Others do not believe that there ever was very advanced civilization on earth and all of the superb knowledge was handed down to us by ancient astronauts who came down from heaven.
Most scientists do not subscribe to any of these ideas and try to explain ancient enigmas on the ground of religion and ritual...
In this section we are presenting few sites that are still causing controversy in the scientific community.
A Link between Egypt and Americas?
According to the official view there was no contact between the Old World and the New World before Columbus. Yet numerous similarities have been found that suggest a link between Egypt and the Americas:
- Both have huge pyramids, aligned to the cardinal points
- Both have structures built with megalithic stones and extremely fine joints
- Both exhibit intriguing bumps on many unfinished stone blocks (Picture 1)
- Both employed a unique style of construction using "L" shaped corners
- Both use the same style of metal clamps to hold the huge stones in place
- Both used the process of mummification to preserve and honor their dead
These compelling similarities suggest that both ancient cultures were influenced by a sophisticated common source.
A Link between Mesopotamia and Americas?
Some researchers reject the dogmatic idea that the New World was isolated prior to 1492. There is some compelling evidence that the ancient peoples from Mesopotamia may have crossed the Atlantic ages before Columbus.
The images below represent very much the same religious symbolism. Also, please notice the object held by the deity (it looks the same).
Mythic Olmec figure with Jaguar mask on Quetzalcoatle, sculpture from the ancient Olmec site of La Venta (Mexico). Olmecs are considered to be the oldest Meso American Culture.
The Olmecs
Holding a cone or sponge in his right hand and a bucket in his left, this genie is performing a ritual involving the Assyrian sacred tree. The sacred tree was an extremely important symbol in the palace of Ashurnasirpal, appearing on reliefs in virtually every room of the palace. It was also used in textile patterns, on stamp and cylinder seals, and in ivory carvings. It represented both the king and Ashur, the chief god of Assyria, and was also a symbol of the fertility of the land.
Winged Eagle-Headed Being (Genie); Neo-Assyrian period, reign of Ashurnasirpal II (r. 883–859 B.C.)Mesopotamia;excavated at Nimrud (ancient Kalhu)
Alabaster (gypsum)
Alabaster relief from Nimroud, in the Louvre.
Drawing, offering by the god, by Saint-Elme Gautier. In Egyptian mythology Horus, the son of Osiris, is shown as a hawk and a winged solar disk.
Winged Genie. Iraq, from Nimrud (Kalhu), Northwest Palace, Room L. Neo-Assyrian Period, reign of Ashur-nasir-pal II, circa 883–859 B.C. Alabaster, 93 1/16 x 80 13/16 in. (236.3 x 205.3 cm).
The Brooklyn Museum
Bearded, Winged Being (Genie); Neo-Assyrian period,
reign of Ashurnasirpal II (r. 883–859 B.C.)Mesopotamia;
excavated at Nimrud (ancient Kalhu). Alabaster (gypsum)
Mysterious Alignment
Another puzzle is mysterious alignment of world's ancient sites. Easter Island is exactly aligned along a straight line around the center of the Earth, with the Nazca lines, Ollantaytambo and the Great Pyramid of Egypt. Other world wonders that are within one tenth of one degree of this alignment include: Perseopolis, the capital city of ancient Persia; Mohenjo Daro, the ancient capital city of the Indus Valley; the Oracle of Zeus-Amon at Siwa; and the lost city of Petra. The Ancient Sumarian city of Ur and Angkor temples in Cambodia and Thailand are within one degree of latitude of this alignment.
The alignment of these sites is easily observable on a globe of the Earth with a horizon ring. If you line up any two of these sites on the horizon ring, all of the sites will be right on the horizon ring. 3-D world atlas software programs can also draw this line around the Earth. Start on the Equator, at the mouth of the Amazon River, at 48° 36' West Longitude; go to 30° 22' North Latitude, 41° 24' East Longitude, in the Middle East, which is the maximum latitude the line touches; then go to the Equator at 131° 24' East Longitude, near the Northwest tip of New Guinea; then to 30° 22' South Latitude, 138° 36' West Longitude, in the South Pacific; and then back to 48° 36' West Longitude, at the Equator.
