This easy five-part multi provides a welcome diversion for geocachers traveling north on Interstate 55 in Missouri. Visit some interesting road cuts and learn about the geology south of St. Louis along a 40-mile stretch between Perryville and Festus.
All the stops are close to exits, but you won’t have to park your car, or walk, on the interstate to view them. At Stops 1 through 3 you’ll be looking for an orange plastic matchbox. Note the special instructions at Stop 4. Each contains information you’ll need to find the regular cache at Stop 5.
STOP 1: PERRYVILLE (N 37º 43.657’, W 89º 54.210’)
Take Perryville exit 129 (MO Rt 51). Turn left (south) on MO 51 and cross over I-55. Immediately after crossing over I-55 turn right (north) on Lake Drive. Go 1.48 miles north on Lake Drive and park on shoulder. You are standing on the Joachim Dolomite, an Ordivician rock unit that is about 450 million years old. It has some good shell fossils.
Get back on I-55 North and proceed 11 miles to exit 141. Between here and the next stop you’ll see mostly two rock units. The Joachim is visible on the hills. In the valleys you’ll see the St. Peter Sandstone, which lies beneath the Joachim.
STOP 2: FAULT ZONE (N 37º 50.841’, W 90º 00.122’)
Take exit 141 (State Rt Z). Turn right (east) on Z and park on the shoulder. Walk to the road cut on the east side of I-55. The clue is here.
This is an exposure of St. Peter Sandstone, which is described in greater detail at Stop 5. This cut is significant because it exposes the Ste. Genevieve fault zone, a major branch of the New Madrid fault system. This fault has been mapped for a distance of 125 miles, from St. Clair, Missouri to Anna, Illinois. I-55 crosses the fault at the Route Z overpass, where jumbled blocks of the St. Peter lie at various angles. When you find the clue, you’ll be standing right on the fault.
Just a quarter mile to the north, undisturbed beds of the much younger Burlington and Keokuk formations are exposed. All of the Silurian, Devonian and upper Ordovician units that should be here are missing, the result of millions of years of erosion. This represents a vertical displacement of about 1,800 feet.
There are lots of shell fossils in these rocks. At one time (about 500 million years ago) this was a tropical island beach with sparkling white sand and azure seas filled with clams, weird bugs and corals. There were no fish in the seas and no trees on the land – just moss and maybe grasses.
At this point I invite you take a short side trip through some of the most beautiful and historic country in southern Missouri. It’s a combination of the Ozark mountains and the Mississippi River hill country. You can, of course, get back on I-55 and exit at the appropriate spots to find the clues, but the following descriptions presume that you are taking the back roads described below.
Go east on Rt Z to US61 north and enter the village of St. Marys. This is an old farming town that’s seen better days. On the east (right) side of US61 is the floodplain of the Mississippi River, some of the richest farmland in the country. There are some nice historic homes here. For you out-of-staters, Missouri and Wisconsin are the only states I know of that use letter designations for secondary (county) highways. What makes it confusing, in Missouri anyway, is that every county in the state probably has a Rt Z, a Rt H, etc. Keep that in mind as you roam the Ozark hills.
STOP 3: LIME KILN ROAD (N 37º 58.407’, W 90º 05.208’)
Stay on US61 north through Ste. Genevieve, pass the Mississippi Lime Company plant, then turn left (south) on Lime Kiln Road (intersection is at N37° 58.740', W90° 4.723'). Go about 0.65 miles and stop at the chalky white limestone road cuts. This is the Salem Limestone, +98% pure calcium carbonate. It is cross-bedded and oolitic, which means it is composed mainly of tiny round carbonate grains, smaller than the head of a pin. These are the same beds that Mississippi Lime mines to produce quicklime and ground limestone that goes into foods and pharmaceuticals. If you’ve ever taken a Tums tablet, you have eaten some of the rocks you are looking at now – bon apetit!
To continue to Stop 4, go back to US61 and turn left (north). Drive 10.76 miles, through the village of Bloomsdale. I would be doing you a disservice, however, if I didn’t invite you to spend an hour or so in beautiful, historic Ste. Genevieve. To get downtown, go back to the Mississippi Lime Company plant entrance, but turn the opposite direction (east) on Market Street, then follow it downtown.
Founded in 1735, Ste. Genevieve was settled by lead miners, farmers and fur traders and the French influence is visible everywhere you look. Visit the Great River Road Interpretive Center, on South Main Street, or tour the historic Church of Ste. Genevieve to learn about the area’s fascinating history. The Ste. Genevieve Museum contains varied collections of local memorabilia, including prehistoric Indian relics, old documents and Spanish land grants.
Historic homes provide a glimpse into the pioneer spirit of the city's earliest settlers. The Guibourd-Valle House was built about the 1784's by a pioneer French settler. Handsomely restored, it is the most elegantly furnished museum house in town. The Felix Valle State Historic Site, built in 1818, was home to one of Ste.Genevieve's premier colonial families. This important Federal style limestone building features the authentically restocked mercantile store of the historic trading firm of Menard & Valle. The Bolduc House, built in 1770, is regarded as the first, most authentically restored creole house in the nation. There are also several nice family restaurants downtown.
STOP 4: FOURCHE DU CLOS (N 38º 01.311’, W 90º 13.989’):
Welcome to the Fourche a du Clos Roadside Park in the village of Bloomsdale. I picked this spot more for the great view than for the geology. You are looking up the valley of Fourche du Clos Creek. There is a granite elevation marker here – a good place to check the altitude accuracy of your GPS.
Muggles seem to be watching this stop, probably from across the road. My container keeps disappearing, so I'm changing this to a virtual clue. Find the elevation marker. Add the three numbers, then divide the sum by two (example: if the elevation is 684, then 6 + 8 + 4 = 18. 18/2 = 9. The value for Clue D would be 9).
You are standing on the Joachim Dolomite. A few hundred feet north of this park on US 61 there is a good exposure of Joachim in a road cut. Further north, you pass through the Joachim into the underlying St. Peter Sandstone.Proceed north on US61 for 12.80 miles to the cache.
STOP 5: CACHE (N 38º 09.AB2, W 90º 21.CD3)
Now you’ve visited four great road cuts and collected the clues that will take you to the cache! Take the numbers you’ve been writing down and substitute them for the appropriate letters in the cache coordinates. About 200 ft before I-55 exit #170, then turn right (north) on Cement Plant Rd. Go 0.3 mi north on Cement Plant Rd to Castle Acres Rd. The cache is about 0.4 mi NW of this intersection. Castle Acres is a dead end road, so you’ll have to retrace your steps back to I-55, then you’ll be on your way again.
CHECKSUM: 09.AB2 + 21.CD3 = 31.735
As you approach I-55 (exit 170) you’ll pass through a great exposure of St. Peter Sandstone (N38° 9.435', W90° 21.124'). This is a good place to stop if you want to get a close look at the St. Peter. Note that you can easily break small chunks of this sandstone with your fingers. That’s because it is almost pure silica sand with very weak clay cement holding the grains together. Nearby Crystal City got its name from the plate glass factories that have operated here since the late 1860s. The glass is made from the same clean silica sandstone that you see here.
BONUS: email me with a one-word description of the red stamp on each clue to claim your bonus find! At Stop 4 send me the elevation on the granite marker.