This field is south of the great crater Arzachel (100km dia.) and well known to the versed lunar observer for its many and varied features. The first feature is Arzachel itself, the southernmost of the trio with Ptolemaeus, Alphonsus that dominates the center of the visible disk of the Moon. It is noted for its spectacular terracing and interior rimae one of which that arcs north-south just east of the oddly off center peak on the floor and roughly concentric with Arzachel A (8 km). Below or south of Arzachel is the crater Thebit with the interesting configuration of Thebit A (19 km) on the northwestern portion of the wall and Thebit L (10 km) to the northwest of that. Then west of this is the iconic “Straight Wall” which is neither straight nor a wall per se. It is seen here as a dark diagonal slash on the eastern edge of Mare Nubium, running up from the a curious set of peaks below Thebit northwest to the little crater Thebit D (5 km). The “wall” is made up of a number of small faults 8-50km in length, with the cliffs being some 250-300m high and 2.5-3km in width, angled at less than 10o. The curious peaks are colloquially called “The Stag’s Horn Mountains” and are the remnants of previous features destroyed in the massive Nubium impact event.
Just to the right of image center is a particularly well defined good sized crater, Werner (70 km). Between it and Thebit is a larger crater, less well defined, Purbach (118 km) with remnants of now buried craters on it’s northwestern floor. Above Werner is a very poorly defined crater, more of an oval plain, Blanchinus and north of it is LaCaille (both 68 km dia.). These are interesting because of their intersection. The raised terrain between the eastern wall of Purbach and the western walls of Blanchinus and LaCaille form what is known as the “Lunar X” at low sun angles. Can you see it here? Those familiar with the feature when it is on the terminator probably can.
Before leaving this region notice the feature to the upper right of LaCaille. It appears like a deer hoofprint in snow, more hoof-like than the popular Aries Hoofprint! In LROC images it appears to be the juxtaposition of two badly ruined craters that once shared a common straight wall. This is Delaunay (roughly 46km dia) and a much modified version of the two craters in the uppermost right corner of the image, Azophi (48 km) at bottom and Abenezra (41 km) above. You can see how the shared straight wall is flattened between them as in Delaunay thought these latter craters were not as ruined. This is truly a region of straight and not-so-straight walls!
