sentences:  With a population density of 4,955 people per square mile (water excluded), Union County was the 15th-most densely populated county in the United States as of the 2010 Census, and third-densest in New Jersey, behind Hudson County (ranked 6th nationwide at 9,754 per square mile) and Essex County (ranked 11th at 6,126).All of present-day Union County was part of the Elizabethtown Tract, which was purchased in 1664, by English colonists from the Lenape Native Americans that lived in the area of present-day Elizabeth, New Jersey.Union County was formed on March 19, 1857, from portions of Essex County; it was the last of New Jersey's 21 counties to be established.