Bighill Exposure EarthCache EarthCache
Bighill Exposure EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
THE BIGHILL EXPOSURE - AN IMPROVEMENT PROJECT FOR US
421
An artifact that remains long after the US 421 improvement project
was completed is the "Bighill Exposure." On display is a
cross-section of the earth's geology representing vast spans of
time.
gary_f_jackson at the Bighill Exposure
A project undertaken by the Kentucky Transportation Cabinet (KYTC)
has improved a section of US 421 near Bighill in Madison and
Rockcastle Counties. This project is a part of the Kentucky
Transportation Cabinet’s (KYTC) FY 2003-2008 Six Year Highway
Plan. The previously existing US 421 corridor was a 2-lane
roadway through rolling terrain with 9-foot travel lanes and 2-foot
shoulders. The posted speed limit ranged from 35 mph to 55
mph. Traffic along US 421 is forecast to increase over 60%
from 2003 to 2025. Traffic volume during this period is
predicted to rise from 6,500 to 8,500 vehicles per day. While
taking into consideration a number of environmental factors and
sensitive land use issues, the completed project has provided
improved regional access along an improved US 421 route that will:
- Support economic development initiatives in Jackson County by
opening up this low-income area,
- Improve access to jobs, hospitals, services, shopping, and
schools in Berea, Richmond, and Lexington, and
- Improve the highway to help alleviate public concerns about
safety and travel time along the existing US 421 corridor by
providing improved roadway geometrics.
A truly massive engineering feat was the restructuring of US 421 at
the project's north terminus in Rockcastle County. An entire
escarpment was sculpted using heavy, earth-moving machinery,
explosives and the latest civil engineering technology. A
mountain was lowered and a valley raised so vehicle traffic and
commerce can flow unhindered. Ultimately, nearby Jackson
County will enjoy economic gains from easier, safer access to
Richmond, Lexington and the Interstate 75 corridor.
Southeastern cut at Bighill, showing transitional parts of the
Nada Member, Borden Formation, and clear views of the Renfro
through Mill Knob Members of the Slade Formation. The verdine
facies in the Nada, as well as the Science Hill Sandstone
equivalent and Ringgold Bed(?) in the Renfro, are also shown.
Darkened areas at the top of the St. Louis and Ste. Genevieve
Members reflect melanization accompanying Mississippian pedogenesis
and subaerial exposure.
The layers visible are the following:
NADA MEMBER
The Nada is the upper member of the Borden Formation, and at least
37 feet are present at the base of the middle exposure. In
contrast to the greenish gray, dark gray, or grayish red silty
mudstones and shales that comprise underlying parts of the unit,
the upper part of the Nada is largely composed of pale blue-green,
silty mudstones, siltstones, and shales, in what some have called a
verdine facies. Other parts of the unit are rich in
glauconite, comprising a glaucony facies. Phosphatized
gastropods, brachiopods, and fish bones, as well as bioturbation,
are present in the Nada Member.
RENFRO MEMBER
The Renfro Member is unusually thick and well exposed in this cut,
consisting of 56 feet of grayish orange to pale orange dolostone
with a few, thin, interbedded limestones and shales. The
contact with the underlying Nada is sharp and represents another
glaucony- and phosphorite-rich lag horizon, but elsewhere in
east-central Kentucky the contact may be gradational, with
intertonguing between the two members. The Renfro in this
exposure exhibits many complexities and, for ease of description,
is subdivided into three parts by the two prominent limestone
intervals.
ST. LOUIS MEMBER
The St. Louis Member sits unconformably on top of the Renfro at the
first major bench in the exposure. The member is up to 12
feet thick and consists of three widespread subunits. The
lowermost unit, A, is composed of thin- to medium-bedded, light
gray, skeletal calcarenites containing clasts of reworked Renfro
dolostone and sparse, colonial rugose corals. As is typical
of the St. Louis in other places, the unit contains large
elliptical nodules and ball-and-pillow structures of medium gray to
pale green dololutite. Unit B consists of thin-bedded, light
gray, fossiliferous calcarenites and calcilutites with interbedded
greenish gray shales; chert nodules occur locally; brachiopods,
bryozoans, and crinoid debris are common. Unit C, at the top,
is composed of massive, medium to thick-bedded, fine-grained
calcarenite to calcilutites; irregular chert nodules, sometimes
containing colonial rugose corals, are common. The dark gray
or brownish gray color is a product of melanization, and along with
rare subaerial exposure crusts and erosional truncation, is a
product of subaerial exposure and pedogenesis on an unconformity
below the Ste. Genevieve Member. At the north end of the
exposure, unit C is up to 5 feet thick, but toward the south it is
truncated below the Ste. Genevieve to less than 2 feet.
Colonial rugose corals in the St. Louis have been identified as
Acrocyathus floriformis and A. proliferus, which are common St.
Louis guide fossils.
