LBL Conglomerate EarthCache
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Conglomerate is a
sedimentary rock composed of rounded pebbles, cobbles, and boulders
surrounded by a matrix of sand and finer material, and cemented
with silica, iron oxide, or calcium carbonate. The EC brings you to
one of the few examples of conglomerate geology found at Land
Between the Lakes. Park in a turn-off just a short-walk south of
the earthcache.
CONGLOMERATE
GEOLOGY
Conglomerates are sedimentary rocks consisting of rounded fragments
and are thus differentiated from breccias, which consist of angular
clasts.
Classification:
Texture:
Conglomerates are clasified by variation of included rocks and
their size.
Clast composition: Conglomerates are classified for the
lithologies of the clasts
• Monomict- clasts with only a
single lithology
• Oligomict - clasts of only a
few different lithologies
• Polymict - clasts of many
different lithologies
• Intraformational - clasts
derived from the same formation in which they are found
• Extraformational - clasts
derived older rocks than the formation in which they are
found
Clast size: Conglomerates are also
classified by the dominant clast size.
• Granule conglomerate 2–4 mm
• Pebble conglomerate 4–64 mm
• Cobble conglomerate 64–256 mm
• Boulder conglomerate >256 mm
Rounding and Sorting:
In addition to the factors described in this section, conglomerates
are classified in terms of both their rounding and sorting.
Sedimentary
environments: Conglomerates are deposited in a
variety of sedimentary environments.
• Deep Marine - In this setting,
the matric bed is typically coarse-grained sand and the
conglomerates are normally very well sorted and well-rounded large
boulders.
• Shallow Marine - Coarse sand,
gravel, and cobbles are deposited here. Smaller material stays in
suspension/solution and moves offshore
• Glacial - Glaciers carry a lot
of coarse-grained material and the sediments deposited directly by
a glacier, are typically poorly-sorted, matrix-supported
conglomerates. The matrix is generally fine-grained, consisting of
finely milled rock fragments.
• Fluvial - Conglomerates
deposited in fluvial environments are typically well-rounded and
well-sorted. The matrix is composed of medium to finegrained
sediments with medium sized cobbles to boulders.
• Alluvial - Deposits are formed
in areas of high relief and are typically coarse-grained and are
associated with the thickest deposits of conglomerates.
LBL
GEOLOGY
LBL is an interior peninsula within, but at the western edge of the
Western Highland Rim of Tennessee or the Mississippian
(Pennyroyal) Plateau of Kentucky. The bedrock is cherty
limestone of the Mississippian Period covered with fossils of an
ancient sea. Surface exposures or rock are uncommon except along
the major waterways, such as riverbanks and creekbeds. The hilly
borderland between the Mississippian Plateau and the Jackson
Purchase has been known as "the breaks." Much of this region has
been flooded by damming of the Kentucky and Cumberland Rivers,
forming the area known as the Land Between the Lakes. The Jackson
Purchase (Davis, 1923) is a region of flat, low plains dissected by
a dendritic network of low-gradient creeks and small rivers,
situated at the head of the Mississippi Embayment. The
‘breaks’ and the western part of LBL was the shoreline
of the great interior seaway that bisected North America during a
period of geologic time from 145 to 60 million years ago.
At this Earthcache location, you are standing along the Tennessee
Ridge which parallel with the two rivers and bisects LBL. The
Tennessee Ridge is generally dominated by coastal plain gravelly of
Tertiary and Cretaceous (KT) age. However, at this location, the
ridge is overlain by gravel of the Quaternary and Tertiary (QTg)
age.
IN
ORDER TO LOG THIS FIND YOU MUST:
A. Click on my profile and e-mail the answers for the following
questions to me. Do not post your answers when you log in your
find. Logs which do not meet the requirements to claim the find
will be deleted.
1.Estimate the height of the conglomerate outcropping from the base
sandstone layer to its highest point. Answer must be within 1/2
foot of the actual height.
2.Using your keen skills of observation, determine the clast size
and composition of the conglomerate, and sedimentary environment in
which the outcropping formed.
3.Describe a unique feature of the outcropping that you find.
B. Post a picture as instructed below
1. Just west of the top of the exposed surface of the main ledge,
take and log a picture of your GPS and your finger pointing are one
of the smaller exposed surfaces.
2. (Optional, though greatly appreciated) Take and log a picture of
you(and your group) standing atop the exposed ledge.
If you would like to visit another example of a conglomerate rock,
visit
Meteorite Rock by Cadiz Big Dogs (GC1BJR5) in the Tennessee
portion of LBL.
Additional Hints
(No hints available.)
Treasures
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