Name ______________________________
Midterm #1
Geology 103A
Spring, 2009
Short answer questions. Answer each question. If the question asks for a sketch or diagram, be
sure to include it. (5 points each).
1) What is the difference between a primary sedimentary structure and a secondary sedimentary
structure? Name two of each.
2) What is the difference between a linguiod ripple and a cuspate ripple? Draw a simple birdseye sketch of each.
3) What terms are used in the Wentworth scale to describe sand-sized materials? List them all!
4) What is the difference between a quartz arenite in Folk’s sandstone classification system, and
a quartz arenite in Gilbert’s classification system?
5) What is the diameter (in mm) of a grain that has a phi size of –2.0? Show your work.
6) What are the four main families of clay minerals? Why is illite more common in older rocks
than it is today?
7) Describe the difference between an oscillation ripple and a current ripple. What current
conditions are responsible for the formation of each type of ripple, and what depositional
environments might produce these conditions?
8) Rank the following grains in order of increasing stability: chert, quartz, pumice, orthoclase,
plutonic rock fragments. Give a VERY brief explanation for your rankings.
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9) What is the difference between cement and matrix in a sandstone? Name at least four
common mineral cements. What temperature/pressure realm do these cements form in?
10) Give an equation for the maximum projection sphericity of a particle, and define the
variables.
11) How does the composition of a typical sandstone differ from the composition of a typical
conglomerate?
12) Draw a cross-section of a ripple. Label the lee side, stoss side, bottom set laminae, foreset
laminae and wave height.
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13) How would you distinguish between quartz, orthoclase, plagioclase and microcline in a
coarse sandstone sample? Use mineralogic properties that you could determine with a hand lens,
dissecting microscope, or simple field tools in your answer. What other methods could you use
to help with your diagnosis? (10 points)
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14) How do flute casts form? Describe or draw the sequence of events that must occur for a flute
cast to be preserved, then exposed to view at a later time. (10 points)
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15) A sandstone is poorly to moderately sorted, with 65% fine to medium sand grains, 20%
matrix (hard to tell!), and an estimated 15% chlorite cement. The grains consist of gray feldspar,
volcanic rock fragments, biotite mica, and a dark accessory mineral that may be hornblende.
60% of the grains are a gray feldspar that is subangular to subrounded, with size range from 0.3
to 1.2 mm. 20% of the grains are biotite mica that forms abraded, subhedral booklets from 0.6 to
1.5 mm diameter. 10% of the grains are subrounded, black, volcanic rock fragments that may be
basalt, and range from 0.8 to 1.5 mm. 10% of the grains are a dark black, lustrous silicate?
mineral that ranges from 0.3 - 0.8 mm, and has two well-defined cleavage planes at angles
greater and less than 90°. This accessory mineral may be hornblende. The matrix consists of
silt and clay-sized feldspar and clay particles.
Describe the textural and compositional maturity of this sandstone: (5 points)
Name this sandstone using Gilbert's classification system. Draw a picture of the classification
scheme, and put an "X" on the diagram to show where this sandstone would plot. (15 points)
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