The Watchung Mountains (once called the Blue Hills) are a group of three long low ridges of volcanic origin, between high, lying parallel to each other in northern New Jersey in the United States. The Watchung Mountains are known for their numerous scenic vistas overlooking New York City, Newark and New Jersey skylines, as well as their isolated ecosystems containing rare plants, endangered wildlife, rich minerals, and globally imperiled trap rock glade communities. The ridges traditionally contained the westward spread of urbanization, forming a significant geologic barrier beyond the piedmont west of the Hudson River; the town of Newark, for example, once included lands from the Hudson to the base of the mountains. Later treaties moved the boundary to the top of the mountain, to include the springs. The Watchungs are basalt uplifts, geologically similar to the Palisades along the Hudson river. In many places, however, the mountains have become sinuous islands of natural landscape within the suburban sprawl covering much of contemporary northeastern New Jersey. Parks, preserves, and numerous historical sites dot the valleys and slopes of the mountains, providing recreational and cultural activities to one of the most densely populated regions of the nation.