Harrodsburg Area Geodes
Monroe County, Indiana
December 27, 2003
By Mike Streeter
mcstreeter@charter.net

We woke up early on Saturday excited to start our full day of geode hunting. We left our motel room in Bloomington with great expectations. We were excited by the weather forecast that called for a high temperature of 51 degrees, a full 15 degrees above normal - lucky us! We drove south on Hwy-37 in the direction of Bedford to a high road cut just north of the Harrodsburg exit. This location is well known for having plentiful geodes in the limestone walls. We drove up a muddy two-track road on the north side of the west cut and parked in a level grassy field. Although the rocks are essentially identical on both sides of the highway, we chose to work the western side to take advantage of the sun that was rising in the eastern sky. Also, the ice that was hanging down both cuts in large sheets and icicles was far less pronounced on the western cut since it receives considerably more daily sun exposure.

Harrodsburg Cut - looking North on Hwy-37

As a professional geologist and author of "A Rockhounding Guide to North Carolina's Blue Ridge Mountains" (www.mcrocks.com), I am keenly interested in the geology of any area where we rockhound. I discovered by studying the Indiana Geologic Survey's website (http://igs.indiana.edu/) that the rock formation containing most of the area geodes is the Harrodsburg Limestone. Limestone is a broad term that refers to sedimentary rock in which calcium carbonate is the major constituent. Calcareous sediments, including the remains of marine animals, were deposited in a shallow sea that covered most of what is now the continental United States during the Mississippian Period that extended from 360 to 325 million years ago. The sediments would become buried and compressed by overlying materials and cemented by calcium carbonate to form limestone. Certain stratigraphic units within in the formation contain different types of fossils and geodes. The fossils are the remains of extinct marine animals, including brachiopods, crinoids and bryozoans. But how did the geodes form?

It is thought that many varieties of Indiana geodes began as cauliflower-like growths of gypsum that had either been trapped or formed in the soft calcareous sediments. The outside of the gypsum growths would gradually become encrusted with an outer shell of silica. The remaining interior gypsum would later be dissolved by groundwater solutions leaving a quartz-lined cavity in the limestone. At some later time, mineral-rich groundwater would make its way through rock fractures and the crystallization of minerals would take place from the outside in. The two most common minerals to crystallize in the geodes are quartz and calcite. The crystallization of these two minerals took place during separate phases with calcite generally being the latter. Other more exotic minerals found in Indiana geodes include millerite, celestite, strontianite, barite, pyrite and amethyst. It has also been postulated that geodes formed by the chemical replacement of calcite nodules and iron concretions by silica. Knowing how geodes formed may be interesting academically but finding them is the real fun.

According to the Indiana Geological Survey, there are many locations in Monroe County where geodes may be found. Geodes may be found along the shores of Monroe Lake. Geodes may also be found in and along streams that empty into that lake, including Stephens Creek (six miles east of Bloomington) and Ramp Creek (east of Smithville). Other streams where geodes may be found in Monroe County include Clear Creek (three to five miles south of Bloomington), and Jackson Creek (two miles southeast of Bloomington to four miles south of Bloomington). Geodes can also be extracted from the bedrock on Hwy-37 road cuts south of Bloomington. Geodes are commonly found in ravines about the town of Bedford. Small geodes are reportedly abundant at the western end of the railroad tunnel west of Unionville. Geodes may also be found in Brown County along the banks of Bear Creek in the northwestern part of the county. Access to the stream is available along Bear Creek Road north of the town of Trevlac. In Lawrence County, geodes can be found along the banks and in Little Salt Creek, which runs through the northern part of the county. Guthrie Creek and Back Creek, south of Leesville in eastern Lawrence County, are also good places to hunt geodes.

It had heard that millerite and calcite could be found in geodes collected from the first ledge above Hwy-37 on the Harrodsburg cut. So, that's where we headed along with Opal.

Looking North from the first ledge on Hwy-37 cut

We made our way down from the top grassy level at the southern end of the cut to the beginning of a horizontal flat ledge that is about 7 feet wide and 20 feet above the road. As we continued northward on the ledge, we came upon a few small cedar trees in our path that had to be carefully scooted around. Beyond the trees, the ledge slopes markedly downward toward the road as it is covered with a rock talus that includes countless broken geodes. Chrissy's pronounced fear of heights prevented her from getting past the cedar trees, so she retreated from the ledge and opted to collect in other areas where she didn't have to worry about taking a 20-feet header if she tripped. Opal doesn't share Chrissy's fear of heights, so my canine friend kept me company on the ledge. Her "four-wheel drive" serves her well on uneven and treacherous terrain.

Opal Streeter

Just past the cedar tree, I found a couple fair sized geodes sticking part way out of the vertical limestone wall. I noticed obvious chisel marks surrounding a 6" diameter geode indicating that another rockhound had attempted to recover it. Although I should have taken the "hint" that this geode would not be easily liberated, I began banging away around its sides with a pointed chisel and 6-pound sledgehammer. After about 40 minutes, during which it seemed that I was getting nowhere fast, I realized that maybe it would be a good idea to look around a little bit to see if there were easier pickings. I am not generally one to quit but I didn't want to be there two hours later with only one geode to show for my trouble. I discovered an area farther north on the ledge where geodes were also partially exposed and where the rock seemed to be less dense with more fractures. I discovered that many of the geodes in this section of the wall had already been broken but were still in place and some contained excellent calcite crystals. So, I went to work again trying to expose and recover geodes.

Close-up view of broken geodes in the dolostone wall

I worked for about 5 hours with a 4-feet long pry bar, pointed and flat chisels and 6-pound sledgehammer. It appeared to me that one section of limestone had been weakened as water flowing through internal fractures froze and swelled. The recent warm temperatures melted the ice leaving behind expanded fractures and weaker rock. This enabled me to pry loose large vertical blocks and sheets of limestone from the wall which, in turn, exposed geodes. Unfortunately, the melting of the ice had another unpleasant effect that wasn't at all helpful to my cause. I had to spend much of my time on the ledge working beneath a melting sheet of ice that continuously dripped water and dropped ice fragments onto my head and shoulders. It was a good thing that one of the larger icicles didn't let loose and hit me or I would have likely ended up falling with it to the road below. This probably would have delighted any one of the many chuckleheads in passing cars who felt compelled to honk his horn or holler an unintelligible expletive at me while I was working. My efforts and perseverance were rewarded with some fantastic geodes that contain an assortment of minerals including quartz, calcite, pyrite and dolomite. I carefully ferried each delicate specimen to the south end of the ledge where Chrissy felt comfortable enough to tread. She collected the specimens, one in each hand, and carried them in many trips up the hill to the truck. Chrissy did her usual bang-up job gently wrapping and securing all the nice specimens.