Stonehenge is widely believed to have begun sometime after the last Ice Age, during the rise of the first civilizations as people began domesticating animals and establishing agricultural production. Its construction timeline begins around 10,000 B.C. with large pine “Totem Poles” placed in the ground near the site. The monument is usually considered to have undergone three separate phases: Phase I, from 3100 to 2700 B.C., was marked by the construction of a large ditch or ring; Phase II, from 2700 to 2500 B.C., included the digging of 56 holes of unknown purpose, and the Heel Stone was probably set during this period; and Phase III was divided into three sections, with the Blue Stone Horseshoe, the beginning of the Sarsen Ring, and the setting of four external lunar stones from 2600 to 2500 B.C., completion of the Sarsen Ring from 2400 to 2300 B.C., and completion of the entire structure from 2200 to 1800 B.C. From 1800 B.C. to the present, various stones have fallen or been removed from the original Stonehenge.