In other areas, irregular and steep-sided volcanic piles dominate. Elsewhere, clusters of domes occur as in the Marius and Rumker Hills. Other typical features on the maria are sinuous rilles and wrinkle ridges. Many sinuous rilles originate in craters near the higher margins of the mare basins and flow into the lowlands. Apollo 15 collected samples from the margins of Hadley Rille and confirmed the hypothesis that sinuous rilles are basaltic lava channels.
Wrinkle ridges occur in all mare regions and form circumferential or medially transecting patterns.
Ages of the maria are being determined by two methods. Absolute ages are given by radiometric techniques. From these we know that the sampled lunar basalts are much older than their terrestrial counterparts. The basaltic lava flows range in age from 3.15 to 3.85 billion years, so the episode of lava filling on the Moon must have continued for at least 700 million years. Relative ages can be established by counting craters in mare surfaces.