| Egg Rock | |
|---|---|
 View of Egg Rock around 1900, from "The History of Concord, Massachusetts", 1904 | |
  Location of Egg Rock in Concord, Massachusetts | |
| Location | Middlesex, Massachusetts, United States |
| Coordinates | 42°27′54.36″N 71°21′30.6″W / 42.4651000°N 71.358500°W |
| Length | .1 km (0.062 mi) |
| Width | .05 km (0.031 mi) |
| Elevation | 39 m (128 ft) |
| Named for | Located on egg-shaped intermittent island; rock outcropping may appear egg-shaped from some perspectives |
Egg Rock is an outcrop of Silurian Straw Hollow Diorite[1][2] at the confluence of the Assabet and Sudbury rivers, where they form the Concord River in Concord, Massachusetts. The outcrop is located on a roughly oval intermittent island of about 100 by 50 meters. Egg Rock is usually accessible using foot trails over land, but during high river levels the island is separated from the mainland by a narrow channel. The highest point of Egg Rock is about 39 meters above mean sea level and about 6 meters above normal river level.
The United States Geological Survey (USGS) Geographic Names Information System (GNIS) includes Egg Rock as GNIS feature 617309, classified as an island. In the GNIS database as of February 2010, the listed position (latitude 42.4645383, longitude -71.3592266) is misplaced by about 125 meters to the southwest, and is not actually located on the intermittent island. A more correct position is latitude 42.4651, longitude -71.3585.[3]
- ^ Introduction by John McPhee to H.D.Thoreau's “A Week on the Concord and Merrimack Rivers,” [Note the inscription on Egg Rock is misquoted.]
- ^ USGS state geological data: map and detail of rock type
- ^ Satellite imagery incorporating GNIS data (such as that available via "Google Maps") clearly shows the divergence between the GNIS feature location and the actual Egg Rock land form