion-year rock record, a grand narrative that includes rifting and colliding continents, global catastrophes, rising and falling sea levels, and eons of slow, steady erosion.

A dark charcoal-brown is used to identify the oldest rocks in the state, including a relatively small plug of 2.7 billion-year-old gneiss in the Pioneer Mountains southwest of Mackay, and a large swath of 2.67 billion-year-old granitic gneiss, schist and quartzite located between Bovill and the St. Joe River.

They date from a time when modern-style plate tectonics might not have operated, when Earth's mantle was much hotter and when continental crust might have been created in enormous melting events.

At the opposite end of the geological time scale are the roughly 15,000- to 17,000-year-old Lake Missoula and Lake Bonneville flood deposits. They're found in the Rathdrum Prairie and along the Snake and lower Clearwater River drainages, marked in yellow.

Idaho rocks haven't changed much since the last statewide map was produced, but a substantial amount of new mapping has been done since then. The work provided details in areas that were previously just conjecture.

Age-dating technology has also improved dramatically, so the new map provides more accurate information about formation.

THE STORY OF ROCKS

Keegan Schmidt, a geology professor at Lewis-Clark State College who has worked on a number of IGS mapping projects, said geological maps tell a story.

"It starts the first day at the first outcrop," Schmidt said. "You walk up to a rock and mark its location on a map. That rock represents an event."

If it's a type of lava, he said, then you know there was a volcanic eruption. If it's a limestone, there was deposition in a marine or lake environment. A sandstone could be anything from a flood to a beach to a wind-blown dune, but features within the rock will likely distinguish which it is.

"Then you move on to the next rock that's different," Schmidt said. "That represents a different event, so you have to determine the relationship between the two."

Rocks can be folded and faulted as well, he said, so you have to consider the sequence of events. Which rock came first? Was there a time gap before the second rock? When and how were they deformed?

"As you walk along, you're filling in the story," Schmidt said.

USING THE MAP

The practical applications of the story include protecting groundwater, identifying minerals and spotting hazards.

Near Riggins, for example, IGS has done detailed mapping to help identify specific rock units that are causing landslide problems along the U.S. Highway 95 corridor.

The U.S. Geological Survey conducted a number of large mapping projects in Idaho in the '80s and early '90s. And over the past decade or so, IGS has produced more than 160 maps under the auspices of the StateMap project.

Those large-scale maps provide the detailed information needed by geotechnical engineers, industry officials, resource managers and government agencies, he said. They served as the foundation for the new state map.

The Idaho Geological Survey, created in 1919, published its first geological survey map in 1947, followed by the 1978 map, and now by the latest version.

"Geological maps have a certain useful life," Lewis said. "We used to say they were good for 30 years or so, but with the advances in technology I'm beginning to wonder if it's shorter than that. It's a moving target. You can never say you have it all mapped and it's time to go home. This is just a snapshot in time."