Dating the site
One of the most difficult aspects of
research at Happisburgh has been establishing the age of the
artefacts found there. The site is far too old for radiocarbon
dating, which can only be used to date sites that are younger than
about 50,000 years old. Instead, a combination of several
types of evidence has allowed us to establish the age of the site
with some certainty.
These include the geological context,
measurements of the earth’s magnetic field at the time the deposits
were laid-down (palaeomagnetism), as well as evidence from plants
and animals (some of which are now extinct). The diagrams below
show how these methods have been used to date the site.
The artefact-bearing deposits are sealed by glacial deposits. These glacial deposits record the advance and retreat of an ice sheet that covered most of Britain during the Anglian Stage, about 450,000 years ago. Tills and outwash deposited by this ice sheet is a useful marker horizon in the British Pleistocene sequence that can be traced in the cliffs around the coast of East Anglia.
The fine-grained sediments at the site are magnetized and retain the direction of the Earths’ magnetic field at the time of their formation. Detailed laboratory measurements by Barbara Maher and Vassil Karloukovski at the University of Lancaster have shown that the Site 3 sediments indicate reversed polarity, consistent with a major period of magnetic reversal in the Earth’s magnetic field between about 1.8 million and 780,000 years ago (known as the Matuyama Chron). At present, the north magnetic pole is in the Canadian Arctic, but during the Matuyama reversal it was located in the southern hemisphere.
The palaeomagnetic evidence is supported by the biological remains, which include several plants that disappeared from northern Europe during the Early Pleistocene. These include hop-hornbeam (Ostrya-type) and hemlock (Tsuga), which have been identified from pollen grains. The mammals include a mixture of ancient forms, such as ancestral mammoths (Mammuthus cf. meriodionalis), ancestral water voles (Mimomys spp.) and an early elk (Cervalces latifrons), as well as more modern groups, such as red deer (Cervus elaphus) and grass voles (Microtus species). The latter group includes animals familiar to us today. The biostratigraphical evidence suggests that the deposits date to the end of the period of reversed magnetism, between 1 million and 780,000 years ago.
The Pollen succession
Analysis of the vegetational succession by R.G West in the 1960s and during the current project by Sylvia Peglar and Mark Lewis has shown that the channel fill contains a pollen sequence indicative of an interglacial cycle. This record shows that an early phase of open vegetation was replaced by a succession of coniferous woodland that developed into fully-temperate deciduous woodland during the peak of the interglacial cycle. Deteriorating conditions are represented by a return to coniferous woodland (contemporary with human occupation) at the top of the sequence. Site 3 probably dates to one of the prominent warm stages (i.e. those most likely to have supported deciduous forest and other thermophilous plants), either 866,000-814,000 years ago or 970,000-936,000 years ago.