February 3, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Back in 1830, The Catawba Iron and Coal Company got an outrageous deal on a plot of land at the foot of North Mountain in western Botetourt County, Virginia. Within about 1 square miles, coal, iron ore (oxide), and high-purity limestone could be mined, and thick layers of quartz pebble conglomerate could be quarried for much sought-after millstones. While this surprisingly small area appears to have everything early industrial America could have wanted, the Catawba operations shut down in 1865, apparently due to transport issues in post-Civil War Virginia. Most of the property is entirely reclaimed by forest, but LiDAR-derived imagery shows the old workings and some details of the geologic circumstances that create the interesting resource abundance.
I imagine that the folks running Catawba Iron and Coal were pretty pleased with their setup. Extractable iron ore and coal measures extend for miles along the foot of the mountain, and in one spot a coal mine was located only 660 feet (200 m) from an iron ore pit on the same wagon road. These resources aren’t worth anyone’s time in the modern world, but in the 19th century this was a real bonanza.
The high-purity limestone sourced 1 mile (1.6 km) from the iron pits was used as flux in the on-site blast furnace that processed the iron ore. The ore was known to produce good quality, easily-cast steel. The furnace was fired by charcoal made from the property’s hardwood forests, and the coal was shipped elsewhere for fuel. This place offered a very unique combination of resources within an unusually small geographic area. There are plenty of iron, coal, and limestone resources within the Virginia Valley and Ridge and Appalachian Plateau, but I think this location is quite unique due to the presence of high-quality deposits of all of the materials right next to each other.
Geochemistry and structural geology are behind the wide variety of rocks and ore present along the foot of North Mountain. The mountain itself is developed on Mississippian-aged sandstone and conglomerate (the millstone source), with a little bit of shale and coal mixed in. The broad valley southeast of the mountain is developed on much older Cambrian- to Ordovician-aged dolomite and limestone (carbonate rock). This older rock was thrust over the Mississippian-aged layers along the Pulaski Thrust Fault during the late stages of Appalachian Mountain assembly. The iron ore deposits occur right along the fault, where the dolomite meets the sandstone and shale.
The fault and the carbonate rock are key to the presence of iron ore in oxide form so close to the coal. The carbonate rock above the fault neutralized the acidity of iron-rich fluids/waters associated with black shales located beneath the fault. Iron oxides precipitated with the loss of acidity, collecting along the fault zone at the edge of the carbonate rock. The Pulaski Fault appears to have actually been slipping in the upper coal layers in this area, so the neutralization zone at the fault and the coal beds are literally right next to each other.
The position of the Pulaski Fault within the sedimentary layer sequence also controls the presence of the high-purity limestone on the property. Within the region, the Pulaski Fault is typically positioned higher in Mississippian-aged shales above the coal zone. As a result, most of the rock above the Pulaski Fault has already eroded away, leaving only a thin horizon of Cambrian dolomite at the surface that does not extend particularly deep. In the vicinity of North Mountain, the fault is positioned in deeper Devonian shales. This arrangement preserves much more of the rock above the fault, including the high-purity Ordovician limestone and younger layers. The fault steps up from the Devonian shales into the Mississippian-aged coal at the Catawba Iron and Coal site.
No one knows if the iron-rich fluids were sourced from shales near the coal or older, deeper Devonian-aged shales beneath the Pulaski Fault. A similar iron resource along the Pulaski Fault occurs well southwest of Botetourt County along Draper Mountain in Wythe County. In this location, black Devonian shale is clearly the iron source as no Mississippian-aged rock (and thus no coal!) is present.
Devonian-aged black shales produce several other iron occurrences in the area where they are in stratigraphic contact with Devonian-aged limestones (Oriskany-type ores; read about them at this link). In these cases, the two rock types have not been brought together by a fault; they are already next to each other within the sedimentary layer sequence in the area. These Oriskany-type deposits are a bit different from North and Draper Mountain because the iron-bearing and acid-prone shale is above the carbonate rock (limestone). Water can percolate down from the surface through the shale and reach the limestone, where the neutralization occurs.
At North and Draper Mountain, the carbonate rock is above the shale, suggesting a different mechanism may have formed the iron ore deposits. Iron-rich, acidic fluids may have been pushed out of the Devonian shale under the fault as the carbonate rock was thrust over it, setting the stage for the neutralization to occur. It is also possible that groundwater flow paths down the tilt of the bedrock layering formed the ore deposits in the typical Oriskany style–no one knows for sure.
I highly doubt the 19th century owners were too worried about the particulars of ore generation; they just wanted to sell it along with the many other products available on the slopes of North Mountain. The site was well known within pre- and post-Civil War Virginia, as its coal was high rank (sub-anthracite) and its steel of very desirable quality, presumably due to particular trace chemical characteristics of the iron ore. An interesting description of the site can be found here. I was fortunate enough to produce a geologic map of this area in 2017, and the iron pits and coal spoil piles are still very clearly visible. Plant fossils are easy to find among the coal spoil, and I think the entire area is in Jefferson National Forest so it can be visited by walking west-southwest from the aptly named Stone Coal Road.
(LiDAR data from US Geological Survey)
Coal mine marked on the hillshades is at 37.488190N 80.011347W
This post was originally published on The Geo Models blog.