Summary of site:
Interstadials are short periods of warmer climate interrupting extended periods of glacial conditions: they last for a few thousand years as opposed to the multiple tens of thousands that would qualify for the term interglacial.
Woodgrange has given its name to the Woodgrange Interstadial, a period dating from around 13,000 to 11,000 years ago. It was followed by a brief cold period, the Nahanagan Stadial, that lasted for around a thousand years before the general climatic improvement that has kept the ice at bay ever since.
The evidence for this geologically brief warming was provided by a series of borehole cores drilled into the deposits between a group of drumlins west of Derry Hill about 4 km west of Downpatrick. It is an area of drained estuary with a surface cover of fen and marsh. The boreholes averaged less than 10 m in depth to clean gravels.
The first zone above the gravels yielded pollens of grasses, sedges, docks, willow, juniper and members of the rose family, the first plants to colonise the are at the close of the Midlandian glaciation (which lasted roughly 65,000 years). This flora diversified from the initial tundra vegetation with the addition of more willows, pinks, umbellifers, members of the goosefoot family, the green waterweed Pediastrum and ferns. The following zones saw an increase in rock roses, meadow rue and sea buckthorn. The sedge, Cladium, is represented by a few pollen grains. Birch trees, while not common, were present in this open landscape. Then came meadowsweet and crowberry joined by nettles and Cladium again. Grassland predominated at this time and these prairies proved to be the ideal environments for the Giant Irish Deer, Megaloceras giganteus that thrived throughout Ireland.
About 10,000 years ago, after the brief Nahanagan Stadial, there was a sudden increase in juniper pollen and a progressive increase in birch as the juniper declined. This marks the onset of post-glacial times with sustained warming with later fluctuations leading to the present day.
Above this level in the cores marine algae appear in the record indicating brief periods of marine conditions (this entire inter-drumlin basin is below the 10 m contour) before the inundation of the area with a rising sea level lead to the deposition of an estuarine clay. The first indications of this marine phase have been carbon dated at around 6,500 years ago and the maximum level is dated at a little over 3,000 years ago and is represented in the cores by a sandy raised beach.
This is a key site because it establishes firm dates for the first appearance of land plants at the close of the Midlandian glaciation and clearly exhibits the floral progression into the post-glacial period. It also records the rise in sea level into these post glacial times and carbon dates for the sea level maximum.
Although the succession at Sluggan Bog in County Antrim largely supersedes the Woodgrange record in its length and precision, the international importance and status of this site remains, centred on its historic importance.