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Bog Iron Formation in the Nassawango Watershed, Maryland

By Owen P. Bricker, Wayne L. Newell, and Nancy S. Simon

U.S. Geological Survey
Open-File Report 03-346
Online Only
Version 1.0

The accompanying report was presented at a poster session at the Gordon Conference on Catchment Science: Interactions of Hydrology, Biology and Geochemistry , July 2003, Colby-Sawyer College, New Hampshire.The study was conducted in cooperation by the USGS Eastern Earth Surface Processes Team and Water Resources National Research Program.


Bog iron deposits occur at a number of localities in the Pocomoke River basin (Figure 1) (Singewald, 1911). The most extensive deposits are situated along Nassawango Creek northwest of Snow Hill, a town on the Pocomoke River. After the discovery of these deposits an iron furnace was built in 1830 on the west side of Nassawango Creek, five miles northwest of Snow Hill, at a location known as the Furnace. The furnace exclusively smelted bog iron obtained along Nassawango Creek for a mile or so north of the furnace site; iron was produced from 1830 to 1850 at a rate of approximately 700 tons per year. Smelting technology of that period used mollusc shells for lime flux and locally burned charcoal for fuel; they were unable to remove phosphorous and the iron was rendered brittle when cold ("cold short") because of the high phosphorous content inherited from the ore. For that reason, and because of the limited size of the deposits and the discovery of the much larger and higher quality ore deposits in other parts of the country, production ceased about 1850. The bog iron deposits have been forming throughout the Holocene and are still forming today. Singewald (1911) stated, that their chief interest today lies in the fact that the deposits are now forming and at such a rate as to be observable. "Deposits which were once exhausted are again workable after an interval of a few years" (for more on rate of formation of bog ore at different locations see Starkey, 1962 and Moore, 1910). In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite (FeO(OH)). The ores in the Nassawango, in addition to goethite and other ferric oxyhydroxides, contain a significant amount of magnetite.

Field Study

Figure 2 shows the surficial deposits in the Pocomoke River watershed. Sandy, quartz-rich barrier beach type deposits occupy east (ocean) side of the watershed. To the west are organic-rich fine sand to clayey silt estuarine/back bay deposits.
Figure 2

Figure 3 is a map showing the distribution and extent of ditching in the Pocomoke River basin. Drainage ditches have been dug since colonial times. However, most of the ditches were dug during the middle of the 20th century and are actively maintained. There is more ditching in the areas of fine grained sediment where natural drainage is poorer. The water table in the basin before ditching was close to the surface and interfered with agricultural practices. Ditching lowered the groundwater table, but caused bypassing of the natural riparian wetlands, swamps and tidal marshes where processing of nutrients and trapping of sediment occurred. Prior to ditching, there were approximately 800 miles of natural drainage through areas that permitted long contact time of water with environmental materials. Ditching created an additional 1200 miles of water ways and substantially decreased the contact time for the waters with natural materials that mitigated nutrient concentrations and trapped sediment. The shorter residence time may limit the concentration of dissolved iron. By contrast, the Nassawango Creek has long reaches with natural channeling and limited ditching along the tributaries. This original configuration may result in the apparent higher concentration of dissolved iron and the thicker present day accumulation of ore.
Figure 3

Geomorphological models of bog iron accumulation in the Nassawango Creek watershed are shown in Figure 4.
Figure 4

Iron-bearing groundwater typically emerges as a spring (Figure 5). The iron is oxidized to ferric hydroxide upon encountering the oxic environment of the surface. "Bog ore" often combines goethite, magnetite and vugs or stained quartz. It is not clear whether the magnetite precipitates upon first contact with oxygen, then oxidizes to ferric compounds, or whether the ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment surface and reoxidized upon exhumation at the surface (Figure 6).
Figure 5 and Figure 6

Study of the reduction-oxidation potential of spring waters in the watershed showed that most waters are consistent with equilibrium between dissolved ferrous iron and solid ferric hydroxide (log K=-37.1). Two of the measurements (Figure 7) are consistent with equilibrium between dissolved aqueous iron species. It is not known why the electrode responded to the dissolved species in these two cases rather than the dissolved-solid equilibrium that most measurements reflected, since all of the springs looked similar in physical characteristics.
Figure 7

