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Limestone: The Building Block for Tilting Brooks Range Peaks

To glimpse the Brooks Range in northern Alaska, I recently drove eight hours up the dusty Dalton Highway, cringing as oncoming trucks lobbed rocks at my formerly pristine windshield. The drive was worth the wincing. After about 200 miles, Sukakpak Mountain gave me a dramatic welcome to the southern Brooks Range--it rose at an angle like a ghostly, 4,000-foot wave frozen in the act of crashing on a beach.

The drastic slant and pale complexion of many Brooks Range peaks is due to the drift of continental plates and the presence of limestone, said Gil Mull, a geologist with the Alaska Division of Geological and Geophysical Surveys. Mull, a man who has picked around the Brooks Range with his rock hammer for 30 years, shared a few hours of his time with me shortly after I returned from my trip to the Brooks Range. The range lies above the Arctic Circle in a 600-mile, east-to-west band that seals off the North Slope from the rest of Alaska.

Many peaks appear limestone white on satellite photos of the Brooks Range. When geologists see limestone, they often think of salt water. Limestone portions of the Brooks Range were once waist-deep oceans; the oceans, which existed about 400 million years ago, were about the same depth as the ocean surrounding the Bahamas today, Mull said.

Limestone, a light gray rock, is often composed of the remains of countless sea organisms with calcium-rich shells and other body parts. After they died, the hard coatings the creatures wore broke down into immense banks of shell debris and limestone mud. Over time, the mixture hardened into stone.

The limestone mountains of the Brooks Range were formed by the movement of several of the earth's plates, Mull said. The earth's crust is made up of about a dozen or so plates--gigantic slabs of rock that are more than 1,000 miles across and are up to 40 miles thick. The North American Plate, which covers most of the continent, and the Pacific Plate, which underlies the Pacific Ocean, collide in Alaska.

Where the Brooks Range lies today, the Pacific and North American plates began crashing together about 145 million years ago. Moving at about the speed our fingernails grow, the Pacific Plate acted much like a snowplow, Mull said. The Pacific Plate shoved itself over the top of the North American Plate, folding and stacking up layers of limestone, sandstone, and other rocks. During the next 30 million years, the limestone that forms some of the more dramatic scenery in the Brooks Range was forced upward, and mountains were born.

The snow-plow effect of the Pacific Plate makes the Brooks Range relatively simple for geologists to map, Mull said. Fragments of easy-to-identify rock types found hundreds of miles away can be mated, making it easier to see where the Pacific Plate gouged up rock formations and pushed them for millions of years, setting them where they now rest.

Peaks such as Sukakpak Mountain tilt more than other mountains because limestone is the primary building material. Unlike shale or other sedimentary rock types that tend to crumble and form more rounded peaks, limestone is rigid enough to jut into the air at crazy angles without breaking. The resulting mountains make a trip to the Brooks Range worth a few star cracks on the windshield.