GEOLOGY 445 - GLACIAL GEOLOGY

LAB 7 - GLACIAL EROSION AND DEPOSITION

Introduction

Glaciers are commonly visualized as massive engines of destruction, bulldozing their way down the mountains, tearing off pieces of resisting rock, inexorably moving all that they encounter. But how efficient are they at removing rock, and, once removed, how far does that rock material travel?

Problem

To answer this question, even for a single valley, we must make major assumptions. We must assume that we can reconstruct the land surface as it was prior to glaciation. For example, on the east flank of Longs Peak (Longs Peak, CO, 1:24,000) we can reconstruct a uniform slope by connecting contours on bedrock across the valley. Assume bedrock to be exposed south of the prominent south lateral moraines and north of Alpine Brook. Ridges which extend above those contours reflect net deposition. The Roaring Fork valley below those contours reflects net erosion, but how much was glacial and how much fluvial?

Procedure

Make the appropriate assumptions and calculate the volume of material eroded by fluvial (and associated mass movement) processes, the amount eroded by glacial processes, the amount deposited by Pinedale glaciers in the terminal and lateral moraines, and the amount deposited by Bull Lake and earlier glaciers in their terminal and lateral moraines. [See "Reading Between the Lines", Volume chapter, for a graphical discussion.]  The method of volume determination is to:

1. Draw the reconstructed contours on the former bedrock surface by continuing them across the moraines and Roaring Fork valley.
2. Mark the difference in elevation where index (200') contours on the former surface cross those on the present surface.
3. Color in the areas of 0-200, 200-400, 400-600 foot differences, and so on.
4. Determine the area of each of the colored zones and multiply them by the mean depth/thickness, and
5. Sum the negative volumes to obtain the approximate volume of erosion.
   1. Estimate the amount of fluvial erosion by fitting an angle-of-repose V-shaped valley into the width of the modern Roaring Fork valley.  What is that volume?
   2. Assume the remaining erosion to be glacial.  What is that volume?
6. Sum the positive volumes to obtain the volume of deposition.  Multiply by 0.75 to represent the difference between sediment (density ~ 2) and rock (density ~ 2.65).
   1. Identify the Pinedale (last glaciation ~ Wisconsin) moraines and deposits by their continuous sharp lateral crests and hummocky, locally undrained, terminal deposits.
   2. Estimate the volume of rock equivalent deposited during the Pinedale.
   3. Assume the remaining positive volume to be pre-Pinedale.  What is that (rock-equivalent) volume?

Questions

• If the Pinedale Glaciation included 80,000 of the last 120,000 years, what was the annual volumetric rate of glacial debris delivery (m³yr^-1)?
• If debris was removed from the entire eroded area at an equal vertical rate, what would that rate of erosion have been (mm/thousand years)?
• If glaciation in the Northern Hemisphere began about 2,000,000 years ago, what was the average annual rate of glacial erosion across this period (m³yr^-1)?
• If river erosion included 1/3 of that time, which is more effective as an erosive agent - the river or the glacier?
• How far did the glacier transport eroded material?
• How far did the river transport outwash?
• Which is more efficient as a transporting agent, the river or the glacier?

Discuss your results, stating your assumptions!