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70 results

Page 1 of 4

  • Surface exposure dating of the Pierre Sublobe of the James Lobe, Laurentide Ice Sheet

  • Stephanie L. Heath, Thomas V. Lowell, Brenda L. Hall
  • Journal:
    Quaternary Research / Volume 97 / September 2020
    Published online by Cambridge University Press:
    13 April 2020, pp. 88-98

  • The Laurentide Ice Sheet of the last glacial period terminated in several lobes along its southern margin. The timing of maximum extent may have varied among the terminal lobes owing to internal ice sheet dynamics and spatially variable external controls. Some terminal ice lobes, such as the westernmost James Lobe, remain poorly dated. To determine the timing of maximum ice extent in this key location, we have mapped glacial deposits left by the Pierre Sublobe in South Dakota and applied 10Be surface exposure age dating on boulders on moraine ridges associated with three distinct late Quaternary glacial drifts. The oldest and most extensive “Tazewell” drift produced variable 10Be surface exposure ages spanning 20–7 ka; the large range is likely attributable to moraine degradation and subsequent boulder exhumation. The oldest ages of about 20 ka are probably limiting minimum ages for the Tazewell moraine surfaces. By contrast, exposure ages of the youngest “Mankato” drift of the easternmost Pierre Sublobe tightly cluster at about 16 ka. This age for the Pierre Sublobe is consistent with the nearby Des Moines Lobe, suggesting both acted together.

Greek Astronomy
  • Thomas L. Heath
  • Published online:
    05 November 2014
    Print publication:
    20 March 2014
  • From its beginnings in Babylonian and Egyptian theories, through its flowering into revolutionary ideas such as heliocentricity, astronomy proved a source of constant fascination for the philosophers of antiquity. In ancient Greece, the earliest written evidence of astronomical knowledge appeared in the poems of Homer and Hesiod. In the present work, first published in 1932, Sir Thomas Little Heath (1861–1940) collects some of the most notable essays and discussions of astronomical theory by Greek astronomers and mathematicians, presenting them in English translation for the modern reader. With chronological coverage, Heath's book features a thorough introduction, a doxography of what ancient authors said about the earliest theorists and longer excerpts exploring fundamental ideas. Among the pieces are extracts from Plato's Republic and Ptolemy's work on the impossibility of a moving Earth, alongside material from Aristotle, Euclid, Strabo, Plutarch and others.

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