Narrow steep-sided valley or canyon that may or may not have a river at the bottom. A gorge may be formed as a waterfall
retreats upstream, eroding away the rock at the base of a river valley; or it may be caused by rejuvenation
, when a river begins to cut downwards into its channel for some reason for example, in response to a fall in sea level. Gorges are common in limestone country (see karst
), where they may be formed by the collapse of the roofs of underground caverns.
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