|There are two primary sources of iron ore in Maryland: igneous and sedimentary. I choose to focus on one of the sedimentary iron ores, the so-called "Arundel ores," or "bog iron ores," of the Potomac Group. They occur in a belt about 5-8 kilometers wide running from the northeastern corner of the state in Cecil County through Baltimore and on into Washington, D.C. and Virginia (Clark and Bibbins, 1897; Clark and others, 1911). Their location follows and largely coincides with the Fall Line, I-95, U.S. Route 1, and the Baltimore-Washington Parkway (Fig. 1). |
Figure 1. Distribution of the "Arundel iron ores" in Maryland. The ore-bearing clay is confined to the Arundel Formation (adapted from Kranz, 1989).
By the late 1800's, a debate had developed about the age of the ores and the beds in which they were found. Yale University paleontologist O. C. Marsh believed them to be of Late Jurassic age on the basis of a similarity of the dinosaur bones found in association with the ores to the Late Jurassic dinosaurs of the Western United States (Marsh, 1888). However, botanists such as Lester F. Ward and William M. Fontaine felt the plants indicated a later Early Cretaceous age (Ward, 1888, 1897; Fontaine, 1889). By the beginning of the twentieth century, the latter view prevailed. Modern work by palynologists Gil Brenner and E. I. Robbins confirm the age as very late Early Cretaceous, Late Aptian-Early Albian time approximately 115-105 million years ago (Brenner, 1963; Robbins, 1991).
There has been very little controversy on the source and origin of the "Arundel" ores. Ground water seeping through the sands and sandpockets carried dissolved iron (Fe++) and manganese (Mn++). Upon reaching oxidized environments such as grottos, springs and swamps, these dissolved metals encountered so-called "iron bacteria" (e.g., Lepothrix), causing the chemical reactions that precipitate the ore masses. Such processes can be seen today in nearby locations such as Hundley Meadows State Park in Virginia and Chesapeake Beach, Maryland (Robbins and Norden, 1994). The ore tends to accumulate in horizons, but occurs in discontinuous lumps as might be expected from such a process (Fig. 2). Robbins (personal commun., 1995) believes the ores probably formed in Cretaceous times, as she sees no evidence of ore formation in the Potomac Group beds at present.
Figure 2. Typical stratigraphy of the "Arundel ore," as shown in the Reynolds Mine, Anne Arundel County, circa 1900 (Ries, 1902).
The ores are composed of siderite, which is an iron carbonate ("white ore"), and various iron oxides ("brown ore"). Some of the ore masses can be larger than an automobile, but many are small. The miners laboriously dug the lumps from the clay, broke them in to smaller pieces, loaded them on wagons, and hauled them to the furnaces or rail depots. The miners were paid directly for the ore by weight. Much of the ore came from small pits on local farms, operated intermittently, and often used black agricultural workers as miners (Shannon, 1921, 1922). When these agricultural workers were not involved in planting, harvesting, or putting-up crops, often in winter, they were able to supplement their meager incomes through mine work. For others, mining was their primary occupation (Fig. 6i). They were also known to have burned the lignitized logs associated with the ores for warmth (George Smith, personal commun., 1992). Of course, they had no notion that their fires were fueled by trees which had died more than 100 million years ago.