Bighill Exposure schematic
STE. GENEVIEVE MEMBER
The Ste. Genevieve Member unconformably overlies the St. Louis and
begins with a transgressive lag of reworked, dark, upper St. Louis
clasts. The unit is 28 to 36 feet thick and thins to the
northwest because of erosion from the overlying Warix Run
Member. The unit can be separated into two parts by an
erosional hiatus with a dark paleosol horizon. The lower part
is an intertonguing and internally truncated facies complex of
thin-bedded, light greenish gray calcilutites; massive, dark
brownish gray, clotted, birdseye calcilutites; and skeletal-oolitic
calcarenites. The calcarenites contain, high-angle,
planar-tabular crossbeds, some of which show a herringbone
pattern. The complex is 18 feet thick, and facies are
uniformly truncated along a melanized, paleosol horizon up to 3
feet thick with breccias and thin, wispy, subaerial exposure
crusts. The upper part of the member begins with breccias
eroded from the lower unit and consists of 10 to 18 feet of
crossbedded, skeletal-oolitic calcarenite and interbedded dark
calcilutites. Platycrinites penicillus, a Ste. Genevieve
guide fossil, has also been found in the upper unit. The top
of the unit is unconformable with the overlying Warix Run Member,
and contains remnant soil teepees, root tubules, breccias, faint
crusts, and melanization, all indications of a major
paleosol. In northern parts of the cut, the Ste. Genevieve
shows several relatively recent solution pits, some of which are
lined with flowstone.
WARIX RUN MEMBER
This member is very similar to the Ste. Genevieve from which it was
formally separated in 1984. The main difference is the
presence of quartz sand and peloids. The Warix Run also
typically occupies deep erosional lows cut into underlying
units. At this exposure, the Warix Run is separated from the
Ste. Genevieve by a green sandy breccia, up to 1 foot thick,
containing reworked clasts of the underlying Ste. Genevieve
paleosol; most of the quartz sand is concentrated near the base of
the unit. Overall, the unit is a dark brownish gray, peloidal
calcarenite with large, planar-tabular crossbeds. The unit
occupies a channel cut into the Ste. Genevieve here and thickens to
the north as the channel deepens in that direction.
Accordingly, thickness varies from 3 to 13.5 feet because of the
erosional contact with the Ste. Genevieve. The dark color of
the unit is the product of melanization, which together with
sparse, wispy exposure crusts and root tubules indicates periodic
exposure. Contact with the overlying Mill Knob member is
gradational. Fossils of any kind are extremely rare.
MILL KNOB MEMBER
The Mill Knob Member is composed of about 45 feet of cyclically
alternating shaly calcilutites, oolitic/skeletal calcarenites, and
birdseye calcilutites or dolostones, locally capped with
paleosols. Bedded or nodular cherts are common.
Although every cycle is slightly different, most of the cycles are
shoaling-upward, and five such cycles are present in this exposure,
two of which are capped with paleosols. The upper 5 to 6 feet
of the member are unusual in the presence of a coarsening-upward
cycle containing much shale. This cycle begins with a basal
zone that reflects reworking of the underlying paleosol and grades
upward into greenish gray mudstones and shales interbedded with
thin calcilutites, which are laminated and contain possible rare
mudcracks. The top of this cycle is erosionally truncated and
capped with a major paleosol, exhibiting melanization, exposure
crusts, breccias, and soil teepees. Complete fossils are
uncommon throughout the unit.
DIRECTIONS
From the Kentucky Artisan Center at Berea (exit 77 off of I-75),
proceed east on KY 595. Proceed through Berea, and then turn
left (east) onto KY 21 until it intersects US 421 in Bighill.
Turn right (south) onto US 421 and proceed for 2 miles to the
beginning of the exposure. There is room to pull off at the
side of the road. Please exercise care with young children,
as traffic coming down the mountain tends to move fast.
DO NOT LOG AS A FIND UNTIL YOU HAVE A PICTURE READY TO POST.
To get credit for this EC, post a photo of you (I do not accept
pictures of just a hand) at the posted coordinates with the Bighill
Exposure in the background and please answer the following
questions.
- How tall is the exposure? An estimate is fine.
- What is the elevation at the posted coordinates?
- How many different layers do you see?
Logs with no photo of the actual EarthCacher/Geocacher (face must
be included) logging the find or failure to answer questions will
result in a log deletion. Exceptions will be considered if
you contact me first (I realize sometimes we forget our cameras or
the batteries die). Logs with no photos will be deleted
without notice. I have used sources available to me by using
google search to get information for this earth cache. I am
by no means a geologist. I use books, internet, and asking
questions about geology just like 99.9 percent of the geocachers
who create these great Earth Caches.
Reference:
"The Bighill Exposure and a Little Beyond," a
joint field trip of the Kentucky Society of Professional Geologists
and the Kentucky Section of the American Institute of Professional
Geologists. October 21 - 23, 2004. Compiled by Richard
A. Smath.
Congratulations to
for the
FTF!
Additional Hints
(No hints available.)