Bombardment history of the Moon, illustration. This bombardment history is deduced from the lunar surface features. This sequence starts at upper left with the Moon sweeping up debris after it had formed. The impacts of this debris and internal heat, melts the Moon's surface (upper centre). At upper right, the Moon's surface re-solidifies into highland terrain. At lower left, major impacts form the Moon's basin features. At lower centre, after the Moon became tidally locked, a period of volcanism caused by impacts only on the nearside of the Moon resulted in lava flooding the nearside. Finally, the quieter current period sees low-level cratering caused by occasional meteorite impacts (lower right).