I was looking at the moon, as I am wont to do, and while observing the Mare Frigoris region noticed the thinnest of black lines running through the middle of the plain. It started at the point between craters Plato and Archytas, in the middle of the Mare, then proceeded west. I assumed it was a rille. It was very thin and rather depended on the seeing, which was good. I think I was using 553x but I'm not sure; it could have been 277x (I failed to note down the eyepiece I was using and I can't remember it now). After a few minutes the feature seemed to lengthen and its western tip took a turn to the south toward a peninsula of mountains southeast of crater Birmingham. Studying the area in Rukl today, I don't find any rilles here, just what appear to be wrinkle ridges.
The Kaguya Lunar Atlas, plate 31, has a partial picture of the area, and show what they identify as mare ridges. The portion of the "western tip" I saw curving to the "peninsula" is partially shown in the photograph. That ridge continues west beyond the peninsula, but I did not see that. Mare ridges usually mark buried rims of circular basin and usually have circular patterns; Mare Frigoris' mare rims are unusual in that they have a random pattern. The angle of light must have been just shallow enough for the ridge to cast a small shadow for me to see.
I also spent a fair amount of time trying to see the rille running the length of Vallis Alpes but could not.
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