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How many time have you been on a roadtrip and thought to yourself, "I wish I knew exactly what those rocks out there are, and their ages"? Okay, never. This map is perfect for this question, and you will ask it at some point in your life. Before we start looking at it though, we need a quick refresher on geologic time.
Geologic time spans the formation of the earth 4.6 billion years ago to the present. This is a lot of time, and it is divided up into sections. The two biggest divisions are:
Precambrian time from 4.6 billion years ago (BYA) to 544 million years ago (MYA). The name Proterozoic is often used to mean a particular part of Precambrian time.
and
Phanerozoic time from 544 MYA to the present.
It's no suprise that there are lots and lots of subdivsions, and sub-subdivisions, and sub-sub-sub-subdivisions and on and on. The Phanerozoic is broken down into three main parts:
Paleozoic from 544 MYA to 245 MYA
Mesozoic from 245 to 65 MYA
Cenozoic from 65 MYA to the present.
Here are the breakdowns we care about:
The Paleozoic contains the following periods:
Cambrian
Ordovician (ord-UH-vee-shee-ann)
Silurian (sih-LUR-ee-ann)
Devonian
Mississippian
Pennsylvanian
Permian
The Mesozoic contains these periods:
Triassic
Jurassic
Cretaceous (kruh-TAY-shuss)
Finally, the Cenozoic contains:
Tertiary (TER-sherry)
Quaternary (kwah-TER-nury)
Now just to hassle you, we make the following further divisions:
The Tertiary contains:
Paleocene
Eocene
Oligocene
Miocene
Pliocene
The Quaternary contains:
Pleistocene
Holocene
The way I listed the time periods just now go from oldest to youngest. When you look at the geologic map, you will see them listed so that the youngest time period is on top, and as you move down the column, you go back in time. This is a rule that all geologic maps follow.
The buttons give you a handy format for viewing time.
You can also get these in the "Helpful Documents" folder under Lab 7.
You are going to use this side of the map to find a route and figure out what types of rocks (or sediment) you are driving across. For example, find Monroe, LA and Alexandria, LA on the map:
Monroe is sitting in the pink-colored formation. If you look around in the pink, you'll see the symbol "Q." This color represents river deposits, and "Q" is the youngest period on the map. So these are river floodplain deposits that are still building.
Now let's drive south to Alexandria on US 165, making note of EVERY SINGLE formation we drive through, no matter how small on the map. If you can see it on the map, no matter how small, it's worth noting. The tiniest blob on the map is still many times larger than the entire ULM campus. So here is what I see:
We stay in the Q until Columbia, when we enter the Tec;
Then we enter the Qt;
Then we enter the Tej and the town of Tulles;
Then back in the Q again;
Then the Qt, then the Tm, then the Qt, then the Qm;
It's hard to see but we enter a slightly different color labeled Qb;
Then we enter the Q and the town of Alexandria.
When you do this exercise you'll see a lot of crazy alterations, like you'll drive Q/Tec/Q/Tec/Q/Tec/Q and so on. In the answer keys that I give you, I make note of every one of these switches.
Okay, we have seen colors and labels for rock types on the map. What the heck do you do with this information? read on.
Look to the left of this section and you will see the timescale. If you look across, you'll see the colored columns, which represent rocks. Notice that the colors are consistent across columns. For example, find the Mississippian time period, then look across at all the rock columns. Everything is some shade of blue. It's a rule that we follow when making maps that we keep a consistent color scheme for each rock "system." So Mississippian rocks are blue, Ordovician rocks are purple, Cambrian rocks are a reddish-brown, Cretaceous rocks are green, Tertiary and Quaternary rocks are orange-to-yellow, and the youngest stuff, the sediment being carried by running water, is gray.
Now, it's important to note that these are the colors used on the MAP. Real ROCKS can be whatever color, no matter how old they are. So if you look on the map and see the curving, green colors in the states of Mississippi and Alabama, you know those are Cretaceous in age.