Centered on: |
0.00° N 48° 36' W | 30° 22' N 41° 24' E | 0.00° N 131° 24' E | 30° 22' S 138° 36' W |
The circumference of this line around the center of the Earth is 24,892 miles. Along this line, the great circle distance from the Great Pyramid to Ollantaytambo is 7,472 miles, 30.0% of the circumference. Ollantaytambo is 2,579 miles from Easter Island, 10.3%. Easter Island is 10,096 miles from Angkor Wat, 40.6%. Angkor Wat is 2,490 miles from Mohenjo Daro, 10.0%. Mohenjo Daro is 2255 miles from from the Great Pyramid, 9.1%. In addition to calculating the distances between these sites as a percentage of the circumference of the Earth, the distances may also be calculated in degrees of the 360° circumference, by multiplying the percentage by 3.6. For example, the Great Pyramid is 108° away from Ollantaytambo. Near Ollantaytambo, Machupicchu is within one quarter of a degree and Cuzco is within one third of a degree of the alignment.
The Adirondack Mountains:
New Mountains From Old Rocks
Text and Figures adapted from Chapter 4 of"The Geology of New York: A Simplified Account"
by the NYS Geological Survey
Summary
The rocks of the Adirondacks, almost without exception, are metamorphic. They have been subjected to high temperatures and pressures at depths of up to 30 km in the earth's crust. Most of the rocks in the Northwest Lowlands are metasedimentary or metavolcanic and have a complex history. Most of the rocks in the Central Highlands are metaplutonic; granitic gneiss is the most common. Metanorthosite forms several large bodies in the Central Highlands; the largest makes up the High Peaks area. Olivine metagabbro bodies are scattered throughout the eastern and southeastern Adirondacks.
The Adirondack rocks have been both severely folded and sheared by ductile deformation and shattered by brittle deformation. Ductile deformation has produced very complicated folds of all sizes throughout the region. Ductile shearing created intensely deformed mylonites, which are found throughout the region but are most abundant in the southeastern Adirondacks and in the Carthage-Colton Mylonite Zone. Long, straight valleys that run north-northeast mark the most prominent examples of brittle deformation. These valleys are the results of accelerated erosion along major faults and fracture zones. In addition, most Adirondack rocks have an abundance of joints. The Adirondack deformation happened when the crust of the region was severely compressed during the Grenville Orogeny.
Almost all Adirondack rocks are Middle Proterozoic in age. The oldest metasedimentary rocks were deposited in shallow seas beginning about 1.3 billion years ago. Metavolcanic rocks of the same age show that volcanoes were active at that time. Some Adirondack metasedimentary rocks contain grains eroded from a much older landmass. Most of the metaplutonic rocks, including the metanorthosite, granitic gneiss, and olivine metagabbro bodies in the Central Highlands, were formed from magmas that were intruded about 1.15 to 1.1 billion years ago.
All these rocks were then buried as much as 30 km below the surface during the Grenville Orogeny. The crust was severely deformed and thickened, and the rocks at depth were intensely metamorphosed. Deformation and metamorphism peaked between 1.1 and 1.05 billion years ago. Over the next several hundred million years, erosion stripped away more than 25 km of rock, and major faults were formed. The region was then covered by shallow seas, in which sediments accumulated through the Cambrian and Ordovician Periods. Sediment accumulation probably continued into the Pennsylvanian Period. Most of these sedimentary rocks have been removed by erosion, but traces can be found in grabens. From the Middle Ordovician into the Tertiary Period, there was no significant tectonic activity in the Adirondack region. Sometime in the Tertiary, the Adirondack dome began to rise, possibly because of a hot spot near the base of the crust. Erosion then carved the region into the separate mountain ranges we see today.
