22. Where are the youngest rocks exposed in this area? What is their age (geologic period)? 23. Where are the oldest rocks exposed in this area? What is their age (geologic period)? 24. Study the pattern made by the rock units in Michigans Lower Peninsula (between Lake Michigan and Lake Huron). What major structural feature is shown? Provide reason(s) to support your answer. Hint: note the spatial outcrop pattern and the age relationship between different units (as shown by the map legend). Remember from the lecture that domes and basins are circular features, and we tell the two apart by the age relationship of the rock units. In a dome, the oldest rocks are in themiddle, with younger rocks around the edges. In a basin, the youngest rocks are in the middle, while the oldest rocks are around the edges. This is where the legend that accompanies the map comesin. The different rock units, shown in different colors, are arranged in order in the legend from youngest at the top to oldest at the bottom. So the legend allows you to determine the relative agesof the rock units on the map (which are older, which are younger) and hence whether this circular feature is a dome or a basin. 25. Note the outcrop pattern of Silurian and Devonian rocks in northwest Indiana. What major structural feature is present in this area? How can you tell? Hint: Notice that in this area the rock units are repeated like stripes. This indicates the presence of a fold. You would then have to look at the relative ages of the rocks, using the legend, to tell whether the fold is an anticline or a syncline. Synclines have the youngest rocks in the middle of the fold; anticlines have the oldest rocks in the middle of the fold. 26. What is the oldest possible age of the folding event in NW Indiana? How can you tell? Hint: remember the principle of original horizontality: the rock units were horizontal when they were deposited, and only folded some time after they were deposited. Hence, the fold can be no older than the youngest rock unit involved in the fold. 27. What is the linear feature shown on the map between Lansing and Detroit? Hint: note that this feature is shown with a thicker line than the contacts between units shown everywhere else on the map. If you dont know the significance of a contact drawn with a thicker line, then you need to re-read the preceding introduction to geologic maps. Geologic maps show the distribution of rock units at the surface as well as structural features, such as faults and folds. A geologic map is usually printed on top of a regular topographic map (the base map) to help you locate yourself on the map. The base map is printed with light colors, so it doesn't interfere with seeing the geologic features on the map. The geology is represented by colors, lines, and symbols unique to geologic maps. Each color used on a geologic map represents a different geologic unit. A unit is a specific type of rock of a specific age range (for example, a Perm.

1. 22. Where are the youngest rocks exposed in this area? What is their age (geologic period)?
23. Where are the oldest rocks exposed in this area? What is their age (geologic period)?
24. Study the pattern made by the rock units in Michigans Lower Peninsula (between Lake
Michigan and Lake Huron). What major structural feature is shown? Provide reason(s) to support
your answer.
Hint: note the spatial outcrop pattern and the age relationship between different units (as shown by
the map legend). Remember from the lecture that domes and basins are circular features, and we
tell the two apart by the age relationship of the rock units. In a dome, the oldest rocks are in
themiddle, with younger rocks around the edges. In a basin, the youngest rocks are in the middle,
while the oldest rocks are around the edges. This is where the legend that accompanies the map
comesin. The different rock units, shown in different colors, are arranged in order in the legend
from youngest at the top to oldest at the bottom. So the legend allows you to determine the
relative agesof the rock units on the map (which are older, which are younger) and hence whether
this circular feature is a dome or a basin.
25. Note the outcrop pattern of Silurian and Devonian rocks in northwest Indiana. What major
structural feature is present in this area? How can you tell?
Hint: Notice that in this area the rock units are repeated like stripes. This indicates the presence of
a fold. You would then have to look at the relative ages of the rocks, using the legend, to tell
whether the fold is an anticline or a syncline. Synclines have the youngest rocks in the middle of
the fold; anticlines have the oldest rocks in the middle of the fold.
26. What is the oldest possible age of the folding event in NW Indiana? How can you tell?
Hint: remember the principle of original horizontality: the rock units were horizontal when they
were deposited, and only folded some time after they were deposited. Hence, the fold can be no
older than the youngest rock unit involved in the fold.
