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As you drive the length of New Jersey, from its southernmost point to its northwestern tip, you pass through a series of distinctive landscapes. Each one owes its special character to the geological deposits below the surface soil and to the geological forces that shaped them over the course of thousands or millions of years. From the seashore to the mountains, New Jersey's variegated scenery reflects the state's geological history. The deposits of clay, sand, and rock form a remarkably representative sample of geologic periods and contain equally good samples of the organisms that lived in those times. The progression of New Jersey's geological record is fairly straightforward; the youngest deposits are in the south and east, while the rocks are generally older and older as one goes northwest to High Point. So, if you start at Cape May Point and travel northward, you are traveling back in geologic time.
Cape May and the barrier beach islands of the coastline from Wildwood to Sandy Hook are the most recent additions to New Jersey. They are products of the last 10,000 years, built by waves and wind as sea level rose after the great glaciers of the Ice Age melted and released their water back into the ocean, This encroachment of the sea is only the most recent inundation of the ocean waters over previously exposed land; as we shall see, New Jersey has been covered by marine waters many times in the geologic past.
The Pinelands are largely underlain by the Cohansey Formation and along the northwesternoak-pine fringing forest by the Kirkwood Formation. Geologic formations are distinctive bodies of sedimentary rock that can be mapped on the basis of their composition; formations are named after geographic localities where they are well exposed. The Cohansey Formation, for example, is largely composed of quartz sand that can be seen (and mapped) throughout the Pinelands; the formation is named after a town in Cumberland County. Both the Cohansey and the Kirkwood were laid down in the Miocene Epoch, between 20 million and 5 million years ago. The Kirkwood Formation contains fossils of sea creatures. In Monmouth and Salem Counties abundant mollusk shells, the teeth of giant great white sharks, and the bones of prehistoric whales indicate that the sea flooded the land early in the Miocene. The Cohansey sand records the retreat of that Miocene sea across southern New Jersey. The white sand of the Cohansey aquifer holds abundant supplies of groundwater, but very few fossils. In the eighteenth and nineteenth centuries, the sand was used as a source of silica for making glass, and in some of the old sandpits a few poor molds of fossil seashells have been found.
The higher hills of the Pinelands are topped by yellowish and orange gravel thought to have been deposited between 5 million and 100,000 years ago in swift-moving streams. Some of the pebbles in these gravels contain the fossils of very old animals, sea creatures that lived around 400 million years ago. The presence of these fossil-bearing chert pebbles in the midst of much younger sediments shows that the gravels were derived from the older rocks of the mountains to the northwest, where the bedrock contains fossils of the same kinds of prehistoric organisms.
The inner part of the coastal plain is separated from the outer Pinelands by a low ridge that forms an intermittent drainage divide. Such a low coastal ridge is called a cuesta. The highground of the cuesta gives its name to such places as Atlantic Highlands, Cream Ridge, Arney's Mount, Mount Holly, and Mullica Hill. The inner coastal plain, home to deciduous forests of broad-leafed trees, farms, suburbs, and cities, stretches from Raritan Bay in the northeast diagonally across the state to the Delaware River near the top of Delaware Bay. This area is underlain by soft loose sediments of clays, sands, and marl, deposited from about 100 million years ago to 50 million years ago. In places, these loose sediments are richly fossiliferous, yielding remains of animals and plants from around the end of the age of dinosaurs. In fact, the South Jersey marls were the scene of the first major dinosaur discoveries in North America.
Along a line stretching from Trenton to New Brunswick, the sands and clays of the coastal plain are abruptly replaced by reddish rocks, primarily shales and sandstones. These are the beds of the Newark Supergroup, stretching from the Hudson River southwesterly across the state to the Delaware River, and continuing across southeastern Pennsylvania toward Maryland. These hardened sedimentary rocks were laid down in a great trough called the Newark Basin from about 220 million years ago to 185 million years ago. The redbeds are mostly devoid of fossils, but here and there they reveal traces of animals from the beginning of the age of dinosaurs. This sequence also contains gray shale, the hardened remains of the mud laid down at the bottom of large lakes; fish and aquatic reptile fossils have been found in these gray beds.
The northwestern margin of the Newark Basin is abruptly bounded by very old rocks of the Highlands province that are very much like some of the ancient rocks of New England. The hard crystalline rocks here form low mountains and hills. These are igneous and metamorphic rocks of the Precambrian age, one to two billion years old. No fossils have been found in these rocks, but younger deposits of Paleozoic sediments found in the intervening valleys do have occasional fossils in them. These fossils are the same age as the fossils found in the Paleozoic rocks in the northwestern part of the state, in the valleys and ridges of the Appalachian Mountains. The remains found in the mountain valleys are mostly of primitive sea creatures such as trilobites and brachiopods. They represent ancient ecosystems that flourished on prehistoric seafloors which have in the long course of geologic time been turned into rock and thrust up into mountain ranges.
So each part of the state has its distinctive kinds of rocks and, embedded in them, unique groups of fossils. The nature of the rocks and the fossils found in them depends upon the age of the deposits. In layered sedimentary rocks, the rocks at the bottom of the stack, deepest down in the earth, are the oldest rocks. The youngest rocks in the sequence will be at the top of the stack, closest to the surface. This concept of correlating depth with age of rock strata is known as the Principle of Superposition, and while there are some exceptions to the usual order (for instance, special cases associated with mountain building), it is the fundamental idea behind dating sedimentary rocks.
