"Graph of Cratering Rates over Time. The vertical axis is labeled ""Rate of Formation of Impact Craters"" in arbitrary units. The horizontal axis is labeled ""Time before Present (in Billions of Years)"" in intervals of 1 billion years, beginning at 5 on the left and ending at zero on the right. The plotted curve begins near the top-left at about 4.5 billion years and is labeled ""Hypothetical cratering maximum"". The curve drops very steeply to about 4.1 billion years, with this portion of the curve labeled ""Cratering of highlands"". The curve then becomes more shallow as the cratering rate further decreases, until it reaches 3.2 billion years at about 25% of the maximum cratering rate. This part of the curve has two labels, near 4.1 billion years as ""Imbrium and Orientale impacts"", and near 4.0 billion years as ""Formation of maria"". At 3.2 billion years the curve becomes horizontal out to zero billion years, representing a constant, very low cratering rate. This part of the curve is labeled ""Recent craters (Tycho, Copernicus)""."