Sentinal Bridge In The White Mountains, NH

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Some info here<<


More info on the >>Flume Gorge << which is very a very interesting attraction:

How the Flume Was Formed

Nearly 200 million years ago in Jurassic times, 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.

The gorge was covered by glaciers during the Ice Age, but the ice sheet did not greatly change the surface.  It partially filled the valley with glacial debris and removed soil and weathered rock from the vicinity. After the Ice Age, Flume Brook began to flow through the valley again.

The highly fractured granite and basalt have been eroded by frost action as well as by the brook’s water. As you walk through the Flume, look at the floor of the Gorge and you many notice remnants of the main basalt dike, and on the walls of the gorge, small trees are growing. Erosion is still occurring.