Stonehenge

The Y and Z Rings were not discovered until 1923. Subsequent radiocarbon dating showed they were the last work done in the area.

An excerpt from the article 16 facts about Stonehenge
According to the researchers, the so-called bluestones (most likely diabase) were placed in them. However, since they were constructed at the last stage, the version that they served as holes to help construct other sanctuary elements has been firmly ruled out.