Glaciers and the Ice Age

Leading ideas about landforms in the 1830s

• Puzzles
  • Misfit valleys and streams; “rock basins” (cirques); wind gaps; U-shaped valleys
  • Widespread deposits of very recent sediments: detrital silt, boulder clay, diluvium, drift
  • Widespread erratics
    • Around Alps, blocks clearly derived from them
    • Northern Germany, blocks sourced in Scandinavia across the Baltic Sea
• Two basic models
  • Diluvial
    • Features due to multiple violent flooding events
      • Wind gaps, moraines , scratched surfaces seem to fit
      • Can only move large blocks short distances
      • Source of water unclear; possibly related to sudden uplifts (?)
    • Non-actualistic
  • Marine inundation
    • Cover continental areas with sea, leave mountains
    • Mountain glaciers spawn icebergs
    • These emplace sediment and erratics
    • Requires recent marine inundation without any supporting evidence

Early studies of glaciers

• Swiss naturalists (Charpentier and Venez)
• Features associate with modern glaciers
  • Moraines, striations, erratics
  • Trace back to modern glaciers
• Extrapolate that glaciers were more extensive and covered the Swiss Plain
  • Limited scale but well beyond present
  • Not explain the widespread deposits and erratics

Agassiz

• Proposes continental glaciation using a catastrophic idea
  • Earth cools and is covered by ice (at least over all of Europe)
  • Rapid uplift of Alps (Elie de Beaumont model)
  • Ice slides off mountains onto adjacent lowslands and plains
  • Subsequent erosion reduces elevation, climate warms and ice melts
• Evidence
  • Widespread distribution of glacial features (erratics) across Europe
• Both the Directionalists and Lyell (steady state) objected
• Agassiz went into the field to demonstrate his evidence

Reactions and (eventual) resolution

• Studer and others
  • Worked to extend glacial model to northern European plain
  • Found evidence that ice sheets forming around lower ranges (Vosges, Jura, etc.)
  • Postulated that climate was cooler in the recent past
• Lyell and Buckland (1840)
  • Joint field trip with Agassiz in Scotland
  • Reinterpret features (moraines, striations) as glacial
    • Buckland had favored diluvial models; Lyell marine inundations
  • Presentation at Geological Society
• Later Lyell and Darwin
  • Modified earlier enthusiasm
  • Argued for local glaciers around mountains (Alps, Pre-Alps, Scottish Highlands) due to fluctuating climate
  • Moved away from any continental ice sheet idea
  • Returned to marine inundation model
• Consensus by 1850s (as predicted by Buckland)
  • Acceptance of the idea of glaciers as significant geological agents
  • Majority idea was that these occurred in the vicinity of mountains
    • Waxing and waning under the influence of climate
  • Agassiz remained committed to continental glaciation
• Later developments
  • Studies of Greenland and Antartica
  • Show how continental scale ice sheets can work
  • Lead to acceptance of continental ice sheets in the recent past, although not to the enormous extent postulated by Agassiz
  • Attributed to climate fluctuations, not catastrophic uplift of Alps