Geology Reference
In-Depth Information
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Figure 2
This photograph of the Munising Formation shows the
shoreline projection called Miner's Castle. The two turrets at the top
of Miner's Castle formed by wave action when the water level of Lake
Superior was higher. The contact between the Miner's Castle and
Chapel Rock sandstone members is located just above lake level.
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Figure 3
This photograph shows the same area as Figure 2,
but it was taken shortly after the right-hand turret of Miner's Castle
collapsed into Lake Superior.
On April 13, 2006, one of the two
turrets of Miner's Castle collapsed into
Lake Superior (
sudden. Although it is disappointing
to lose such a prominent landmark, no
one should be surprised that it hap-
pened. The forces of nature helped
shape the structure, and the same
forces caused its collapse. It is only a
matter of time until the remaining tur-
ret meets the same fate as its twin.
Figure 3). Witnesses
described the collapse as loud and
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a particular environment) migrate in a landward direction
(see Figure 6.22).
This time-transgressive facies relationship occurred
not only in the Grand Canyon area (see Geo-inSight
on pages 534 and 535) of the craton during the Sauk
Sequence, but elsewhere on the craton as well, as the seas
encroached from the Appalachian and Ouachita mobile
belts onto the craton interior (
of the craton was submerged beneath a warm, equatorial
epeiric sea (Figure 20.2a).
As the Sauk Sea regressed from the craton during the Early
Ordovician, a landscape of low relief emerged. The rocks
exposed were predominantly limestones and dolostones
deposited earlier as part of the Sauk transgression. Because
North America was still located in a tropical environment
when the seas regressed, these carbonates experienced
extensive erosion at that time ((
Figure 20.7). Carbonate
deposition dominated on the craton as the Sauk trans-
gression continued during the Early Ordovician, and the
islands of the Transcontinental Arch were soon covered by
the advancing Sauk Sea. By the end of Sauk time, much
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Figure 20.8). The resulting
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