New Haven's best-known geographic features are its large deep harbor, and two reddish basalt trap rock ridges which rise to the northeast and northwest of the city core. These trap rocks are known respectively as East Rock and West Rock, and both serve as extensive parks. West Rock has been tunneled through to make way for the east-west passage of the Wilbur Cross Parkway (the only highway tunnel through a natural obstacle in Connecticut), and once served as the hideout of the "Regicides" (see: Regicides Trail). Most New Haveners refer to these men as "The Three Judges". East Rock features the prominent Soldiers and Sailors war monument on its peak as well as the "Great/Giant Steps" which run up the rock's cliffside.

In addition to the New Haven Harbor, what additional natural element is a prominent geographic feature of the area, flanking the northeast and northwest boundaries?