By the beginning of the nineteenth century, it was widely recognized that fossils were the remains or in some cases traces of prehistoric organisms. At about this time British geologists realized that distinctive suites of rock had characteristic fossils contained within them. Trilobites of a certain species, for example, could be found only in one or at the most several rock formations, but the vertical range of their occurrence in the sedimentary stack was definitely limited. By carefully studying and mapping the various rock formations in England and Wales, the early geologists demonstrated that unique assemblages of fossils occurred in a predictable order within stratified sequences of sedimentary rocks. This set of observations was formally called the Principle of Faunal Succession.
Around the same time in France, the famous anatomist Baron Georges Cuvier was acquiring a reputation for identifying animals from portions of their skeletons. Cuvier identified many fossil bones, and he demonstrated that fossil elephant skeletons excavated in Paris did not belong to any living species of elephant. Cuvier thus showed that animal species could become extinct, and in fact he went on to postulate that numerous groups of vertebrate (backboned) animals had succeeded each other in the rock record. He even hypothesized that a series of mass extinction events, or revolutions as he called them, had wiped out large portions of prehistoric faunas in the past. When an old assemblage of animals became extinct, a new group replaced them, only to become extinct in turn. One of Cuvier's triumphs was the identification of the "monster of the Meuse." Along the Meuse River in Belgium in 1770, ancient chalk mines yielded the well-preserved skull of a gigantic prehistoric reptile. Originally it was identified as a crocodile. When the French Revolutionary Army invaded Belgium in 1795, this skull was brought back to Paris as one of the spoils of war. Cuvier studied it and pronounced it a giant sea-going lizard related to the modern monitor lizard family. He named it Mosasaurus, meaning "lizard of the Meuse" in Latin.
As geologists worked all over Europe to determine the vertical order of the rocks and the fossil faunas contained in them, they began to appreciate the enormous length of geologic time needed to lay down the thousands and thousands of feet of finely layered sediments now exposed as sedimentary rocks. They could use the Principle of Faunal Succession to identify equivalent rock successions separated by great distances, utilizing fossils to correlate outcrops of sedimentary rock that were separated by many miles. The Principle of Superposition allowed them to judge which rock layers were older and which were younger, relative to each other. Various estimates were made of the length of geologic time, but no direct way of determining absolute dates for rocks and fossils was available until the twentieth century. As research into the phenomenon of radioactivity progressed during the early twentieth century, scientists began to realize that the property of radioactive half-life decay could be used to provide absolute dates for minerals containing the right radioactive substances. Each kind of radioactive element emits energy at its own characteristic rate; as this radioactive energy is emitted, the radioactive element is gradually converted to a stable nonradioactive substance. This happens at a rate known as the radioactive half-life. After one half-life, one half the original amount of radioactive substance is present; after two half-lives, one quarter of the original amount is left; after three half-lives, one-eighth of the original remains; and so on. The ratio of the amount of original radioactive element to its nonradioactive product depends on time, in particular, how many half-lives have passed since the mineral crystal formed in the rock. This principle gave geologists a more accurate, absolute way of dating rocks and the fossils contained in them. Perhaps the most widely known radioactive dating techniques employs the decay of carbon-14 in organic remains, but there are various other radioisotopes that can be used for the purpose of dating rocks of different compositions and ages.
In the early 1820s, a series of fossil finds in England led to the discovery of an entirely new, previously unknown group of extinct animals. At Oxford, the Reverend William Buckland and his colleagues were studying the giant bones and teeth of a prehistoric reptile, which they named Megalosaurus ("giant lizard"). In the southeast of England, a country doctor named Gideon Mantell had dug up many fossils, including the remains of a large extinct plant-eating reptile, which he eventually called Iguanodon ("iguana tooth").
By 1842, several more such creatures had been found. A young British anatomist named Richard Owen inspected the bits and pieces and found some common features uniting these prehistoric reptiles. He deduced from the structure of the hip and limb bones that these animals stood, as do modern mammals and birds, with their legs underneath their bodies, and not sprawled out to the sides, like modern reptiles such as lizards and crocodiles. Owen felt confident enough to declare that they belonged to no known branch of the reptiles: they required a new taxonomic category. He called them the Dinosauria, meaning "terrible lizards." The erect posture of the limbs was what distinguished the Dinosauria from all other reptiles.
Working under Owen's supervision, the British artist Benjamin Waterhouse Hawkins constructed enormous "lifelike" statues of Megalosaurus and Iguanodon for the Crystal Palace Exposition of 1854 in London. Hawkins followed Owen's instructions and made his dinosaurs into giant four-legged elephantine reptiles. The interior of the unfinished Iguanodon model was the scene of one of the most unique New Year's Eve parties in history; Owen invited twenty-one scientists to a dinner party inside Iguanodon on December 31, 1853. With a horn upon its nose, the quadrupedal dinosaur model looked like a gargantuan reptilian rhinoceros.
There matters stood until 1858, when a famous discovery in New Jersey revolutionized our image of dinosaurs and changed the course of dinosaur paleontology.
|List of Illustrations|
|Preface and Acknowledgments|
|Ch. 1||Fossils, Strata, and Time||1|
|Ch. 2||The Deep Past: New Jersey Before the Dinosaurs||9|
|Ch. 3||Who Are the Dinosaurs?||19|
|Ch. 4||New Jersey: Birthplace of American Dinosaur Paleontology||27|
|Ch. 5||The Earliest Dinosaurs||41|
|Ch. 6||Heyday of the Dinosaurs||59|
|Ch. 7||Cretaceous Sea Life||71|
|Ch. 8||The Last Dinosaurs||89|
|Ch. 9||The Great Extinction||113|
|Ch. 10||After the Dinosaurs||129|
|App. A||Where to See Dinosaurs and Other Fossils in and around New Jersey||147|
|App. B||Methods for Studying Dinosaur Footprints||149|
|App. C||How to Find Fossils in New Jersey||153|