The Big Picture
The Adirondack Mountains make up a roughly circular region about 200 km in diameter in northern New York State (see Figure 1.1). The region is divided into two subregions, the Central Highlands and the Northwest Lowlands. They are separated by the Carthage-Colton Mylonite Zone, a narrow belt of intensely deformed rocks (Figure 4.1; see also Plate 2 of the Geological Highway Map, on which the Carthage-Colton Mylonite Zone is labeled CCMZ).
The metamorphic bedrock in the Highlands resists erosion well. It was left towering over the rest of the countryside when the sedimentary rocks that once covered it were worn away. The highest elevations are found in the High Peaks area of the Central Highlands; there, numerous summits rise above 1200 m. The highest peak, Mount Marcy, is more than 1600 m high. Elevations fall off rapidly north and east of the High Peaks and more gradually to the south and west.
The Adirondack region is part of a much larger area called the Grenville Province (Figure 4.2). The Grenville Province is a broad belt of mostly metamorphic rock of Middle Proterozoic age; it extends along the western side of the Appalachian Mountains from Labrador to Mexico. Around the Adirondacks and south of the region, this belt is almost entirely covered by younger sedimentary rocks.
The Adirondack region was once flat and was covered by the same sedimentary layers that now surround it (see Plate 2). However, in relatively recent geologic time, the Adirondack region was uplifted, forming a dome. During uplift, erosion removed the sedimentary layers from the ' ion. This erosion eventually created a "window" through the sedimentary rocks that permits us to see the much older basement rocks beneath. The Adirondack basement extends into Canada at the surface along a narrow zone called the Frontenac Arch (Figure 4.2). The Frontenac Arch crosses the St. Lawrence River at the Thousand Islands.
Seen from space, the Adirondack Highlands look cracked and wrinkled (Figure 4.3). We can see three prominent types of features on the satellite image:
- Long, straight valleys that run north-northeast are the most prominent. Throughout the Adirondacks, these valleys contain streams and lakes (Figure 4.1). Many of the larger Adirondack lakes, such as Lake George, Schroon Lake, Indian Lake, and Long Lake, follow this north-northeast trend. Figure 4.4 A shows one example. In the High Peaks region, these valleys divide the area into a number of long, straight mountain ranges (Figure 4.4 B). These long, straight valleys have formed along faults and fracture zones where the broken rocks are less resistant to erosion.
- Gently curved ridges and valleys. These ridges and valleys are usually more subtle than the deep, fault-related ones. They are most prominent in the central and southern Adirondacks, where they make an east-west arc. They follow the layering in folded rocks. Harder, more erosion-resistant rocks (such as granitic gneiss) form the ridges, while softer layers (like marble) form the valleys.
- Radial drainage patterns. Streams and rivers in general flow out from the central and northeastern parts of the Adirondack dome toward its edge. We can see this pattern most clearly in the outer parts of the dome; elsewhere, the rivers tend to follow the dominant north-northeast valleys. Figure 4.5 shows this radial drainage pattern in some detail and compares it to structures in the underlying bedrock.
Rock becomes metamorphosed when it is subjected to elevated pressures and temperatures. In a continent-continent collision, mountain-building forces bury rock many kilometers beneath the earth's surface. The weight of the overlying rock subjects the buried rock to enormous pressures. The internal heat of the earth gradually heats the buried' rock to extremely high temperatures. Under these conditions, the minerals in the buried rock react chemically with each other to form new mineral assemblages.
The original composition of the rock, together with the temperature and pressure to which it is subjected, determines what kind of metamorphic rock will form. It is difficult to reconstruct what conditions were like during metamorphism in the Adirondacks because metamorphism takes place deep below the surface of the earth. However, we can use laboratory experiments to estimate the pressures and temperatures that produced the rocks we see at the surface today.
