Geology and Geography   (Previous << Contents >> Next)

From Warren History, Volume One, No. 10, Fall 1993

At the end of the Paleozoic period, about 200 million years ago, what is now New Jersey looked vastly different from what it does today: To the northwest, great mountains stood proudly, their majestic peaks 30,000 feet above sea level; eastward from the mountains lay a shallow sea that covered all of south and central New Jersey.

New Jersey Geology (click on image to enlarge)

During the Mesozoic, periods of torrential rain alternated with scorching dry spells, as many as 10,000 of them, some a century in length. As the rain tore at the bone-dry mountains, erosion carried unimaginable quantities of silt, sand and clay easterly toward the sea, gradually building up a plane that spread south and east from the mountains, displacing the Atlantic. These sediments, known as the Newark sediments, created the Triassic Basin. The uppermost of them, called the Brunswick (so named for its outcroppings along the Raritan River at New Brunswick) is estimated to be 10,000 feet thick. A soft red shale mixed with sandstone, it is the loose sediment worn by the rains and rivers from the shale and sandstone mountains to the west, and then compressed by the forces of nature and time into stone again.

"Sometime near the end of the Late Triassic Epoch, as the great basin that is today central and northeastern New Jersey became filled with the thousands of feet of sediments that are now identified as the Newark series. another significant geologic process was in operation," writes Christopher Schuberth in his geology of this area. "At least three major periods of volcanic eruptions resulted in the flow of extensive lava sheets of basaltic composition. Broad areas of the Triassic Basin were successively covered by lava flows ranging in thickness from 350 to 850 feet and extending up to 30 miles in length across the then unconsolidated Brunswick sediments. The first series of lava flows, estimated to have an aggregate thickness of approximately 650 feet, forming the present-day First Watchung Mountain, were subsequently buried by sand and mud of the Brunswick formation. That is to say, the eruptions were followed by a period of volcanic quiescence during which time sedimentation continued as before and buried the preceding lava flows with about 600 feet of sandstone and shale. Then a second series of flows, totaling approximately 850 feet thick and forming the present-day Second Watchung Mountain, covered these sandstones and shale. These flows also were buried, but this time by about 1200 feet of sediments during a rather long interval of volcanic inactivity. Finally, a third episode of volcanic activity spewed lava over these still younger Brunswick shale. Today, Riker Hill, Long Hill, Hook Mountain, and Packanack Mountain are the remnants of these last flows. This series of flows eventually was covered by the youngest Brunswick sediments."

"It should be emphasized that each of the three ridges consists of several distinct lava flows rather than just one several-hundred-foot thick flow. There is abundant evidence that each lava flow underwent considerable weathering and erosion before it was buried by the next flow during the major eruptive cycle. The three composite sheets of extrusive igneous rock today stand out in bold relief in the central and western part of the New Jersey Lowland -- the Watchung Mountains and the lesser ridges, such as Riker Hill or Hook Mountain, are in sharp contrast to the easily eroded red shale of the Brunswick formation."

Had nothing further occurred, the great volcanic flows might have remained buried forever beneath of the Newark sediments: There would have been no Watchung Mountains, or the lesser ridges to the west. But in the Late Triassic this entire region, including the Newark sediments and buried lava flows, was tilted westerly by enormous tectonic forces so that, as now, it was inclined 15-20 degrees from the horizontal toward the northwest. Then, gradually, imperceptibly, during the millions of years that followed this massive shifting of the earth, water eroded away the Newark sediments, exposing the jutting edges of the buried lava flows. These exposed basaltic edges, now much weathered by millions of years of rain, are what we call the First and Second Watchungs. Between them, in what we know as the Washington Valley, lays the crumbly red Brunswick shale. Basalt, known locally as trap rock (from the Swedish word "trapp," meaning stairs), is extensively quarried for use in road construction, ballast and rip rap. It is also rich in minerals, formed as water flowed over and through the cooling lava year upon millions of years. Amethyst, smoky quartz, prehnite, stilbite and chabazite are only a few of the 60-odd minerals found in the Watchungs. Copper is also relatively abundant. One of the finest collections of these minerals may be seen today at the American Museum of Natural History's Hall of Minerals and Gems.

The geologic does not end with the Late Triassic, l25 million years ago, or with the uplifting and tilting of the land during the Cenozoic. In relatively recent time, l50,000 to 200,000 years ago, great masses of ice advanced southward, scouring the land, blocking rivers, creating new lakes. Moving south at the rate of a foot each day, an impenetrable mountain of ice, perhaps 2000 feet thick, spread over New York City and environs. These glaciers advanced and retreated in ponderous cycles, finally departing the region 10-20,000 years ago.

"When the Wisconsin glacier pushed across New Jersey it also blocked the exits of such rivers as the Passaic," writes Schuberth. "With the ice margin acting as a dam, melting water was not able to flow freely away from the glacier and necessarily had to accumulate against high land on the south. In this way, several areas became flooded and turned into marginal glacial lakes. Perhaps the best-known of these coldwater lakes was Glacial Lake Passaic. With the Watchung Mountains along the eastern and southern flanks and the Ramapo Mountains along the northwestern border, Lake Passaic filled the natural basin south of the terminal moraine in the vicinity of Summit and Morristown.... It occupied an area that is today the valley of the lower Passaic River, and its southern shore followed the recurved basaltic rock of the Second Watchung Mountain."

At its greatest size, Glacial Lake Passaic was about 30 miles long, 8-10 miles wide and 240 feet deep at its maximum. The lake endured for thousands of years, depositing some 80 feet of glacial clay on its bottom. When the Wisconsin glacier finally retreated past the Paterson gap, the Passaic River flowed again to the sea and the lake gradually drained, aided by an uplifting of the land now freed from the weight of the ice. The Great Swamp and nearby meadows are the clayfilled remnants of the glacial lake's bottom. 

Warren's geologic history extends from the unimaginable past to the relatively recent. The traveller from Green Brook to Bernards journeys first up the sharp edge of a basaltic lava flow and down its gently sloping side (First Watchung Mountain), then across a valley of Brunswick shale and up again (Second Watchung Mountain) and down again toward the Passaic and Dead Rivers. In his journey of a few minutes, he has crossed mountains of great antiquity, the reddish remnants of mountains even older, and the clay bed of a youthful lake formed by New Jersey's last glacial visitor. Few communities in the state can boast of such great geologic diversity, or the natural beauty left behind as the legacy of nature's wonderful forces.

REF: Kemble Widmer, The Geology & Geography of New Jersey, Princeton, l964; Christopher J. Schuberth, Geology of New York City and Environs, New York, l968.