New Haven’s Sentinels: The Art and Science of East Rock and West Rock

2. Topography and Geology of the New Haven Area

Topography and Geology of the New Haven Area • Pass from the forest—here we stand upon Rocks piled on rocks, and hills promiscuous thrown; A sheer, bare cliff, that downward looks so far, All objects seem but half the size they are. W. T. Bacon, 1848 47 New Haven was settled in 1638 on a relatively flat plain at the southernmost end of Connecticut’s “Central Valley,” a giant geologic “ditch” which originated some 230 million years ago and slowly filled with sediments and volcanic rocks over a period of about 50 million years (Image 23). This northerly-trending Early Mesozoic trough, which extends into Western Massachesettes, is shown on Connecticut’s LiDAR map (Image 24A) in green and on Connecticut’s geologic map (Image 24B) in yellow. Between 230 and 200 million years ago, a period known as the Late Triassic, as much as 3000 feet of sandstone accumulated in a geologic depression below the present site of New Haven. To the north, the city is bound by the Four Rocks—igneous masses of Early Jurassic age, ca. 201 million years old—that form the curved chain of hills rising rather abruptly above the plain and reaching heights of more than 600 feet above sea level (Image 3). Volcanism resulted from the spreading and thinning of the Appalachian crust when the North American and African continental masses began to separate. Initially only older, preexisting faults were reactivated, but in the Early Jurassic new sets of fractures sliced through the geologic terranes and became sub-vertical pathways allowing red-hot magma, which had accumulated at depth, to rise and flow out onto the surface. Locally, voluminous batches of magma remained at depth and were squeezed into contiguous sandstone formations. The Four Rocks in the New Haven area belong to the latter group, commonly referred to as diabase sills (or laccoliths) and dikes. Long geological periods of 23. LiDAR (Light Detection And Ranging) map of the New Haven and Hamden region,2000. (CLEAR,Center for Land Use Education and Research,Haddam CT.) Courtesy Emily Wilson. This photo provides a detailed view of the lowlands of New Haven bordered by the Eastern and Western Highlands and the Four Rocks,its Sentinels.The northerly, elongated hills are glacial drumlins.They formed to the south of Pine Rock and Mill Rock and show the direction in which the land ice moved.Note the Yale Bowl and rowing channel. 48 | New Haven’s Sentinels 24. Geological map of the southern-most part of the Central Valley. (Geology of Natural Parks,United States Geological Survey.) This sketch map shows the distribution and extent of the Early Jurassic (~200 million year old) intrusive diabase bodies of East Rock and West Rock sensu lato (in orange) and contemporary lava flows of the Metacomet Ridge complex (in red).Of special interest is the S-shaped pattern of the intrusive units. Topography and Geology of the New Haven Area | 49 fracturing, tilting and erosion followed resulting in their present-day configurations. The northerly elongated Sachem’s Ridge and Beaver Hills on Image 23 are glacial drumlins. They formed “downstream” Pine Rock and Mill Rock when those hills resisted the southward advance of land ice in the Late Pleistocene. The earliest geologic map that clearly indicates the locations of the igneous rocks was published in 1842 by James Gates Percival as part of his extensive report, The Geology of the State of Connecticut. Percival surveyed the entire state from east to west at two-mile intervals traveling on foot and horseback, and mapped most major geologic formations with astounding accuracy. Half a century later, James Dwight Dana, Silliman’s successor at Yale College, published a booklet On the Four Rocks of the New Haven Region: East Rock, West Rock, Pine Rock and Mill Rock, in which he described these igneous masses and provided detailed maps. For his fieldwork, Dana relied to a large degree on Percival’s geologic map (Image 4) and the U.S. Coast and Geodetic Survey’s topographic map published in 1877. The United States Geological Survey published a sketch map showing the distribution of igneous rock complexes in the New Haven region in 2000. Intrusive bodies (sills) dominate in the western part of the rift valley and tilted lava flows are common in its eastern section. • 25A. LiDAR map of Connecticut,2000. (CLEAR...