The oldest lunar features are called maria, which are large, dark plains of solidified volcanic basalt. They are relatively smooth and flat compared to the surrounding highlands.

While they are the oldest features on the Moon, they are not the oldest formations. The lunar highlands predate the maria and are believed to be remnants of the original lunar crust formed from the accretion of planetesimals.

Here's a breakdown:

* Maria: These formed from volcanic eruptions between 3.8 and 3.2 billion years ago, filling the vast impact basins that were created during the early bombardment period of the solar system.

* Highlands: These are heavily cratered and consist of anorthosite, a rock type rich in plagioclase feldspar. They formed even earlier, about 4.5 billion years ago, during the initial formation of the Moon.

So, while the maria are the oldest visible features on the Moon, the lunar highlands are actually older.