David P. Harper

This guidebook is set up a little differently than those for the less populated western states. Driving slowly along New Jersey highways to gaze at passing geology is a good way to get rear-ended by a semi; according to author David Harper, it’s illegal to stop on any New Jersey highway (not just the interstates) to examine outcrops. Therefore, rather than the odometer and milepost logs used in the West Roadside Geology of New Jersey uses more of a point-to-point system, explaining the geology around successive towns along the route and directing the user to state or local parks for more detailed examination.


The large geological province divisions in New Jersey geology are (from east to west) Cretaceous/Cenozoic coastal plain formations; Jurassic rocks of the Newark Basin, associated with the rifting that formed the Atlantic Ocean; Proterozoic rocks of the New Jersey highlands associated with an active oceanic plate margin that caused the Grenville mountains during the formation of the Rodina supercontinent; and Paleozoic rocks of the Valley and Ridge province associated with a backarc basin during the Taconic, Acadian and Alleghanian orogenies during the closing of the Iapetus Ocean. Superimposed over all this are various glacial features from the Pleistocene; ancient river channels, flood outlets from periglacial lakes, outwash plains, moraines, kames, eskers, and wave-cut shorelines from when sea level was much higher during interglacials.


New Jersey was thus at the center of two continental collisions – in the Proterozoic to form Rodina from Laurentia and Amazonia, and in the Paleozoic to form Pangea from just about everything; and two rifting episode, in the late Proterozoic to form the Iapetus Ocean and in the Jurassic to form the Atlantic. You can see how earlier geologists interpreted this using the “geosyncline” concept, with great trenches forming along a continental margin, filling with great heaps of sediment (which actually did happen, since they were active subduction zones), then somehow inverting to form mountain ranges (which required various armwaving explanations), then repeating. The east coast of the US, within easy reach of local American and travelling European geologists, was more or less the “type locality” for the geosyncline concept.


Continental suture zones are often centers of mineralization due to hydrothermal activity along fractures and New Jersey is no exception; you wouldn’t normally think of New Jersey as a mining state, but to my considerable surprise, it was. In the 1890s, just before the discovery of the Mesabi Iron Range in Minnesota, New Jersey was the largest iron producing state in US. The Franklin Mining District was one of the world’s largest zinc producers; mining started in the 17th century and the last mine didn’t shut down until 1986 (the tailing heaps, where accessible, remain the world’s largest producer of fluorescent minerals). New Jersey being what it is, leftover mine shafts used to dump chemical waste eventually became some of the world’s most complicated environmental remediation sites; I had enough trouble with little Diesel leaks from underground storage tanks to appreciate what it must have been like to remediate a shaft sloping at a 30° angle, 750 feet deep, and filled with assorted rusting drums of every chemical waste product generated in the 20th century.


Harper mentions a couple of geologic influences on the state’s borders. The New Jersey – New York land border was originally supposed to be a straight line; when it was surveyed in the 18th century it was done with nautical compasses (even though astronomical methods were known to be more accurate). Unfortunately the border area contains large magnetite deposits and the compass-surveyed line ended up subtle bends, as more than 2000 feet too far south in some places. New Jersey wanted the boundary resurveyed in the 19th century, but even though almost all the land swaps would have favored them, New York demurred. On the other border, there’s a peculiar circular land boundary between Delaware and Pennsylvania, based on a 12-mile radius circle from the cupola of a courthouse (the actual border isn’t a perfect arc because the technology to survey one didn’t exist in 1750). Although the land border arc doesn’t extend into New Jersey, the Delaware arc stops right at the New Jersey low tide shore of the Delaware River, not at the center of the channel - which makes up the Pennsylvania and the rest of the Delaware borders. Thus the river border follows the centerline until it hits the continuation of the Delaware arc, then follows the arc to the New Jersey edge of the river, continues to follow the shoreline until it reaches the other end of the arc, then moves out to the centerline again; for a short distance Delaware controls the entire river channel, not just its side of the centerline. Delaware has used this arrangement to block several New Jersey river projects, and New Jersey has brought Supreme Court cases several times to attempt to have the boundary rationalized; the last time the Court specifically prohibited the states from ever raising the issue again. Given the state’s reputation for less than peaceful solutions to turf wars, it’s not that surprising that various New Jersey lawmakers made vague threats to reactivate the museum ship New Jersey and send it to deal with Delaware; however as of this date the New Jersey remains docked in Camden.


Quite well done given the constraints. There are a lot of pictures of outcrops, which should help to make sure you’re looking at the right place (no GPS coordinates, though). A few rock types are illustrated by stone walls or buildings, in cases where the necessary outcrop is covered, on private land, or otherwise inaccessible, and there are directions to various mining and mineral processing museums since the original mines are long since closed. The geological maps are in all cases excellent; the reference list is extensive and mostly recent (although there’s one reference section for the entire book, rather than for individual geological provinces).… (más)