February 3, 2020

The mountain that has it all (at least it did in the pick-and-shovel days)

Posted by larryohanlon

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.

The blast furnace is located about 1 mile (1.6 km) from the coal mine and iron pits. Limestone outcrops are about 1.25 miles (2 km) away. The massive limestone quarry seen today is modern and very active. Significantly, the blast furnace was downhill from the iron mines, making ore transport that much easier.

The iron pits line up along the ore zone, which is thin but stretches a long way parallel to the trend of the mountain. The coal and associated shales make a very smooth zone on the flank of the mountain. Most of the mountain side is extremely rocky and rugged where sandstone and pebble conglomerate, the millstone source, are exposed. The sandstone/conglomerate layering is clearly visible in this image.

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.

Today, even in the forest, you can stand at a coal spoil pile and see the iron pits just downhill. A single wagon road actually connected the coal mine and the nearest iron pits.

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.

This is a geologic map draped over the LiDAR-derived hillshade image. The light green area is Mississippian-aged sedimentary rock containing the coal; the light pink area is dolomite above the Pulaski Thrust Fault (called the Catawba Branch Fault in the map report). The trend of the fault is shown by the toothed black line; the iron ore occurs right along it.

This block diagram was produced from the geologic map shown above. At its right edge, the pink/peach-colored dolomite (Ce) is thrust over the light green Mississippian rock. The Pulaski Fault separates the two rock units. This diagram shows interpreted geology down to 1,000 ft (300 m) below sea level. Since North Mountain is just shy of 3,000 ft (900 m) in elevation, about 3,600 ft (1.1 km) of rock is shown here. If the diagram extended further down and to the right, the dolomite would be in contact with the dark blue Devonian shale (Dm).

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.

A zoomed out structural diagram. The transparent layers at the top represent rock that has already been eroded away to expose what is at the surface today. The coal occurs in what is only a tiny bit of Mississippian-aged rock still preserved in this area; the majority has already been eroded away. The dark blue material is Devonian shale, the likely iron source (shale between coal beds could be as well). The orange stuff above it is Cambrian dolomite, where fluids from the shale were neutralized to form iron ores right by the coal.

Because the rocks that support North Mountain were beneath the Pulaski Thrust Fault, they are pretty messed up! The coal is actually folded into the core of a tight syncline (U-shaped fold). As a result, the coal itself is folded and contorted, and the coal beds show irregularities and discontinuities. The sandstone layers uphill and downhill of the mine are actually the same layer on different limbs of the fold. Downhill of the coal mine, the sandstone layers are particularly damaged where they meet the Pulaski Thrust Fault.

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.

The Catawba Syncline’s structure preserves the Ordovician limestones. If the Catawba syncline were not slightly U-shaped (and thus not a syncline), everything but the deepest part of the orange layers (Cambrian dolomite) would be eroded away already. The dark blue zone behind “DDF” is a possible source of iron-rich fluids.

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.

The sandstones supporting this mountain are older (Silurian) and don’t contain coal. The landscape towards the bottom of the image is underlain by Cambrian dolomites very similar to those at the foot of North Mountain. The low but gnarled and rugged landscape at the top of the image is underlain by Devonian sandstone-shale interbeds that lie just above the black shale horizon.

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.

Fenwick Mines is not far from North Mountain and is a classic Oriskany-type ore (it’s really close to Oriskany, Virginia, the namesake of the ore type). Here, the valley in the foreground is underlain by black shale, which sits right on top of a thin limestone layer that crops out at the foot of the mountain in the distance. Water percolated down through the black shale and neutralized against the limestone, forming the iron ore.

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.

A possible model for ore formation at the Catawba Iron and Coal site. Iron-rich fluids were expelled from the black shale during emplacement of the Catawba Syncline on the Pulaski Thrust. When the fluids interacted with the dolomite, neutralization and iron oxide precipitation occurred. Because of the arrangement of rock units here, downward percolation of iron-bearing water is less likely, unless a black shale near the coal is a source.

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.

The layout of features at the site shows how good surface imagery can allow for geologic inferences to be made remotely. Sinkholes and low topography are a dead giveaway for carbonate rock, the aligned iron pits show right where the Pulaski Fault goes (no one would waste their time digging where there wasn’t iron), and the number and texture of sandstone beds on North Mountain is typical of the Mississippian section in this part of the Valley and Ridge.

Draper Mountain and the Catawba Iron and Coal site at North Mountain are about 60 miles (96 km) apart.

(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.