The study has shown that Pocomoke River tributaries in the central part of the watershed exhibit turbidity even during dry years (Figure 8). The turbidity results primarily from iron oxyhydroxide floc due to precipitation of ferric compounds as ferrous-rich ground water emerges as river base flow. Note that the internal generated turbidity has nothing to do with runoff and sediment transport from upland areas.
Figure 8


The ground water of the Pocomoke basin is rich in reduced iron. This is particularly true in the Nassawango sub-basin where bog iron deposits along the flood plain of Nassawango Creek were dug in the mid-1800's to supply an iron smelter near the town of Snow Hill. The rate of bog iron formation was so rapid that areas could be re-mined in a matter of few years (Singewald, 1911). Bog iron is still forming in this area, and in other parts of the Pocomoke basin. Ground water has been measured with ferrous iron concentrations in excess of 20 ppm. When this water emerges at the surface or is discharged into the river system it rapidly oxidizes to an amorphous particulate iron oxyhydroxide which in time crystallizes to goethite. The iron in this system is important for at least two reasons: 1) iron oxyhydroxides strongly sorb phosphorous and many trace metals. Early reports on the composition of the Nassawango bog ore indicate that it commonly contained 10% P which made the pig iron smelted from this ore brittle when cold (Singewald, 1911); 2) the iron precipitating in the rivers causes turbidity which reduces light penetration to rooted aquatic vegetation and may impact other organisms, for instance, by coating gills and interfering with oxygen transfer. The first effect will play a role in the behavior and cycling of P in the system, while the second effect will impact biota in the system. In the fall of very dry years (1999 and 2001), we found the rivers in the central part of the Pocomoke basin quite turbid although there had been no storms to wash sediment-laden runoff into the rivers. Samples of the particulate matter creating the turbidity were iron-rich and displayed a weak x-ray diffraction pattern of goethite. There also seemed to be some organic material, probably algae, contributing to the turbidity, but this has not yet been investigated. Whatever the mix of materials that cause the turbidity, they are authigenic in the rivers and are not contributed by runoff. If all of the sediment erosion and runoff could be eleminated, it would have no effect on the turbidity generated by these chemical processes. Any practice recommended to reduce suspended sediment in these waters must take authigenic precepitates into consideration. Best management practices for sediment control in the watershed will have no effect on the turbidity resulting from oxidation of ferrous ions. Our project studies at other locations in the Chesapeake Bay have observed that vivianite (Fe3(PO4)2* 8H2O) is forming in sediment at heads of tributary estuaries where tidal water is non-saline and anoxic. Presumably, the phosphates originate from degradation of organic materials in the anoxic environment. Vivianite is commonly encountered in modern sediments we have cored from similar depositional environments. Vivianite in the estuarine sediments of the Kent Island Formation and Omar Formation may be the source of phosphates in early bog iron that was first mined in Nassawango Creek.


Moore, E.J. 1910. The occurence and origin of some bog iron deposits in the district of Thunder Bay, Ontario. Economic Geology 5, p. 528-537.

Mixon, R.B., Berquist, C.R. Jr., Newell, W.L., and Johnson, G.H. 1989. Geologic map and generalized cross sections of the Coastal Plain and adjacent parts of the Piedmont, Virginia. U.S. Geological Survey Miscellaneous Investigations Series Map I-2033, 3 sheets.

Owens, J.P., and Denny, C.S. 1978. Geologic map of Worcester County. Maryland Geological Survey, Baltimore, MD.

Owens, J.P., and Denny, C.S. 1984. Geologic map of Somerset County. Maryland Geological Survey, Baltimore, MD.

Singewald, J.T. Jr. 1911. Report on the iron ores of Maryland. Maryland Geological Survey Special Publication, Volume IX, Part III. The Hopkins Press, Baltimore, 337 p.

Starkey, J.A. Jr. 1962. The bog ore and bog iron industry of south New Jersey. N.J. Acad. Sci. 7, p. 5-8.

OFR-03-346 in PDF format (It is recommended that you right-click and save this 59 x 36 inch poster to disk.) (28 MB)
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For questions, please contact Wayne Newell or Owen Bricker.

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