The boundary between the Cretaceous and Tertiary periods (the K–T boundary) marks a major change in fossil plants and animals that occurred 65 million years ago. At this time a number of animal groups, including the dinosaurs, became extinct.

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 that led to extinctions.

One of the first high-iridium sites found in New Zealand was at Woodside Creek in Marlborough, pictured here. Most of the drillholes were made by Canadian geologists in 1977 in order to get samples for chemical analysis.