BIOL 4480
History of Life
FIELD TRIP 1
1. Leave Conway and drive north on US 65. Observe all speed limit signs. NOTE: For
clarity, the above map does not show all of the towns and all of the state and county roads
in the area.
2. About 70 miles north of Conway, roughly 1.5 hours’ drive, you will approach the town
of Marshall. Just before you get to Marshall, you will drive past large roadcuts where US
65 cuts through a high ridge (called Devil’s Backbone Ridge). We won’t stop here this
time (although we will on our second trip), but as you drive past, slow down if you can
and take a look at the rock layers. How are they arranged? How does their position and
arrangement compare with the layers that you saw in Conway on the Hogan Road cut?
3. As you enter Marshall, turn right on Arkansas 27. Stay on 27 when it splits with
Arkansas 74, and continue to the town of Harriet. At Harriet, take Arkansas 14 north
towards Yellville. Just before you cross the Buffalo River, there should be a little car
turnout and canoe put-in, called Dillard’s Ferry. Park here. Your instructor will take the
opportunity to explain a little about the overall geology of the Ozarks.
Hike back up the highway to see the stromatolites exposed in a rock layer known as
the Everton Limestone, early Middle Ordovician in age. What are stromatolites? How did
they form? What do they tell you about the environment when the Everton Limestone
was laid down?
4. Go back to Harriet on Arkansas 14. Bear left and stay on Arkansas 14, passing through
the towns of Big Flat and Fifty-Six. As you approach the intersection of Arkansas 14 with
Arkansas 5 and Arkansas 9, thirty miles from Harriet, you’ll start going down a sizable
hill. About halfway down the hill, pull over and park alongside
the road. Be careful, and set your parking brake! Make sure
you’re not blocking anyone’s driveway. Cross the road to see the
exposed layers of limestone.
Geologists divide the limestones exposed along this roadcut
into three main layers. From oldest to youngest, these are the
Plattin Limestone, the Kimmswick Limestone (both middle
Ordovician), and the Fernvale Limestone (late Ordovician). The
differences between the layers are subtle and not easy to see, but
the Fernvale Limestone tends to look slightly pinkish. You will
take samples of all three rock layers for analysis in a future lab.
Your instructor will also point out fossils exposed in the Plattin
Limestone. What do they look like? What can you tell about their
biological relationships? Do they tell you anything about the
environment?
Lying on top of the Fernvale Limestone is a layer called the
Cason Shale (late Ordovician and Silurian), but here it is deeply
weathered and covered by vegetation. Above it, up to the top of
the hill, is a thick layer known as the Boone Chert. We’ll look at
the Boone Chert on the second field trip.
5. Continue down the hill and turn right, heading south on
Arkansas 5/9/14. Just past the intersection, you will start seeing
roadcuts on the left side of the road. Pull over and park wherever
you can, and cross the road to examine the rocks.
The rocks that you’re looking at are assigned to the Joachim
(pronounced “Joke-um”) Formation, and consist mostly of
limestone. There are few fossils in the Joachim Formation, but
your instructor will point out evidence for ancient microbial
mats. What does this tell you about what the environment was
like when the Joachim Formation was being laid down?
This is a reconstruction of where the continents of the Earth were located around the
times that the rocks that you’ve seen were being laid down. (I haven’t lectured yet about
why and how the continents move; that’ll come later, but suffice it to say that they do.)
What is now North America straddled the Equator. There wasn’t much dry land; the
nearest coastline was in what is now northern Minnesota—although in what is now New
England, a great mountain range called the Taconic uplift was slowly starting to form.
Most of what is now the United States was covered by warm, tropical, shallow seas. The
continental shelf edge at the time was somewhere south of the Ozarks. There are rocks in
the Ouachita Mountains, west of Hot Springs, of the same age as the ones that we have
looked at—but those rocks are generally fine shales, laid down in deep water (possibly
many thousands of feet deep), with no fossils other than planktonic organisms that died
and drifted downwards.
Writeup: Write me clear summaries of what you saw on the three stops, in which you
explain the major features that you saw. Don’t just write a laundry list of “I saw this. . . I
saw this. . .”; put it together into a coherent picture. How do your various observations fit
together? What can and can’t you learn from them? How do you know what you think
you know about these rocks and the evidence for life in them? Finish by integrating all
you’ve seen into an overview of what Arkansas’s environment was like at the time, using
the plate tectonic map provided and anything else you have available.