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The Edison Mines EarthCache

Hidden : 09/06/2007
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3 out of 5

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Geocache Description:

From the Revolutionary War to the turn of the 20th century, iron was mined at the site now named for Thomas Alva Edison. Edison founded an iron mining and processing company here in the 1890's and made important contributions to the field of mining technology. The Edison Mines are rich with historic ruins, numerous hiking trails, abandoned railroads and many abandoned mines to explore in this interesting area.

Figure 1. Monument to Thomas Alva Edison

LOCATION: Sparta Mountain Wildlife Management Area, in the Edison section of Sparta Township, NJ. For a topographic map click here.

WHAT TO BRING: Bring a magnet if you have one.

ACCESSING THE TRAIL: Caution - portions of the trails are a bit challenging, with some narrow overgrown sections, a few downed trees, and moderate slopes. As with all mining areas, please be very careful and be aware that there is no reason to enter the mine workings. Please do not enter any fenced in areas.

The waypoint will bring you to the Edison Monument in the parking lot of the Sparta Mountain Wildlife Management Area. Several trails lead from the parking lot. The Edison Mines area can be easily overlooked if you are not paying attention. All of the buildings are gone except for a few traces. The area has reverted to mostly forest. With the leaves on the trees, it’s hard to imagine that here was once a booming iron mining community using cutting-edge mining technology.


In the 1890's, Edison established an iron mining and processing company in what is now known as the Edison section of Sparta Township, New Jersey. Edison purchased much of the land in the area, including five mines, and built a large processing plant to turn the iron ore into briquettes for iron smelters. Today, these five mines are collectively known as the Edison Mines. They include the Ogden Mine, Davenport Mine, Roberts Mine, Pardee Mine and the Vulcan Mine. A small town sprang up to support his company at this site. At one time, the Edison Mines (not to be confused with the city of Edison in Midddlesex County, NJ)was home to approximately 500 people.

This monument (fig. 1) is a tribute to Edison, and explains the history of his iron mining and processing company, the New Jersey and Pennsylvania Concentrating Works. When you arrive at the parking lot and read about Edison from the monument, you'll get an appreciation for the work that went on here.

If you would like to hike around the area, there are some interesting things to see. Cross Edison Road to the next waypoint:

N 41 03 47 E 74 34 16
Horseshoe Cut, Ogden Mine

The famous Horseshoe Cut is a large, horseshoe-shaped open mine pit of the abandoned Ogden Mine. It's fenced-in and overgrown, but worth a look. Follow the dirt road which bears to the right off Edison Road. You will travel about a half mile up the mountain to the next waypoint:

N 41 03 35 W 74 34 34
Davenport Cut, a.k.a. the South Cut

This iron mine was notched into the mountain. It’s one of the most unique open-cut iron mines in the state.

Two other trails lead from the parking lot to more mine workings and ruins of the Edison Mines. One trail leads to the Roberts Mine area. The other trail will take you to Edison Pond, approximately a half mile walk. A sign next to the Edison monument points you to the Edison Pond trail.

N 41 03 53 W 74 34 13
Roberts Mine

This short walk will take you to the Roberts Mine area, where you will see many other workings, including large open cuts, pits and shafts. (fig. 2) Some of the smaller mine shafts are still open but are not safe. During your walk to the Roberts Mine you can also see some remains of mining equipment and building foundations scattered about.

Figure 2. Mine shaft opening at the Roberts Mine


The iron-bearing mineral in the ore is magnetite (Fig. 3), an iron oxide with the chemical formula Fe3O4. (Fun fact: rust is also an iron oxide, but with a chemical formula of Fe2O3.) Magnetite is a hard, dense, black mineral that crystallizes into cubes (4-sided objects) or octahedrons (8-sided objects). The magnetite ores of New Jersey occur in pre-Cambrian rocks (rocks more than 542 million years old) located in the New Jersey Highlands. The ore was concentrated in ore "belts" and usually mixed with other types of rocks, often granite and limestone. Magnetite is magnetic; Edison took advantage of this unusual mineral property in the design of his Magnetic Ore Separator.

Figure 3. Iron ore from the Edison Mines

Iron ore has been mined at this site since the late 1700's. The oldest of the openings of the Ogden Mine dates back to 1772, making it one of the earliest worked iron mine in the state. During the Revolutionary War the iron ore from the Ogden Mine was transported to Pompton Furnace about thirty miles away and processed into cannon balls for use by the Continental Army.


Thomas Edison is most famous for his inventions and work with electricity, but he was also interested in geology. Edison spent many years involved in mining operations in New Jersey. His foray into iron mining and processing began in 1880 when he developed an electromagnetic ore-separator while working on electric light and power. In this device, sand from crushed rock was poured through a hopper so that it fell in a thin, broad stream in front of an electromagnet. (fig.4) The electromagnet attracted magnetic particles (such as iron) into one receptacle, while non-magnetic particles fell straight through into another bin.

Figure 4. Edison's magnetic ore-separator.
Diagram courtesy of Edison National Historic Site.

The concentrated ore was made into into briquettes to be used in iron smelters. The original rock and ore mined together had an iron content of about 20 percent. Following crushing, separating and concentrating, the briquettes had an iron content of over 60 percent.

Figure 5. The magnetic separator building once located at the Edison Mines in Sparta, NJ. Photo courtesy of Edison National Historic Site.

During 1900, the last year of operation of the Edison Mines, the New Jersey and Pennsylvania Concentrating Works mined and separated 75,206 tons of crude material at its magnetic concentrating plant on site at the Edison Mines in Sparta, New Jersey (fig. 5). Of this, about 10,000 tons of concentrates were shipped in the form of briquettes.

After pouring over $2 million of his own money into this venture, Edison was forced to abandon it when large iron ore deposits were discovered in the Mesabi Range of Minnesota. This discovery made Edison's concentrated low-grade ore too expensive to produce for the steel mills. Edison’s iron mining company is considered to be his biggest failure.


The Iron Mines and Mining in New Jersey, William S. Bayley, (1910).

Thomas Edison's "Ogden Baby", The New Jersey & Pennsylvania Concentrating Works, Rodney P. Johnson, 2004.

Annual Report of The State Geologist For The Year 1900, New Jersey Geological Survey, 1901.

All the Injured Found, The New York Times, August 14, 1892.

To claim this cache: Answer the following questions, and post your answers in your log. Tell us how many people were in your group. (You don't have to wait for a confirmation from us to claim the cache. We trust you!)

1) Take a picture of yourself with the Edison Monument in the background, or take a picture of your GPS receiver propped on the monument.

2) Determine the general direction of the mine workings containing the magnetite ore.

3) (Optional, for extra credit) Can you find a magnetic rock? Describe it. What color is it? Does it seem heavy for its size? Why does it have these properties?

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