Egg Rock, an outcrop of Silurian Straw Hollow Diorite, marks the confluence of the Assabet and Sudbury rivers, where they form the Concord River in Concord, Massachusetts. The rock sits on a roughly oval intermittent island about 100 by 50 meters; most seasons you reach it by foot trails, though in high river levels a narrow channel separates the island from the mainland. The highest point here rises to about 39 meters above mean sea level, roughly 6 meters above normal river level.
An inscription was carved into the rock in 1885 to commemorate the 250th anniversary of the 1635 founding of Concord, a mark visitors have long noted. Officially, the United States Geological Survey lists 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 for Egg Rock is latitude 42.4651, longitude -71.3585.
Look now at the meeting of the two rivers and the oval island beneath your feet.