One laboratory approach is to determine both the mineral assemblage found in a rock, and the chemical composition of that rock and its minerals. Artificial "rocks" of the same composition are then exposed to various temperatures and pressures in laboratory apparatus. If the mineral assemblage produced by the experiment at a certain temperature and pressure matches that in the natural rock, we conclude that the rock formed under roughly the same conditions. Another approach is to study the way in which the properties of minerals change with temperature and pressure, and then use this information to calculate the conditions under which a rock with a certain mineral assemblage was formed. Such experiments (the actual procedures are much more complicated!) allow us to determine approximately what the temperatures and pressures were during the metamorphism.
When we compare Adirondack rocks with experimental results, we conclude that rocks in the Central Highlands were formed under rather extreme conditions-at temperatures of 750-800'C and at pressures 7000 to 8000 times the pressure of air at sea level. These pressures are equivalent to those at depths of 25 to 30 km below the earth's surface.4 Conditions affecting the rocks of the Northwest Lowlands were a little less extreme. Temperatures were about 600-750'C, and burial depths were about 20-25 km. When we learn how deeply they were buried, we realize that the rocks we now walk on in the Adirondacks once lay beneath nearly a full thickness of continental crust.
To reconstruct the geologic history of the Adirondack region, we need to figure out what the rocks were like before they were metamorphosed. The first question is: Were they sedimentary or igneous? For some rocks we need only look at the mineral makeup. For example, we know that the metamorphic rock quartzite (Figure 4.7) must have originally been a quartz sandstone, because both rock types are made almost entirely of the mineral quartz and there are no igneous rocks of that composition. Similarly, metanorthosite has the same mineral composition (chiefly plagioclase feldspar) as the igneous rock anorthosite, which is unlike any known sedimentary rocks. Certain sedimentary or igneous features in the original rock may have survived metamorphism. These features are also clues to what the rock was before metamorphism; some examples are shapes of mineral grains or the presence of sedimentary bedding. Some metanorthosites (Figure 4.8A) and metagabbros have mineral grain shapes that show the original rock crystallized from magma. For other Adirondack rocks, the nature of the original rock is much less clear. We do not yet know, for instance, whether some granitic gneisses are metaplutonic, metavolcanic, or metasedimentary.
Metasedimentary and Metavolcanic Rocks
Metasedimentary and metavolcanic rocks make up well over 80 percent of the exposed bedrock in the Northwest Lowlands. They are less abundant in the Central Highlands, where most of the rocks exposed at the surface are metaplutonic. They include both marbles (metamorphosed limestones) and quartzite, as well as various kinds of gneisses that are the end products of metamorphism of shales and sandstones.
What was the environment like when the original sedimentary and volcanic rocks were formed? An exciting discovery in recent years gives us some help in finding an answer. In the early 1980s, fossils of dome-like, laminated structures called stromatolites were discovered in the Adirondacks. They were found in marbles near Balmat (Figure 4.9). This find was very surprising, because the rock containing the stromatolites had been metamorphosed and deformed. Usually, intense deformation and recrystallization destroy any fossils that are present. In fact, stromatolites are the only fossils ever found in the metamorphic rocks of the Adirondacks. Both ancient and modern stromatolites are formed by cyanobacteria (blue-green algae) that live in shallow, well-lit water. We conclude from the presence of stromatolites in Adirondack marbles that these rocks were originally deposited in shallow marine waters.5
The metasedimentary and metavolcanic rocks of the Adirondacks record a complex geologic history. These rocks were originally horizontal layers. Now, the layering has been complexly folded and faulted, and in places disrupted by magma.
Metaplutonic Rocks
Three major types of metaplutonic rocks are found in the Adirondacks: granitic gneiss, metanorthosite, and olivine metagabbro.
Granitic gneiss.-The most common metaplutonic rock in the Adirondacks is granitic gneiss (see Plate 2). Geologists are still arguing about the origin of these rocks. However, much of the granitic gneiss in the Central -Highlands appears to be metamorphosed plutonic rock, so we have put it in the metaplutonic category. This rock is composed largely of alkali feldspar and quartz, with lesser amounts of other minerals.
