http://www.space.com/36661-late-heavy-bombardment.html
EXCERPT: Early Earth suffered constant threat of attack from leftover planet-building material. From about 4.5 to 3.8 billion years ago, failed planets and smaller asteroids slammed into larger worlds, scarring their surface. Near the end of the violence, during a period known as the Late Heavy Bombardment, impacts in the solar system may have increased. The increased activity most likely came from the movement of the giant planets, which sent debris raining down on the smaller rocky worlds....
EXCERPT: Early Earth suffered constant threat of attack from leftover planet-building material. From about 4.5 to 3.8 billion years ago, failed planets and smaller asteroids slammed into larger worlds, scarring their surface. Near the end of the violence, during a period known as the Late Heavy Bombardment, impacts in the solar system may have increased. The increased activity most likely came from the movement of the giant planets, which sent debris raining down on the smaller rocky worlds....