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Geology 370: Glacial Geology

EROSIONAL LANDFORMS

A. Introduction

abrasion

plucking (quarrying)

meltwater

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B. Micro-Scale Features

Striae

Image credit: K.A. Lemke

Image credit: ©Michael Collier. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

• abrasion

• indicate orientation but not direction of flow

• requirements

• fossilized striations

• superimposed striations

Chatter marks & crescentic gouges

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

Image credit: Natural Resources Canada http://gsc.nrcan.gc.ca/surf/kivalliq/photo1_e.php

• intermittant rather than continuous contact

Micro crag & tails

abrasion form in lee of resistant material (crag) indicate both orientation & direction of ice flow

Image credit: Natural Resources Canada http://gsc.nrcan.gc.ca/surf/kivalliq/photo1_e.php

Potholes

Porcupine Mountain State Park, MN

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C. Meso-Scale Features

Streamlined features

Crag & tail Image credit: Natural Resources Canada, Canadian Landscapes Photo Collection. http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=171

Roches moutonnees Image credit: Natural Resources Canada, Canadian Landscapes Photo Collection. http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=472

• crag and tail: resistant rock with tail forming downstream

• roches moutonnees

  • combination of abrasion (stoss side) and plucking (lee side)

  • stoss side:

    • high effective normal pressure

    • abrasion

• lee side:

• cavity forms

• plucking

• most likely to form in areas of thin, fast moving ice

• idealized morphology: smoothed, gently sloping stoss side and jagged steep lee side

• acutal morphology function of preexisting joint and fracture patterns in rock; not always a reliable indicator of ice flow direction

Rock grooves & rock basins

Glacier National Park, MT

• rock grooves: either glacial abrasion or meltwater erosion

Finger Lakes, New York combination of glacial abrasion and high pressure subglacial meltwater flow lakes contain two-three hundred meters of glacial sediments

Image credit: NASA Visible Earth . New York's Finger Lakes. Courtesy of ISS Crew Earth Observations experiment and the Image Science & Analysis Group, at NASA’s Johnson Space Center. http://visibleearth.nasa.gov/view_rec.php?id=17036

• rock basins

  • structural weaknesses exploited by plucking

  • flow over a depression

    • extending flow on up-ice side increases basal pressure and abrasion

    • compressive flow on down-ice side results in plucking and removal of material

    • form of basin is accentuated

    • positive feed-back mechanism helps basin grow

Meltwater channels

Image credit: Natural Resources Canada, Canadian Landscapes Photo Collection. http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=253

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D. Macro-Scale Features

Cirques, aretes and horns

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

• cirques
  image credit: Steve Hillebrand, US Fish & Wildlife Service Digital Library System http://images.fws.gov/default.cfm?CFID=3302570&CFTOKEN=37370443

  • formation similar to that of a rock basin: combination of abrasion and plucking

  • cirque headwall:

    • arcuate & much steeper than cirque floor

    • formed primarily by plucking and freeze-thaw weathering

• cirque floor may contain a rock basin (tarn)

• morphology:

• grow in clusters; cirques within cirques

• actual shape affected by underlying geologic structure

• elevation indicative of regional snow line

• orientation affected by local & regional climatic factors

• aretes: ridges separating cirques

• horns: pyramid shaped peaks formed when multiple glaciers erode back into the same peak

Glacial troughs

• abrasion dominates on valley walls, plucking on valley floors

• morphology: parabolic shape

  • power law equation Y = aX^b

    Y = vertical distance from valley floor

    X = horizontal distance from center of valley

    a = a constant

    b = measure of profile curvature; typically 1.5-2.5

  • quadratic equation Y = a + bX + cX²

    X and Y as above

    a, b, and c determined statistically

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

• giant stair steps

• fjords

Areal scouring

Image credit: Natural Resources Canada, Canadian Landscapes Photo Collection. http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=108

Image credit: Natural Resources Canada, Canadian Landscapes Photo Collection. http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=106

scoured bedrock

joint & foliation patterns are primary determinant of landscape morphology

E. Summary

Abrasion, plucking, and meltwater erosion

Micro-, meso-, and macro-scale features