Observing Project 5E Geological Forces at Work II The Straight Wall L15

It is called "California's Terrible Wound." The San Andreas Fault extends across hundreds of miles through the mountains of southern California where the Pacific and North American continental plates run together and the Pacific plate is forced beneath the surface and into the mantle of Earth. As the plates rub against each other, unimaginable amounts of energy are building up, waiting to be released. This scenario is played out on fault lines all over Earth. Did these processes ever take place on the Moon?

Located near the Moon's central meridian is the Straight Wall which is in the Lunar 100 as L15. This is the most obvious example on the Moon's near side of a fault line. If you use a map to locate Mare Nubium, the Straight Wall pops into view very easily, most especially at low Sun angles. The wall runs for approximately 155 km across the eastern edge of Mare Nubium. Look for the Wall beginning about eight days after new moon and again as sunset approaches around twenty-two days after new moon. The long shadows created by the low Sun at these times gives rise to a stunning illusion. The wall truly looks like a wall but in fact this so-called "wall" is sloped only seven degrees from horizontal.

We don't know enough at this point about how the Moon's crust is divided and whether or not more of these fault lines exist below the visible surface. It is a fascinating point of geology to note that the location of the fault is near the "shoreline" of Mare Nubium and highlands begin to the east. Most of Earth's faults can be found in similar geological confluences, such as the Pacific's "ring of fire" that runs from the tip of South America, up the Andes and the U.S. west coast, up Canada and Alaska, then all the way down the coast of Asia into the islands of Indonesia. The Moon has been dead geologically for billions of years, but features like the Straight Wall serve as fossilized reminders that the Moon may once have been as geologically dynamic as Earth is today.

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