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

East Rock

92 | New Haven’s Sentinels East Rock The East Rock complex is composed of four hills, Whitney Peak, East Rock, Indian Head and Snake Rock. The tectonic setting of East Rock appears to be similar to that of West Rock. Its magma(s) also rose along a curved fault that dipped east. After gravity resisted its further rise, it too entered the overlying sediments. Both the East Rock and West Rock sills probably intruded at comparable stratigraphic levels in the New Haven sandstone formation, but the former intruded westward and the latter eastward, both away from the border faults along the highlands and toward the center of the basin; the area with the least sediment overburden. East Rock’s northern segment trends northnortheast ; its southern fringe bends east into Indian Head and ends in an east-west trending 45. LiDAR elevation map of East Rock and Mill Rock,2000. (CLEAR,Center for Land Use Planning and Research,Haddam CT.) Courtesy Emily Wilson. This photo shows the topography of the area in detail.Mill Rock,a dike,extends east and connects to Whitney peak.East Rock exhibits the same eastward turned“nose”as does West Rock and is separated from Indian Head by a narrow valley formed by erosion of a northeast trending fault.A similar feature separates Indian Head from Snake Rock.The smooth hillsides north of Mill Rock/East Rock and Indian Head were formed by an accumulation of glacial till and show the direction in which the land ice moved. Tectonic Setting of New Haven’s Rocks | 93 dike. The curvature most likely resulted from the presence of a cross fault that caused a rather localized impediment to magma flow. The cooling columns in East Rock’s western flank, where the hill reaches a height of about 360 feet, dip west and there the body is clearly a sill intruding reasonably parallel to the layers of sandstone below it. Its eastern flank, however, is an eastward dipping dike that cuts steeply through the sandstone layers. The change from feeder dike to sill during its emplacement thus appears to have occurred close to the present erosion level. 46A. John W.Barber (1798–1885), W. Side of the East Rock. (John W.Barber,1831.) In this view,Barber clearly shows the scarp and columnar structure of the mountain.Three figures admire the view,providing a good measure of its height.To the far left is Ithiel Town’s bridge constructed over the Mill River. 94 | New Haven’s Sentinels 46B. Topographic map of East Rock. (Chester R.Longwell and Edward S.Dana,1932.) This beautiful map of the East Rock complex sensu lato clearly shows that the thickness of the original sill varied laterally.No clear evidence exists for a connection between Whitney Peak and the Mill Rock dike.The East Rock slab was intersected and offset along several northeasterly trending faults that were widened by erosion. 47. Cross section of East Rock,showing the change from feeder dike to sill. (James Dwight Dana,1891.) This section shows how the East Rock sill was generated by magma moving up along an eastward dipping fault zone,before using preexisting bedding planes in the older sandstone formation (yellow) to intrude westward.The presence of two dikes separated by an inlier of sandstone suggests two intrusive events,similar to what happened at West Rock. Indian Head is a smaller version of East Rock. It too bends from a steeply east-dipping dike into an eastward tilted sill. Since these intrusive masses are in the eastern part of the geologic Connecticut Valley, one would expect their feeder to have been provided by one or more faults that formed the eastern border of that rift zone; instead it was one of the western border faults of the Gaillard rift that became the feeder for the East Rock complex. The latter secondary tectonic depression is a rift within a rift, rimming the Eastern border fault of the Connecticut Valley. The presence of the Foxon-Fairhaven feeder dike and four (Talcott type) flow units in this secondary depression indicates that there were at least four volcanic pulses inside the Gaillard rift, more or less contemporary with emplacement of the East Rock dike/sill system. • ...