Missouri Boulders

Take a tour of the geology of Missouri with virtual stops at Elephant Rocks State Park, Johnson's Shut-Ins, and Washington State Park. Go out to the southern end of the Green Center's prairie at Brittany Woods Middle School to see the display of Missouri's native rocks. Be sure to use your sense of adventure to make the tour of Missouri exciting.

The Tour

Rock #1 (starting from the west end of the prairie)

Description: Limestone (CaCO3). Light gray, massive. Reacts vigorously with dilute HCl. Veins of white and translucent Calcite criss-cross the rock.

Orogenesis: Limestone is a sedimentary rock deposited in marine settings. 100 million years ago, Missouri and much of North America were covered by an expansive and shallow or "epeiric" sea. This sea allowed the many limestone outcroppings we see along Missouri highways and near the Mississippi River bluffs to be deposited. Once the sea retreated from North America, erosion of these deposits by wind and rain began and still continues today.

Rock #2

Description: Limestone (CaCO3). Medium to light brown, moderately fossiliferous. Crinoid stem segments (like a bony water lily), disarticulated gastropods (snails), and brachiopods (seashells).

Orogenesis: Same as Rock #1, but this section contains fossils of marine creatures including plants, snails, and seashells. Geologists use assemblages of fossils to date these rocks precisely and even tell what the ocean chemistry and environment was like when these organisms were alive.

Rock #3

Description: Quartz Sandstone (SiO2). Light tan, concentrically layered quartz (agate) nodules throughout. Dissolution and weathering evident.

Orogenesis: If the chemistry is correct, then water flowing through sandstones will dissolve weak grains in the rock. In this self-propagating process, the water will dissolve out small caverns, elongated in the direction of flow, in the rock. Eventually, the water will become supersaturated with silica and will deposit the dissolved materials into the crystal forms and layering patterns seen here. This type of formation is present in many of central-eastern Missouri's State Parks (e.g., Washington State Park).

Rock #4

Description: Small rock beside #3. Limestone. Light brown (now green with lichen) with dissolved areas. Finely banded.

Orogenesis: Same as Rock #1. Limestone is even more easily dissolved by running water than sandstone. It is this process that dissolves and carves out the holes in this rock, as well as the many caves found throughout Missouri.

Rock #5

Description: Banded Quartz Arenite (SiO2). St. Peter's Sandstone. Sand grains are well sorted and very well rounded. Some layers are slightly more resistant and may contain more iron (appears reddish-orange).

Orogenesis: Sandstone is a sedimentary rock deposited when material is eroded from mountains by rivers and streams. A sandstone is "well-sorted" if the grains are of uniform size which is indicative of either a) a low-energy, very consistent river, or b) reworking of sediments previously deposited. The rounding of grains provides a similar measure of how far grains have traveled (the more distant from their source, the more rounded). This sandstone is well sorted and well rounded and it is the result of the erosion, reworking, and redeposition of an even older sandstone deposit. The St. Peter's Sandstone unit is one of the most important aquifers of the greater St. Louis region.

Rock #6

Description: Ore Rock. Heavily fractured with veins of Iron (Fe) and/or Lead (Pb) ore. Pinkish granite xenoliths containing orthoclase and plagioclase.

Rock #7

Description: Rhyolite. Dark pink to red with flow banding evident. 1-4 mm plagioclase and quartz crystals.

Orogenesis: Rhyolite and granite (Rock #'s 8 and 9) have similar compositions. However, granite typically forms from slowly cooling magma deep underground (termed "intrusive"), whereas rhyolites form from quickly cooling magma very near the Earth's surface or actually out on the surface as a lava flow (termed "extrusive".) Since rhyolite cools quickly, very few mineral crystals (if any) have time to grow. Usually any crystals that are visible are due to some cooling of the magma while it is held in the magma chamber before being erupted onto the surface. The flow banding evident on this rhyolite is caused by minute compositional differences within the lava flow, and records the direction of flow like a sticky plastic. Many types of rhyolite can be observed at Johnson's Shut-Ins State Park in Southeast Missouri.

Rock #8

Description: Pink Granite (Quartz, Orthoclase). Slightly weathered (many orthoclase crystals are degraded).

Orogenesis: Granite is produced when a large chamber of magma cools very slowly far beneath the surface of the Earth. Individual crystals can grow quite large and a very hard rock is formed due to the high quartz content. Erosion eventually wears away the surface of the Earth to expose these large and once-deep rocks. Time of exposure on the surface can be estimated by knowing how fast certain crystals on the rock surface degrade and the climate of the region where the rock is located. This type of granite is very common in the St. Francois Mountain region of Missouri and large outcrops can be observed at Elephant Rocks State Park.

Rock #9 (ending on the east end of the prairie)

Description: White Granite (Quartz, Plagioclase, Biotite). 5-10% 2-3mm biotite grains, >5mm rounded quartz, >5mm plagioclase (some quite large ~ 2cm)

Orogenesis: Same as Rock #8.

Activity

Press the heel of your hand firmly on rock #7 (dark pink rhyolite). Look at the imprint the rock makes in your hand. Do the same with rock #8 (pink granite) and/or rock #9 (white granite). Again look at the imprints. Compare the imprints between the rhyolite and granite rock(s). Try it again a few times to see if you can observe any differences. You may notice that the rhyolite rock is smooth, so it does not make an imprint in the heel of your hand. And you may notice that the granite rock is rougher, so it makes an imprint in the form of several dozen dots the size of a pinhead. These dots are impressions of crystals in the granite. Rhyolite has crystals too, but they are much smaller and finer than the larger and rougher crystals in the granite.

Rhyolite forms from quickly cooling magma very near the surface. Since rhyolite cools quickly, very few mineral crystals have time to grow. Granite forms very slowly from magna far beneath the Earth's surface. Thus individual crystals of granite can grow quite large. Did you see in the imprint on the heel of your hand, the visible evidence of the size difference of rhyolite vs. granite crystals?

Acknowledgments

Purchase of the Missouri rock boulders was made possible by a 1997-98 Show-Me Conservation Outdoor Classroom Grant from the Office of Environmental Education, Missouri Department of Conservation. Identifications and descriptions of the Green Center's Missouri rock boulders was done in 2002-03 by students and faculty of the Geology Club, Washington University of St. Louis.