Shells that tell a climate story

The Hauriri Terrace was cut by wave action in shallow seas during a warm interglacial period, about 80 000 years ago. It eroded into rocks deposited during the mid-Pliocene. After the terrace was cut, sea-level rose and a layer of shallow-marine sediments was laid on top of the older rocks. The terrace represents, in effect, the fossil seafloor and a fossil beach. Since the formation of the terrace, sea level lowered by about 120m and rose again in response to the last glacial-interglacial cycle that ended about 10 000 years ago. The terrace, with its cover of younger sediments, has since been uplifted and now forms the flat land at the level of the cliff top.
As you walk west along the beach you will come upon a very obvious bed of tightly-packed shells near the base of the cliff. This layer, and the sandy mudstones below and above it, belongs to the Whenuakura Subgroup and was deposited about 3 million years ago. These rocks contain a fantastic record of sea-level changes in the mid-Pliocene (a long time before the creation of the terrace described above). The mid-Pliocene is a period in relatively 'recent' geological history that is being researched in detail to help us understand the potential impact of modern-day global warming on the Earth's ice sheets and sea level.

A bit further along the beach from where you first see the shell layer there is striking blue grey muddy sandstone about 1 m below the band of shells. This mud represents a calm environment, some tens of metres beneath the ocean surface.
Just above it the layers become sandier and are riddled with burrows created by animals (such as worms or shellfish) feeding on organic remains in the sediment. They indicate, along with the more sandy colour, that the water depth was getting less.
The very base of the shell bed is marked by a sharp surface, formed by the erosive forces of wave action in very shallow water. Notice that the lowest part of the shellbed consists of broken shells of many species of shallow-water bivalves, and that very few shells are articulated (i.e. both valves rarely occur together). This means that they are likely to have been moved around and damaged by wave action.
However, the top of the shell bed consists of many large intact shells of now-extinct bivalve species such as large clams, scallops and giant oysters. These were not damaged and many have remained articulated (i.e. with both valves present together). When they were alive they must have been below wave depth, perhaps as deep as about 60 m.
Finally, above the shellbed is another layer of sandy mudstone which was deposited later and shows a slightly greater depth of deposition.
You have just observed a change from deep water conditions to a beach environment, and then back to deep water! This represents global sea-level changes caused by ice sheets growing and melting, primarily in Antarctica, over a 40 000-year glacial-interglacial cycle.