L19 - Alpine Valley
As Mr. Wood states, this is a lunar graben. But, what is a graben? As defined by Webster, it is "a depressed segment of the crust of the earth or a celestial body (as the moon) bounded on at least two sides by faults". Graben are typically formed when the crust gets stretched and breaks apart. Sometimes a block between two faults will drop down to form the graben, sometimes lava will well up into the crack and form the flat bottom of the valley. In the case of Alpine Valley, it was probably lava that welled up into the crack. There is a rille (collapsed lava tube) that runs the length of this valley that is a real challenge to observe from Earth. Alpine Valley is about 79 miles long and 7 miles wide.
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Images © Doug Anderson
This image was taken on 26-May-2004 through a 203mm Celestron C-8 Schmidt-Cassegrain reflector with f/6.3 focal reducer afocally coupled to an Olympus C-700 digital camera through a 40mm eyepiece.