The second member of the Avebury group is the double circle, or rather double oval, on Hakpen hill - Haca's Pen (Haca, or Haco, according to Kemble, was some mythical person with a very Danish name which is found in Hampshire and Berkshire, as well as here. Pen seems to mean merely enclosure, as it does now in English. See Kemble, in 'Journal Arch. Inst.' xiv. p. 134.) this was, according to Stukeley ' 138 feet by 155 feet, and had an avenue 45 feet wide, as compared with 51 feet which Sir R. C. Hoare gives for those of the Kennet avenue of Avebury. The avenue is supposed to have extended in a perfectly straight line for above a quarter of a mile, pointing directly towards Silbury Hill, which is about one mile and a quarter distant.

The third member of the group is the famous Silbury Hill, about a mile distant due south from Avebury. That these two last named are of the same age, and part of one design, seems scarcely open to doubt; but it is quite an open question whether Hacas Pen belongs either to the same age or the same design. Its stones were very much smaller, its form different, and its avenue pointing towards Silbury looks as if that monument existed, and may have long existed before it was built; but of this hereafter.