Mason is the remains of a lunar

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 that lies in the northeastern part of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. It is nearly attached to the eastern rim of the flooded crater Plana

Plana (crater)

Plana is a lunar crater that lies on the boundary between two small lunar mare areas, with Lacus Mortis to the north and the larger Lacus Somniorum on the southern side. It is joined to the crater Mason to the east by a short stretch of rugged ground...

, and southeast of Bürg

Bürg (crater)

Bürg is a prominent lunar impact crater in the northeast part of the Moon. It lies within the lava-flooded, ruined crater formation designated Lacus Mortis. To the south and southeast is the crater pair Plana and Mason. To the west, beyond the rim of Lacus Mortis, is the prominent crater...

. Along the northern rim of Mason is the southern edge of the Lacus Mortis

Lacus Mortis

Lacus Mortis, latin for "Lake of Death", is a plain of basaltic lava flows in the northeastern part of the Moon. It lies just to the south of the elongated Mare Frigoris, being separated by a slender arm of rugged ground...

, a small lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

. To the south is the larger Lacus Somniorum

Lacus Somniorum

Lacus Somniorum is a plain located in the northeastern part of the Moon's near side. It is located at selenographic coordinates 38.0° N, 29.2° E, and has a diameter of 384 kilometers...

.

This is a heavily eroded crater formation that is somewhat irregular in shape, being longer in the east–west direction. The rim is an uneven, disintegrated ring of ridges that have merged with the rough terrain to the south and east. There are clefts or valleys in the western rim that reach the eastern rim of Plana. The interior floor has been resurfaced by lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

, and forms a nearly level basin within the rim. The small crater Mason A lies in the northwest part of the floor.

The crater is named after the English astronomer Charles Mason

Charles Mason

Charles Mason was an English astronomer who made significant contributions to 18th-century science and American history, particularly through his involvement with the survey of the Mason-Dixon line, which came to mark the division between the northern and southern United States...

.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater mid-point that is closest to Mason.

Mason	Latitude	Longitude	Diameter
A	42.8° N	30.1° E	5 km
B	41.8° N	29.6° E	10 km
C	42.9° N	33.8° E	12 km