A magnetic and electrical study of archaeological structures at Loma Alta, Michoacan, Mexico

This paper describes the results of high spatial resolution magnetic and resistivity surveys of the Loma Alta site in Michoacan, Mexico. This site is the largest of a series of man-made earthen islands built up within an ancient freshwater basin. Occupied during three main phases from 100 BC to AD 850, the site now shows no surface detail of the underlying structural complexity. Test pits and trenches excavated during several field seasons revealed a fraction of the large ceremonial complex and provided isolated archaeological data difficult to interpret. Geophysical prospection aimed to investigate the so far unknown settlement pattern and detect architectural remains. Magnetic prospecting of the stone architecture turned out to be excellent due to the high contrast between the volcanic rock and the sedimentary fill. A fluxgate gradiometer survey of the entire mound surface defined a large number of small aligned positive and negative magnetic anomalies. Electrical surveys complemented and verified the magnetic results. A series of verification excavations exposed structural elements predicted by the prospecting and aided a further interpretation of the data. The major results of the study are discussed, as they provide a clear image of the orthogonal layout of the highly structured ceremonial site.