No abstract

No abstract

No abstract

Archaeomagnetism applies many of the techniques of paleomagnetism to samples of anthropogenic origin. The materials most often studied are those heated by past peoples (hearths, burned floors, pottery, etc.) because the heating and subsequent cooling of the material generally preserve a stable and measurable magnetization. These heated anthropogenic materials hold tremendous potential for contributing to the understanding of variations in Earth's magnetic field over the last several thousand years because

The Modoc Rock Shelter is located in Randolph County, Illinois at the base of the Mississippi River bluffs, and is documented as having human occupation from 9000 B.P. to as recently as 800 B.P. Two hundred twenty-eight archaeomagnetic samples were collected from 28 surface hearths in two different excavation areas. Pilot samples from each hearth were A.F. demagnetized to 100 mT to establish the optimum demagnetization level for all samples from a specific hearth. The magnetic directions for individual hearths after appropriate demagnetization have high precision with Ͱ-95 values between 2Њ and 4Њ. Independent inclination and declination records from the two excavations, which were correlated with conventional radiocarbon dates, overlap in time and show good correlation. The composite record shows a prominent eastward declination swing of 15Њ from 6500-7800 yr B.P., followed by a westward swing of equal magnitude from 8000-8900 yr B.P. The inclination record is less varied. This secular variation record correlates well with soft sediment records from lakes in the north-central United States and is perhaps the longest and oldest archaeomagnetic secular variation record from a single site in North America.