Metanorthosite.-Metanorthosite (Figure 4.8) forms several large bodies in the Central Highlands. It is an unusual rock, composed chiefly of a single mineral type, plagioclase feldspar. It is similar to the rock that makes up the highlands (bright areas) of the Moon. The largest metanorthosite mass in the Adirondacks, called the Marcy Massif, underlies roughly 1500 km2, including most of the High Peaks area. Near its southern border, we find ore deposits composed of heavy, black iron and titanium oxides. One such deposit, at Tahawus, has been mined for both titanium and iron. There are also several smaller, dome-shaped masses of metanorthosite in the northeastern and south-central Adirondacks. A number of even smaller bodies are scattered throughout the region.
The metanorthosite originated as anorthosite magma in the earth's mantle and lower crust. The magma rose into shallower levels of the crust, where it cooled and hardened. Later metamorphism converted the anorthosite to metanorthosite.
How do we know that the metanorthosite of the Adirondacks was originally igneous anorthosite? In the less deformed parts of the metanorthosite bodies, we find textures typical of igneous rocks (Figure 4.8A). These textures survived metamorphism. In addition, we find blocks of older rocks in the metanorthosite. These blocks were broken off the surrounding rock and mixed in with the magma as it forced its way up through the crust.
Olivine metagabbro. Olivine metagabbro is less abundant than granitic gneiss and metanorthosite, but numerous masses of this rock are scattered throughout the eastern and southeastern Adirondacks (see Plate 2). Like metanorthosite, olivine metagabbro commonly has textures that show its igneous origin. It also contains features called coronas (Figure 4.10), which show incomplete chemical reactions between minerals. These reactions happened during metamorphism, but so slowly that even in the millions of years before the rock cooled the original minerals were not wholly consumed. Near the edges of some olivine metagabbro bodies, we find spectacular large red garnets that also formed during metamorphism (Figure 4.11). At the Barton Mine on Gore Mountain near North Creek, garnets up to one meter in diameter have been found.
We find two main kinds of deformation in the Adirondack rocks: ductile deformation and brittle deformation. Brittle deformation occurs in rocks that are at shallow depths or at the surface, where they are cold; here they deform by breaking. Ductile deformation can occur in rocks that are deeply buried and hot enough to bend or flow without breaking.
Ductile Deformation
One of the most obvious kinds of ductile deformation in the Adirondacks is folding. We find folds of all sizes in the rocks of the region. The complex patterns on the geologic map (Plate 2) result in part from large, irregular folds. Some of these folds in the southern Adirondacks are tens of kilometers across. Major folds in the northwest Adirondacks generally run northeast. Those in the southern half of the Adirondacks make an east-west arc.
We also see folds in individual rock exposures (Figures 4.12, 4.13, and 4.14). We find folded rocks throughout the Adirondacks; some of them appear to have been folded several times, Clearly, great geologic forces were n, to make such folds. In the folded rocks, we often layer-like arrangement of minerals called foliation 4.15) and parallel streaks of minerals called lineation (Figure 4.16). Foliation and lineation give us clues about the directions in which the folding forces acted.
Rocks at high temperatures deep within the crust may also deform by ductile shear. Ductile shear happens when one block of rock slides past another; the rock between the blocks deforms and stretches like chewing gum or hot plastic, rather than breaking to form a fault a-, it would at lower temperatures. This movement creates a ductile shear zone-a relatively narrow, intensely deformed area between the two blocks. The rock in such ductile shear zones is greatly stretched and flattened and commonly shows strong foliation and lineation.
As movement occurs in a ductile shear zone, the minerals in the rock recrystallize. This process reduces the size of the mineral grains, sometimes drastically. The result is a fine-grained rock called a mylonite with strong foliation and lineation (Figure 4.17). From the shapes of the mineral grains in a mylonite, we can sometimes tell which way the blocks of rock moved along the shear zone.