27. What is the linear feature shown on the map between Lansing and Detroit?
Hint: note that this feature is shown with a thicker line than the contacts between units shown
everywhere else on the map. If you dont know the significance of a contact drawn with a thicker
line, then you need to re-read the preceding introduction to geologic maps.
Geologic maps show the distribution of rock units at the surface as well as structural features,
such as faults and folds. A geologic map is usually printed on top of a regular topographic map
(the base map) to help you locate yourself on the map. The base map is printed with light colors,
so it doesn't interfere with seeing the geologic features on the map. The geology is represented by
colors, lines, and symbols unique to geologic maps. Each color used on a geologic map
represents a different geologic unit. A unit is a specific type of rock of a specific age range (for
example, a Permian limestone, a late Triassic sandstone, etc). All exposures of rocks of the same
age and type are shown with the same color and are separated from other rocks by a line
representing the contact between them. The two main types of contacts shown on most geologic
maps are depositional contacts and faults. Where the original depositional contact between
geologic units is preserved, it is shown on the geologic map as a thin line. If the rocks are in
contact because of a fault, a thicker line is used to represent the fault, and the relative motion
along the fault, if known, is indicated by labeling the upthrown side and downthrown side of the
fault. For low angle (thrust) faults the Thrust teeth are on the upthrown (hanging wall) block. Each

2. geologic unit is assigned a set of letters to symbolize it on the map. Usually the symbol is the
combination of a capital letter followed by one or more small letters. The capital letter represents
the age (geologic period) of the unit. Examples include Q (Quaternary), K (Cretaceous), and C
(Cambrian). The small letters indicate either the name of the unit, if it has one, or the type of rock,
if the unit has no name. Many geologic units are named based on where their characteristics are
bestdisplayed, or where they were first studied. For example, Jm is the map symbol for the
Morrison formation, a stratigraphic unit of Jurassic age first studied near Morrison, Colorado but
exposed throughout Colorado, Wyoming, and Utah (the Morrison formation is the source of many
dinosaur fossils). Js would be unnamed shale (s for shale) that is Jurassic in age. Sedimentary
rocks form in broad, flat layers (beds) that may later be tilted or bent by tectonic forces. The spatial
orientation of tilted rock layers is described in terms of strike and dip. Strike is the compass
direction of a line formed by the intersection of a horizontal plane (the surface of the Earth) and an
inclined surface (the titled rock layer). Dip is the angle between a horizontal plane and the inclined
surface and is always measured perpendicular to strike. On geologic maps "T" shaped-symbols
are used show the strike and dip of each rock unit. The top of the " T " is the line of strike and the
short arm of the "T" illustrates the dip direction in degrees. The higher the number, the steeper the
bed with 90 being perpendicular to the surface (vertical). In the example here, the rock layer
strikes in a southwest-northeast direction. The layer dips relatively steeply (38) to the southeast.
Geologic units can also be bent and warped into wavelike shapes called folds. If the layers dip
toward each other and the youngest rocks are in the center, the fold is a syncline. If the layers dip
away from each other and the oldest rocks are in the center, it is an anticline. Uplift and
subsidence in the crust produces broadly circular domes and basins. A dome is formed by uplift
and subsequent surface erosion which exposes older rocks in the center of the dome, while
younger rocks outcrop at the edges. Basins form by subsidence, allowing younger sediments to
accumulate in the middle while older rocks are exposed at the edges. On a geologic map, a line
that follows the crest or trough of the fold is called the fold axis and indicates the type of fold.
Divergent arrows indicate an anticline, while convergent arrows indicate a syncline. The
orientation of a fold axis is described in terms of trend and plunge, which is analogous to strike
(compass direction) and dip (angle of inclination from horizontal). However, unlike strike and dip,
which are perpendicular to each other, trend and plunge are in the same direction. Trend is the
compass direction, and plunge is the angle with the horizontal. The line for the fold axis represents
the trend of the fold, with an arrow at one end of the axis indicating the direction of plunge.