A soaring mountain range as tall as the Himalayas once towered over the U.S. East Coast. Some 20 miles (32 kilometers) of rock have since transformed into sand and mud, exposing an outcrop of the most extreme rocks in America.

Banded with colorful, unique garnets, the gneiss — a form of metamorphic rock — was pushed as far as rock can go before it melts, to 1,800 degrees Fahrenheit (1,000 degrees Celsius), geologists report in the Dec. 13 issue of the journal Geology.

The find is the first discovery of ultrahigh temperature metamorphic rocks in the United States, said Jay Ague, a professor at Yale University and lead author of the study. The next hurdle is figuring out how they formed, he said.

“The fact that these rocks are there at all challenges all the existing models for mountain building in the area,” Ague said. “These ultrahot [rocks] are becoming an important part of how we think mountain belts form,” he told OurAmazingPlanet.

(via America’s Hottest Rocks)