Mylonites are common throughout the Adirondacks, but are most abundant in the southeastern Adirondacks and along the Carthage-Colton Mylonite Zone, which separates the Central Highlands and the Northwest Lowlands (Figure 4.1). They range in width from a few centimeters to several kilometers. In the mylonites of the Carthage-Colton Mylonite Zone the shapes of the mineral grains tell us that the Lowlands probably slid along this zone northwestward and down relative to the Central Highlands. We can't tell how far the Lowlands moved, but it may have been a considerable distance. In other parts of the world, blocks of crust have moved tens or even hundreds of kilometers along similar ductile shear zones.
Brittle Deformation
Brittle deformation refers to the breaking of rock, in contrast to the flowing of rock that accompanies ductile deformation. In the Adirondacks, we find the most prominent examples of brittle deformation in the long, straight valleys that run north-northeast across the eastern half of the region.
Some of these valleys, such as those occupied by Lake George and Schroon Lake, have steep faults on either side. The central block has moved down at least 400 m along these faults. Such down-dropped blocks of crust are called grabens. In the southern Adirondacks, we find several grabens that contain flat-lying sedimentary rocks of Cambrian and Ordovician age. The most recent fault movement must have happened after deposition of the Cambrian and Ordovician rocks cut by the faults-that is, sometime after Middle Ordovician time. We think that some of these faults originally formed in the Late Proterozoic and were reactivated in Middle Ordovician time. We can see small faults in many outcrops in the Adirondacks (Figure 4.18A). Some faults contain shattered rocks known as fault breccias (Figure 4.18B).
Other straight valleys are the result of erosion along zones of intensely broken rock called fracture zones (Figure 4.19). Valleys form along such zones because the broken rock erodes more rapidly than the surrounding rock. Fracture zones differ from faults: the blocks on opposite sides of the zone have not moved relative to each other, but the rock has simply shattered in place. In addition to the faults and fracture zones that run north-northeast, we find many others that run east-northeast, east, and southeast.
Joints, another type of brittle deformation, are found in every Adirondack rock exposure (Figure 4.20). These breaks look like neat slices through the rock. A joint is different from a fault because there has not been any movement along a joint.
How Adirondack Deformation Happened
What caused the deformation of the Adirondack rocks? Immense tectonic forces compressed the entire region now known as the Grenville Province (Figure 4.2). This compression, or squeezing, of the crust was accompanied by folding of the rock layers. As the crust was squeezed, it thickened and shortened in the same way that a cube of soft caramel candy shortens and thickens when you push on its sides. In addition to the folding, large blocks of crust moved along ductile shear zones and were stacked one on top of the other. As the crust thickened, the lower parts were buried deeper beneath the surface. There, they were subjected to high pressures created by the weight of the overlying rock. These pressures, along with heat rising from the mantle and additional heat from intrusions of magma, thoroughly metamorphosed the rocks.
Where did these forces come from? Our best guess is that they resulted from a collision between two continents. This collision began the complicated sequence of events we call the Grenville Orogeny (see Chapter 3).
We find the age of igneous rocks by radiometric dating (see Chapter 2). However, this task is not simple. Sometimes intense metamorphism, like that which occurred in the Adirondacks, can "reset" some or all of the radioactive "clocks" in the rock. If this resetting happens, radiometric dating will tell us when the rock was metamorphosed. It will not give us the age of the original igneous rock. Radiometric dating has been done on many Adirondack rocks, but we have to be very careful in interpreting the results.
