1. A review by Karen K. Shell ESS 433
Relative Dating Techniques Sierra Nevada, California R.M. Burke and Peter W. Birkeland
A review by
Karen K. Shell
ESS 433

2. Overview Defining Glaciations using RD Techniques
Field Case Studies
Questions
Environmental Factors
Weathering Criteria
Moraine Morphology

3. Environmental Factors
Setting of Deposits
Vegetation
Lithology

4. Environmental Factors Settings of Deposits
Valleys
Elevation of valleys
Constraining bedrock
Location with respect to mountain range

5. Environmental Factors Vegetation
Forests
Brushes
Smaller Trees
Grasses
Soil Development

6. Environmental Factors Lithology
Importance of minimizing variables
Limit to particular grain sizes and weathering resistant minerals
Granodiorite or Quartz monzonite sampled
Correlation between sample sites

7. Weathering Criteria Moraine Surface Boulders
Fresh-to-weathered ratio
Pitted-to-non pitted ratio
Pit Depth
Maximum height of resistant mafic inclusions
Rind-to-no rind ratio

8. Weathering Criteria Moraine Surface Boulders
Average rind thickness
Hammer-blow weathering ratio
Surface boulder frequency
Granitic boulder-to-non granitic boulder ratio
Split-to-non split ratio
Oxidation ratio

9. Weathering Criteria Moraine Surface Boulders (continued)
Fresh-to-weathered ratio
Requires 50% of boulder to exhibit weathering
Rough texture
Pitted-to-non pitted ratio
Has one or more concave depressions (pits)
Pits are not required to be circular in shape
Counts are made separate from boulder counts
Caused by break down of grains

10. Weathering Criteria Moraine Surface Boulders (continued)
Pit depth
Depth is measured from lowest point of pit to current boulder surface
Maximum height of resistant mafic inclusions
Measured from top of inclusion to average position of the adjacent rock surface
Rind-to-no-rind ratio
Discolored parallel to outer surface

11. Weathering Criteria Moraine Surface Boulders (continued)
Average rind thickness
Measured after rind-to-no rind ratio
Measured to nearest millimeter
Hammer-blow weathering ratio
Based on sound heard when striking boulder with hammer. Determines whether boulder is fresh, weathered or grusified
Surface boulder frequency
Measurement of boulder frequency along crest of moraine within an area abundant with boulders

12. Weathering Criteria Moraine Surface Features (continued)
Granitic boulder-to-non granitic boulder ratio
50 samples taken on boulders greater than 50 cm in diameter
Split-to-non split ration
Breaks appear along planar cracks
Not the result of spalling
Oxidation Ratio
Relative visual discoloration within and between 50 samples measured

13. Weathering Criteria Subsurface Features
Grusified granitic boulders beneath surface
Soil properties

14. Weathering Criteria Subsurface Features (continued)
Grusified granitic boulders
Diameters of 30 cm or greater are measured
Grusification is the intense disintegration of grains throughout the clast
Classification
Fresh
Grusified
Grusified and un-oxidized
Grusified and oxidized

15. Weathering Criteria Subsurface Features (continued)
Soil Properties
Measured in field and analyzed in laboratory
Extracted from pits dug along moraine crests.
Field Properties
Horizonation
Color
Texture
Consistence
Structure pH
Other diagnostic features (clays, carbonates, etc.)
A soil horizon is a specific layer in the soil which parallels the land surface and possesses physical characteristics which differ from the layers above and beneath.
B Horizons are commonly referred to as ‘subsoil’, and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium, or organic material. In addition, they are defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colours (ie higher chroma) than the A horizon

16. Weathering Criteria Subsurface Features (continued)
Soil Properties (continued)
Laboratory Properties
Particle size distribution
Percent loss during ignition
Organic matter pH
Dry color

17. Weathering Criteria Moraine Morphology
Width of crest of moraine
Angle of slopes of moraine
Inner and outer slopes
Possible Downfalls
Precision and accuracy require a single person to take measurements

18. Defining a Glaciation Using RD Techniques
How much variation occurs between deposits?
How much variation occurs between a stade?
Splitters vs. Lumpers
Burke and Birkeland’s methodology
A stade is a period of quiescence between glacial cycles that usually lasts less than 10, 000 years.

19. Defining a Glaciation Using RD Techniques (continued)
Splitters
Researchers that make the subdivision at the maximum interval of the sequence, namely at the stade level.
Subtle variations are of greater importance
Good for determining moraine positions and cross-cutting moraine relationships
Lumpers
Researchers that are more inclined to make distinctions between glaciations at larger scale
Consistent recognition of weather features between research locations (valley to valley)

20. Defining a Glaciation Using RD Techniques (continued)
Burke and Birkeland’s Methodology
Methodology largely influenced by the focus of the research on moraines that are in close proximity (adjacent, local)
Determined that no single value could be set for determining glaciations in all locales

21. Defining a Glaciation Using RD Techniques (continued)
Burke and Birkeland’s Methodology
Burke and Birkeland settled for a magnitude of 2 times or greater in variation to separate glaciations
Allows for distinctions to be made between first-order glaciations rather than secondary fluctuations of the first-order glaciations

22. Field Case Studies
Bloody Canyon

23. Field Case Studies Bloody Canyon
Previous Studies
Some studies divided morphological features into four glaciations: Mono Basin, Tahoe, Tioga and Tenaya. (McGee, 1885; Russell, 1887; others)
Other proposed two specific glaciations prior to Burke and Birkeland: Tioga and Tahoe (Putnam, 1949; Kistler, 1966)

24. Field Case Studies Bloody Canyon (continued)
Burke and Birkeland’s Assessment
Tioga
Lie above Tahoe deposits and through RD Techniques is similar in morphology, lithology, etc to the Tenaya deposits.
Determined that Tioga and Tenaya are part of the same glacial event.
Younger than Tahoe deposits as determined by degree of weathering and superposition

25. Field Case Studies Bloody Canyon (continued)
Burke and Birkeland’s Assessment
Tioga
Evidence?
Presence of ash deposits above Till
Absence of clay build-up except near the top of the profile
Granitic boulders are “fresh” throughout the soils

26. Field Case Studies Bloody Canyon (continued)
Burke and Birkeland’s Assessment
Tahoe
Due to extent of moraines, several sites were studied
Vegetation at each of the study sites is crucial to correlation of RD data
Ideal scenario would be the absence of vegetation or if not available, two sites of similar vegetation

27. Field Case Studies Bloody Canyon (continued)
Burke and Birkeland’s Assessment
Tahoe
Evidence?
Little difference in subsurface age between Mono Basin and Tahoe deposits
Minimal soil development
Both have clay development in the B-horizon of the soils. Tahoe deposits exhibits clay in horizons above and below the B-horizon
Clay development is the result of eolian influx and in situ weathering

28. Field Case Studies Bloody Canyon (continued)
Burke and Birkeland’s Assessment
Tahoe
Evidence? (continued)
Surface boulders are limited and when found a moderately to completely grusified.
Since differentiation between the sub-surfaces cannot be distinguished from one another, the Mono Basin and Tahoe deposits cannot be proven to be the result of separate glaciations.
7. Only distinction is the moraine morphology (Mono Basin exhibits less relief than Tahoe Moraine).

29. Discussion Questions
What are the positive/negative aspects of the RD Techniques?
Does the RD technique have differing results when applied to different types of glaciers (i.e. warm-based vs. cold-based, etc)?
Is the elimination of the Mono Basin and Tenaya glaciations valid?
Is the justification of order of magnitude for determining/separating glaciations a valid way to go about distinguishing glaciations?