Saturday, November 29, 2014

The Nature of Hanging Valleys

A hanging valley is generally defined as a stream or glacial valley that ends abruptly atop the steep wall of a deeper valley or sea cliff.  Such formations are especially common in glacial terrain and where the sheer wall of the deeper valley or cliff is composed of resistant rock (e.g. granite, marble, etc.).

In mountainous regions that have been subjected to glaciation, the major glacial tongues erode deep, U-shaped valleys through the range; smaller tributary glaciers feed the primary ice flow from either side.  When the glaciers melt back as the climate warms, the deep, steep-walled glacial valley is lined with hanging valleys on either side, where streams run down the shallower tributary valleys and then plunge into the deep, central valley via magnificent waterfalls; Yosemite Valley offers an excellent example of such post-glacial topography.  Of course, if the glaciated terrain has since become an arid landscape (e.g. ranges of the Great Basin) the streams and waterfalls are seasonal.  Finally, in unglaciated regions of the globe, similar topography may develop when rock falls broaden the central valley, cutting off feeder streams well above the primary river.

Hanging valleys may also form atop sheer sea cliffs, where streams draining higher terrain create valleys that end abruptly at the edge of the cliff; there, the stream becomes a waterfall, plunging to the rocky shore or directly into the sea.  In all of the above scenarios, the feeder stream valley is "left hanging" above the central valley floor or coastal beach.