We have found that almost all rocks in the Adirondacks are of Middle Proterozoic age. Radiometric dating of the metavolcanic rocks suggests that the oldest ones may be as much as 1.3 billion years old. We think the metasedimentary rocks were deposited as sedimentary rocks beginning at about the same time.6
The original sedimentary rocks of the Adirondack basement-sandstone, limestone, dolostone, and shale were probably deposited in a shallow inland sea. Although they were deposited most likely no more than 1.3 billion years ago, some contain grains of the mineral zircon that are about 2.7 billion years old. This fact tells us that the sediments that formed these rocks were eroded from a much older landmass. This landmass was probably the Superior Province, located to the west and north of the Grenville Province (see Physiographic and Tectonic Maps on Plate 4). Metavolcanic rocks that occur with the metasedimentary rocks indicate that volcanoes were present in the region at that time.
Most of the metaplutonic rocks of the Adirondack Highlands are probably between 1.15 and 1.1 billion years old. Shortly before the Grenville Orogeny, large volumes of magma may have risen from the mantle into the crust. Heat from the magma partially melted the surrounding crust, producing molten rock of different compositions. The various kinds of molten rock, such as anorthosite and granite, tended to rise through the crust because they were less dense than the surrounding rocks. Some continued to rise even after they partly cooled and solidified, eventually forming balloon-like domes or spreading out as thick sheets within the crust.
At some point during the Middle Proterozoic, the rocks we now find at the surface in the Adirondack region were as much as 30 km below the surface. Remember that some of these rocks began their existence as sedimentary rocks at the surface, which means that they must have been pushed down that far. For them to be buried so deeply, the continental crust in the region had to be nearly twice as thick as normal continental crust (see Chapter 3). A modern example of double-thick crust is the Tibetan Plateau just north of the Himalayan Mountains. As India continues to collide with Asia, the collision is creating the Himalayas-the world's highest, mountains-along the collision zone, and a double thickness of continental crust under them and to the north. This double-thick crust makes Tibet the world's highest plateau region, with an average elevation of 5 km above sea level. Far below the surface, the rocks are subjected to very high temperatures and pressures.
The Grenville Orogeny, which may have been caused by a similar collision, buried the Adirondack rocks. It is difficult to say when the orogeny began. It was under way at least 1.1 billion years ago. The deformation and metamorphism appear to have peaked between 1.1 and 1.05 billion years ago. Some additional plutonic rocks may have been formed at the time, either by partial melting of the crust or by injection of new magma from below. By about 900 million years ago, the rocks had cooled again. We still don't know the details of these complex events.
Like the collision of India and Asia, the Grenville Orogeny built huge mountain ranges along the collision zone and a high plateau behind it. Over the next several hundred million years, erosion coupled with uplift levelled the mountains and stripped more than 25 km of rock from the plateau. Between 650 and 600 million years ago, the crust of eastern proto-North America was stretched and was broken by major faults. These faults are the ones that run north-northeast throughout the eastern Adirondacks. There are also many smaller faults running east-northeast, east, and southeast. Igneous rocks called diabase dikes (Figure 4.21) show that molten rock was injected and hardened in narrow vertical zones, often along faults. Radiometric dating tells us that these dikes were formed about 600 million years ago.
Beginning in the Late Cambrian, the Adirondack region was gradually submerged beneath shallow seas. Sandstones with trilobite fossils (see Figure A.3) were deposited over much of the region. The contact between these younger rocks. and the underlying basement is visible in several places near the outer edge of the present Adirondack dome (Figure 4.22). Sediments continued to accumulate across much of the eastern United States (with some interruptions) through the Pennsylvanian Period, but no rocks younger than Middle Ordovician remain in northeastern New York.
Later erosion in the Adirondack region stripped off nearly all of the Paleozoic sedimentary rocks. However, there are still traces of Cambrian and Ordovician rocks within the Adirondacks; this fact proves that they once covered the region. In the southern Adirondacks, we find grabens that contain Cambrian and Ordovician rocks formed in these seas. Because these blocks dropped down lower than the surrounding landscape, they were saved from erosion when the other Paleozoic layers were worn off during regional uplift. The Lower Paleozoic rocks that originally covered the region still encircle the Adirondack dome.
