East and West Rock, which dominate New Haven’s northern horizon, have often been referred to as the town’s sentinels. Benjamin Silliman, Yale’s eminent geologist, described them as “composed of precipitous cliffs of naked frowning rock, hoary with time...an immense work of art.” More than a dozen mid and late19th-century artists, who specialized in the back-to-nature movement, fully agreed and ce…