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(Terms in italics are defined elsewhere in the glossary)

underwater reconnaissance Geophysical methods of underwater survey include (1) a proton magnetometer towed behind a survey vessel, so as to detect iron and steel objects which distort the earth's magnetic field; (2) sidescan sonar that transmits sound waves in a fan-shaped beam to produce a graphic image of surface features on the sea-bed; (3) a sub-bottom profiler that emits sound pulses which bounce back from features and objects buried beneath the sea floor. (Chapter 3)

Uniformitarianism The principle that the stratification of rocks is due to processes still going on in seas, rivers, and lakes; i .e. that geologically ancient conditions were in essence similar to or "uniform with" those of our own time. (Chapter 1)

uranium series dating A dating method based on the radioactive decay of isotopes of uranium. It has proved particularly useful for the period before 50,000 years ago, which lies outside the time range of radiocarbon dating. (Chapter 4)

varves Fine layers of alluvium sediment deposited in glacial lakes. Their annual deposition makes them a useful source of dating. (Chapter 4)