Now go back and look at those rock columns. Next to the time period, you'll see a column marked GROUP. This is the name for a set of rocks. So, the Miocene Epoch, which is in the Tertiary Period, contains the Grand Gulf Group.
Now if you follow this to the right, it tells you in four columns what rocks are in the states of Louisiana, Mississippi, Alabama and Georgia. In Louisiana we see the Miocene Grand Gulf Group includes this information:
Fleming
Catahoula
Tm
This means that in LA, the Grand Gulf Group includes the Fleming Formation and the Catahoula Formation. The symbol Tm shows this on the actual map. Look around in LA until you see patches of orange labeled Tm. This means these rocks (sediment) are Tertiary in age (shown by capital T) and are Miocene (shown by the little m). In a symbol like this, the first letter is always a capital and represents the time period. The second letter can stand for the location the rocks are in, or a subdivision of time.
The abbreviation for Cretaceous is K and Cambrian is C with a line through it.
If you look over in Mississippi's column, you see that their Miocene Grand Gulf rocks are called something else. In MS, this group includes the Pascagoula, Hattiesburg, Catahoula and Paynes Hammock Formations.
Now look at Alabama. The Miocene Grand Gulf Group here consists of Catahoula, Paynes Hammock, and something called "HIATUS." The word hiatus means interruption, or break. So, while rocks were being deposited in MS and LA, the conditions were different in AL, and rocks were being eroded away. These rocks aren't here!
In Georgia, we see the Miccosukee and "Undiff. Miocene deposits." This means the rocks this age are so similar, it was pointless to name them separately.
Notice that Florida has its own columns, to the upper right. You'll also notice that igneous and metamorphic rocks have their own charts. So when you are looking for rocks on the map, look in all the different places.
Lastly, you have probably noticed that the colored rock colums contain patterns. The patterns tell you the actual rock type, like sandstone, limestone, etc. If you look in the left-center of the Generalized Chart, you'll see this little section:
So when you need to figure out what type of rock something is, look here.
This shows you how the North American Tectonic Plate is behaving in this region. This region has a long history of geologic activity, as you'll see in a moment. Pay particular attention to the lines on this section, marked E1-E2, W2-W3 and X2-X3. These are cross sections, where a profile has been drawn so you can see what the rocks look like below the surface.
Look all the way down to the bottom left of this side of the map to see the cross sections:
They go all the way across the bottom. Look closely at E1-E2:
The names and colors tell you what the rocks are. The Parish names are listed on top. For example, find Bossier Parish and look to see what's going on below the good citizens there:
At the surface you see the Red River and Bossier City and also blobs of Quaternary rocks, shown by "QUAT." Below that are the Wilcox, Midway, Navarro, Taylor and Austin Formations. There are more below that, but I want you to look at the heavy, almost vertical line. That is a fault. You'll notice that the rocks on either side of it are offset. The colored layers are cut by this fault. The fault has arrows that tell you which side moved up. In this case, the right (east) side went up.
Keep going east till you get to Jackson, MS. Under Jackson is the Jackson Dome, which was pushed up by Cretaceous Basalt. Basalt is volcanic rock, remember. This means that if you were to go back in time to the Cretaceous, you would be seeing a volcano where Jackson will one day be. These rocks were buried in the Tertiary and Quaternary. We don't usually (ever) think of MS as a volcanic region, but it was in the Cretaceous. Cool!
Look above the cross sections and you will see a section called "Land and Sea Changes Through the Ages - Southeastern Region. These images show you large-scale changes in time, going back to the Cambrian.
Now go up to this section of the map:
These symbols tell you what gemstones can be found in a particular area. To the lower right is the verbal explanation. Look at Monroe, and you'll see a yellow dot. The yellow dot corresponds to Agate, a particular gemstone made of quartz.
The map also shows you fossil locations. Look to the right center, and you'll see the fossil map. Baton Rouge is in the area Q3, which identifies Quaternary aged fossils there. In this case, mollusks and plant remains.