ICL

Sand dunes to Sandstone: Revealing the Secrets of Zion’s Navajo Sandstone

Organization

• Key terms: concepts you’ll need to know• Activity 1: Interpreting active aeolian dunes• Activity 2: Difficult Dunes and GPR• Activity 3: Uniformitarianism (present is the

key to the past): Zion’s Navajo Sandstone• Activity 4: Comparing field & GPR

interpretations to USGS-developed models

Key Terms• Slipface: a steeply sloping surface on the lee side of the dunes standing

at or near the angle of repose for loose sand• Angle of Repose: the maximum angle of a slope of a certain material

before it loses stability (~340)• Ground Penetrating Radar (GPR): a non-invasive geophysical

tool/method used to gather detailed information about the subsurface• Saltations: Sediment transport where particles are moved forward in a

series of short leaps or bounces• Foreset: Inclined layers of a cross-bedded unit

• Bedding: An arrangement of sedimentary rock in layers; the general character or pattern of the beds

• Paleoflow: a flow direction of sediment preserved in bedrock and determined by examination of sedimentary structures and textures in the rock formed at the time

Paleoflow & Slipface

Activity 1

• Background: using theory to interpret observations, or vice-versa– In geomorph, link knowledge of process with

observations of landforms• Instructions: Use your knowledge of aeolian

processes to interpret different types of aeolian dune landforms• Remember this?...

Activity 2

• Task 1: Analysis of the topographic map to located similar landforms

• Task 2: Interpreting the GPR image• Task 3: How did these possible dunes form

and why are they not still moving?

Activity 3

• Task 1: Virtual Fieldwork: interpreting photographs

• Task 2: Using GPR to investigate the field site

Activity 4

• Task 1: Your field site description

• Task 2: Which model works best?– Models from USGS website:• http://walrus.wr.usgs.gov/seds/bedforms/pindex.html

Activity 1 Questions

• Which side of the photograph is the lee facing (left, up, right, down)?

• What is the sand’s source—from which side of the photograph is the dominant wind direction?

• What types of dune are these?• What observations led you to this conclusion?

Activity 2

• Difficult Dunes and GPR• Interpret GPR imagery by making measurements

of slope angle• Analyze possible processes involved in possible

dune formation & stabilization

Activity 2, Task 1

• Analysis of the topo map to locate similar landforms– Do you see any other locations of possible sand

dunes in the vicinity of the red oval on the Eau Claire topo map? How many?

Activity 2, Task 2

• Interpreting the GPR image– Geo-nerds love anomalies!– Once anomalies are determined, hypotheses can

be created; Small hills on topo map could be…• Drumlins? Kames? Glacial till? Erosional stream

remnants?

– If these “hills” are aeolian dunes, then angle of repose would be reflected in the slip face

Activity 2, Task 2 (cont.)

• Measure the angle of repose on the GPR image– Protractor (easiest)

OR“rise over run”• Rise = vertical change of slip face; run = horizontal

position– Each tiny “tick” ~0.1m on the horizontal scale and ~0.08m on

the vertical scale– Online here: www.1728.com/distance

Activity 2, Task 3

• Determining the internal structure of the possible Roy Street San Dunes– Possible sand source– Possible saltation from source to location– How sand was stabilized

• Must know wind direction first

Activity 3• Uniformatarianism at Zion National Park• Activity 3, Task 1– Examine ancient sand dunes of Navajo formation– Annotate an image

• Activity 3, Task 2– Use GPR imagery to peer into subsurface• Annotate image• Calculate angle of repose• Answer questions associated with dune “packages”

Activity 4

• Comparing your “field” and GPR analysis with USGS-developed models

• Activity 4, Task 1– Field site description

• Activity 4, Task 2– Choosing a model that fits your analysis