Mare Marginis is located on the northeastern edge of the lunar nearside, right next to Mare Crisium. Where the latter has a circular shape, associated with a large impact crater, Mare Marginis has an irregular outline, suggesting that it is the result of several smaller craters being flooded with lavas.
The mare as such is not in the Lunar 100, but so is some swirlings at its northern edge. At favorable libration the Marginis swirls are not as difficult as its Lunar 100 number might suggest (the list being sorted according to difficulty). Observed at a three days old moon, I actually found the mare more difficult to trace than the swirls. In the sketch the swirls are marked by the large white patch right at the edge in the upper half of the sketch. The swirls had some structure to it, but due to rather poor seeing it wouldn’t stabilize enough to sketch.
The crater just south of the swirl is Goddard, and the two craters at the bottom is Neper and Jansky.