GEOLOGY 445 - GLACIAL GEOLOGY

LAB 4 - Glaciers across Time

Modern glacier extents are but shrunken remnants of those which shaped mountain landscapes during past glacial climates.  Reconstruction of former glaciers and glacial climates is therefore an ongoing challenge in Quaternary and glacial geology.

Reconstruct the Pleistocene glacier that occupied the Roaring Fork valley (topographic map) in Rocky Mountain National Park, using the GLACPRO spreadsheet model.  

1. On the photocopied map, outline the "Pinedale" moraine crests of the Longs Peak glacier. A color version is attached. (HINT: Look for "Mills Moraine", undrained depressions and ice-marginal channels. Also - consider the Geologic Map of Rocky Mountain National Park; 1990, USGS MI-1973, by W. A. Braddock and J. C. Cole, with which I do not necessarily agree, in detail.)   You might also want to view aerial photographs - see Terraserver: http://terraserver.microsoft.com/image.aspx?T=1&S=13&Z=13&X=281&Y=2785&W=1 or Goggle Maps: <http://maps.google.com/?ie=UTF8&ll=40.258864,-105.577154&spn=0.048536,0.109091&t=p&z=14>

2. Input bed elevations at reasonable (short!) intervals up the glacier.

3. Input the lateral moraine elevations as far up-valley as possible.

4. Tweak the basal shear stress/strength to fit the modeled ice surface to the moraine elevations.  (View the topographic map of the McCall Glacier if you don't think this is reasonable.)

5. Estimate the ELA for the maximum Pinedale-equivalent Roaring Fork glacier.  Use several methods (AAR, THAR, highest lateral moraine, cirque floor elevation).  Discuss in question 8.

6. Adjust the mass balance curve of Pierce (1979) to your estimated ELA. (HINT: Change the "Altitude" scale by the difference between Pierce's ELA - for the northern Yellowstone outlet glacier - and yours.)

7. Calculate the "Best estimate" net mass balance for the Longs Peak glacier under those conditions.

   • Estimate the area of the glacier surface (from the topo map) for increments of 500 vertical feet.  See my Geomorphology Manual for topographic map interpretation - Area -  for techniques of area estimate.

   • Multiply each area by the "Best estimate" specific mass balance. 

   • Accumulate these values to yield a net mass balance. 

8. Discuss your results and assumptions!
   • Did the glacier match the assumed 1 bar (100 kPa) shear strength?  Why (not)?
   • Did your ELA estimates agree with each other?  Why (not)?  What do you think is the best estimate, and why?
   • Does Pierce's mass balance curve work for the Roaring Fork glacier?  Why (not)?
   • If not, by how much would have to change it, and in what fashion, and why?