The Great Unconformity
Warning: This rock outcrop is a roadcut along MO-72. There is a very wide shoulder, and plenty of space to keep your vehicle and yourself well off the road - please do so!
This location is a good exposure of the Great Unconformity - a contact between older Precambrian-age crystalline rocks (below) and younger Cambrian-age sedimentary rocks (above). The contact between the two rocks represents a gap in the geologic record of nearly a billion years.
In this case, the crystalline rock is exposed at the bottom of the outcrop on the south side of the road, and also by most of the outcrop on the north side of the road. This crystalline rock is called the Grassy Mountain Ignimbrite, which is part of the 1400 Million year old Butler Hill group of regional rocks. This rock has a dark maroon to black matrix and you'll find visible crystals (called phenocrysts) of two minerals: quartz (smoky color) and alkali feldspar (reddish at this location). This rock was deposited during a violent volcanic eruption. The St. Francois mountains are built by volcanic and plutonic rocks that all have similar chemistry to this one, and were emplaced around the same time.
On top of the crystalline rock is the Lamotte Sandstone, which is Upper Cambrian in age - deposited around 510 Million years ago. This sandstone was deposted in a shallow sea that covered much of North America during this time period. The Lamotte sandstone looks slightly different in different parts of Southern Missouri. In some places, the oldest (lowest) parts of the sandstone have very coarse grains - course enough that the rock is better classified as conglomerate. A bit north of here, at Pickle Springs Natural Area, you can see some excellent exposures of the Lamotte sandstone. There you'll see poorly sorted grain sizes ranging in size from coarse to fine, and beds that are anywhere from from a few centimeters to a meter or more in thickness. Pickle Springs is also a great place to see cross-bedding - remnants of ancient ripples and dunes preserved in the sandstone. You can see some cross-bedding at this location too - but you'll need to look very closely!
An unconformity where sedimentary rocks are deposited over crystalline rocks is sometimes called a nonconformity. You may notice that the unconformity here does not lie completely flat. On the eastern part of this outcrop (closest to Fredricktown), the Lamotte Sandstone beds are at an angle over the ignimbrite. Geologists would say these beds dip down towards the east. This dip is not caused by tectonics but is actually the original angle at which the sandstone was deposited, and reflects the ancient underlying topography of the ignimbrite. An unconformity that reflects ancient topography like this is called a buttress unconformity. When the sandstone was deposited the older ignimbrite was an area of relatively high topographic relief, and the sandstone was deposited on top of it. In the Cambrian, the areas that are now the highest peaks of the St. Francois would have been small islands in a shallow inland sea. Everywhere in this range, if you start at the top of a peak and walk downhill, you'll encounter sedimentary rocks, the beds of which are always dipping away from the peak.
The unconformity between the two units represents a gap in time of about 900 million years. Unconformities between Precambrian and Cambrian rocks are observed all over North America, including Powell's Unconformity at the bottom of the Grand Canyon. The widespread nature of this unconformity is something that is still debated by geologists, but could be related to the global icehouse climates that may have existed from 720 to 635 Million years ago during a time period Cryogenian Period.
In other places around southeastern Missouri you'll find slightly different rocks below and above this unconformity. Typically you'll find ignimbrite, rhyolite, or granite below the unconformity, and sandstone, limestone, or dolostone above the unconformity. But the Lamotte sandstone is the oldest sedimentary rock you'll find in the St. Francois. Why do you think it is missing in some places? As you move away from southeastern Missouri, you can trace this unconformity across North America. The contact between the rocks represents different gaps of time in different places. The rock types change but there is always a crystalline rock underneath and a sedimentary rock above.
In some places around the St. Francois, just at the unconformity you'll find the underlying rock appears eroded into large roundish cobbles called corestones. This represents an ancient soil horizon - one that formed before the sandstone was deposited on top! A dramatic modern example of the spheroidal weathering that produces corestones can be found at Elephant Rocks State Park.
There is one other type of rock at this outcrop, which you'll see towards the western end. This is a dike - a sheetlike vertical emplacement of frozen magma - of a rock type called diabase. This dike would have fed a volcano - one which is long since eroded away. The dike is better exposed on the north side of the road - on the south side of the road it has been quarried out, and you'll see a big hole in the outcrop. How old is this dike? You can figure this out by looking at the surrounding rocks.The principle of cross-cutting relationships was first worked out in the 17th century by Nicolas Steno - a pioneer of geology (and a Saint in the Catholic church).
While you are here keep an eye out for busloads of geology students pulling up to have a look. This location is one of the classic field trip sites in southeast Missouri.
Logging tasks
Observe the sandstone unit. Look closely at the grains and describe them. How big are they? Are the grains angular or rounded? Are the grains all the same? What do you think this implies about their source?
Observe the dike and how it contacts each of the other units. Which units does it cut and not cut? Based on this, what is the age of the dike relative to the ignimbrite and the sandstone?