Flume Gorge
Coordinates: 44°5′59″N 71°40′12″W / 44.09972°N 71.67000°W
The Flume Gorge is a natural gorge extending 800 feet (240 m) horizontally at the base of Mount Liberty in Franconia Notch State Park, New Hampshire, United States. Cut by the Flume Brook, the gorge features walls of Conway granite that rise to a height of 70 to 90 feet (21 to 27 m) and are 12 to 20 feet (3.7 to 6.1 m) apart. Discovered in 1808 by 93-year-old "Aunt" Jess Guernsey, the Flume is now a paid attraction that allows visitors to walk through the gorge.
Nearly 200 million years ago in the Jurassic period, the Conway granite that forms the walls of the Flume was deeply buried molten rock. As it cooled, the granite was broken by closely spaced vertical fractures which lay nearly parallel in a northeasterly direction. Sometime after the fractures were formed, small dikes of basalt were forced up along the fractures. The basalt came from deep within the earth as a fluid material, and because of pressure, was able to force the Conway granite aside. The basalt crystallized quickly against the relatively cold granite. Because of this quick cooling, the basalt is a fine- grained rock. Had this material ever reached the surface, it would have become lava flows.
Erosion gradually lowered the earth's surface and exposed the dikes. As the overlying rock was worn away, pressure was relieved and horizontal cracks developed, allowing water to get into the rock layers. The basalt dikes eroded faster than the surrounding Conway granite, creating a deepening valley where the gorge is now.
...
Wikipedia