• Polished rock pavement
• Striations
  • Nail head striae: striations that widen and abruptly deepen in the down ice direction
• Grooves
• Rat tails (small craig and tail features: tails point in the direction of ice flow)
• Percussion marks and fractures (friction cracks)

  • Simple fractures (rock is fractured but typically not chipped)
    • cresentic fractures: line of conchoidal fractures, convex up ice
    • chattermarks: line of irregular fractures. Commonly found in shallow grooves.
  • Gouges: Chipped fractures (chip of rock is removed along two intersecting fractures)
    • Fracture surfaces typically dip in the direction of ice flow
    • cresentic gouges or fractures (concave up ice)
    • lunate fractures (reversed cresentic fractures) Convex up ice

•  P-Forms (Plastically molded forms)
  • Smooth forms
    • controversy: are they produced by subglacial meltwater or a till slurry?
      • Meltwater theory: most of the features resemble those created by fluvial erosion of bedrock (processes)
      • Till slurry: many features are striated
  • Transverse forms
    • sichelwannen
    • rippled bedrock
  • Longitudinal forms
    • Cavetto forms: undercut channels on steep or vertical walls; generally have sharp edges
    • Grooves or furrows: channel-like features with rounded edges; more commonly found on flat surfaces
    • Spindle flutes
  • Non-directional forms
    • bowls and potholes

• Large grooves
• Streamline features
  • Orientation and morphology is controlled by glacial flow, presence of subglacial cavities, and bedrock lithology and structure.
  • Whalebacks: Hills rounded on stoss and lee sides
  • Rock Drumlins: Blunt end face up ice
  • Craig-and-tail: larger versions of rat tail
  • Roches moutonnees: Gentle abraded stoss side and pluck lee side. Blunt end faces down ice

• Most of these features are formed beneath warmed-base glaciers and are typical of areal scour beneath an ice sheet.
• Many of these small scale features are good indicators of the direction of glacial flow.
• Most presently exposed small scale features were most likely formed by the last ice sheet.
• Striations and small flow indicators typically record the lastest flow direction of an ice sheet occurring during deglaciation.
• Larger features like grooves and roche mountonnees are more likely to preserve earlier flows.

[Glacial and Quaternary Geology] [extended GeoIndex][QkRef][Geological Sciences] [Degree Programs] [Salem State College]
Lindley Hanson (email) Last Modified 3/15/03