The boundary between Cretaceous and Tertiary periods (known as the K–T boundary) marks a major change in fossils 65 million years ago, including extinctions of a number of groups such as the dinosaurs.

In the late 1980s it was realised that the K–T boundary is marked at many sites around the world by a very thin layer rich in elements such as iridium, which are rare on earth but common in meteorites. It is now thought that the K–T boundary represents the impact of an asteroid that triggered a sudden cold period.

One of the first high-iridium sites found in New Zealand was at Woodside Creek in Marlborough.

Most of the drill holes pictured here were made by Canadian geologists in 1977 in order to get samples for chemical analysis.