Tonight the straight wall was prominent. I always look forward to this time of the lunar month and rarely miss a look at this piece of lunar geology. The moon was just past first quarter tonight. The shadow cast on this lunar fault was deep and dark. The slope of the fault is vivid at this time of the month. We set up our 105 mm AP Traveler on our GP mount out on the back deck tonight. We absorbed the lunar views as we traveled the terminator tonight. In between looks at the moon we dined by moonlight on the back deck. It was a beautiful night! It was the night of the great and straight wall.
Looking at the straight wall made me think about faults on the earth. The moon's most prominent fault is only 68 miles or 110 km in length, about 580 feet (300 m) in depth and 1.5 miles (2.5 km) in width. It's an amazing view through the eyepiece. Watching this feature throughout the month is a study in shadow. It's a study in geology too.
I was thinking about the straight wall this weekend while observing another planetary fault system -- the San Andreas fault here in California. The San Andreas fault, separating the great continental slabs we call the North America and Pacific plates is one of the most famous examples of a straight wall on earth. It is much longer than the straight wall on the moon. Its length is 750 miles or 1200 km. From the Mexico border to Cape Mendocino, north of San Francisco, it defines the state. This great gash in the earth spans two thirds of the length of our state of California. From the air it is unmistakable. On the ground, this fault announces its presence as the two mighty plates collide. We feel this collision as earthquakes. You know from a visual glance that something wild is taking place on earth. To feel the earth move is frightening, yet fascinating.
This past weekend some friends gathered and walked on the earth's straight wall. On three separate days we spanned the fault. We walked on our earthly straight wall and observed the weird landscape as it slipped into the sea south of San Francisco at Mussel Rock. We walked on the straight wall a little more inland on another day. Ridges and undulations rose from the flat and slightly concave center of the San Andreas rift zone. And on another day, we walked on the straight wall and saw where the rupture caused by the San Francisco earthquake of 1906 moved fences and offset creek flows. It was powerful and peaceful at the same time.
Earth and moon have straight wall geologic features. One is visible through a telescope, and one is visible under foot. Both take a trained eye and some imagination. Both are beautiful beyond words.