From the Middle Ordovician into the Tertiary Period, there is no evidence of any tectonic activity in the Adirondacks, despite three more mountain-building events that affected New England and southeastern New York (see Chapter 3). The region that is now the Adirondack Mountains was flat, just like the rest of the region west of the Appalachian Mountains. In Jurassic or Cretaceous time, some small dikes intruded in the eastern Adirondacks and Vermont.
Sometime in the Tertiary Period, the Adirondacks began to rise (Figure 4.23). Why? Our best guess is that a hot spot formed under the region near the base of the crust. This hot spot heated the surrounding material at depth, causing it to expand. This expansion raised the crust above, causing the present dome-shaped uplift (Figure 4.23). In the early 1980s, remeasurement of the elevations of old surveyors' bench marks showed that the Adirondacks may be rising at the astonishing (to a geologist!) rate of 2 to 3 mm per year. The mountains are growing about 30 times as fast as erosion is wearing them away. We suspect, however, that the present rapid uplift is a temporary spurt, and the average rate may be much less.
After the Adirondack dome began to rise, stream erosion (and much later glacial erosion) started wearing away the softer rocks and the fractured zones. Eventually, erosion carved the region into the separate mountain ranges we see today. Glacial ice entered the region about 1.6 million years ago; that episode is discussed in Chapter 12.
Footnotes
5The shape of the stromatolites is also very useful in our study of the rocks of the Adirondacks. Their shape tells us whether they are right side up or upside down where we find them in the folded rocks. We can see that the stromatolites in Figure 4.9A are upside down-so we know that the marble that contains them has been folded enough to overturn one limb of the fold. These fossils gave us the first reliable way to tell which way is up in the folded and refolded metasedimentary rocks of the Adirondacks.
6Metasedimentary rocks cannot be dated directly. However, we think that the metasedimentary rocks are the same age as the metavolcanic rocks because they are often found together.
Figure 4.6. This migmatite is a mixed rock-part igneous and part metamorphic. The light layers are composed largely of quartz and alkali feldspar. The dark layers are composed of plagioclase feldspar, biotite, and quartz. The migmatite may have been formed when the rock was metamorphosed at such high temperature and pressure that it began to melt and the melted portion separated into layers.
Figure 4.7. (A) is metamorphic quartzite formed from quartz sandstone. Notice that you can still see the original bedding, even though the rock has been metamorphosed. In a close-up view in (B), however, you can see how the rock has been changed. The original sandstone was made of individual round sand grains. During metamorphism, the grains have completely recrystallized. The final product-a glassy quartz rock.
Figure 4.8. These two photos show Adirondack metanorthosite. The metanorthosite in (A) contains large crystals of plagioclase (medium gray), fine-grained plagioclase (white), and green pyroxene (dark gray). (The ruler is 15 cm long.) (B) shows strongly deformed metanorthosite. The layering is called foliation (Figure 4.15). The large crystal in the left part of the photo is a garnet.
Figure 4.9. These photos show a side view (A) and an eroded bottom view (B) of fossil stromatolites found in marble in the Northwest Adirondacks. (C) shows modern stromatolites at Shark Bay, Australia. This picture was taken at low tide. When we compare the fossils in (A) with the modern stromatolites in (C), we can see that the fossils are upside down. This fact is evidence that the rock in layer (A) has been overturned by folding.
Figure 4.10. These two photographs are of paper-thin rock slices as seen under a special microscope used by geologists. The photos show rings of minerals (called coronas) that formed when the rocks were metamorphosed at very high temperatures and pressures. In (A), the original minerals in the rocks were olivine and plagioclase feldspar. These minerals reacted to form the new metamorphic minerals that make up the coronas: pyroxene, pale plagioclase, and red garnet (black in photo). Plagioclase outside the corona looks dark because it is full of tiny grains of the mineral spinel. These same reactions can be reproduced in the laboratory, but it requires a temperature of up to 800'C and pressures equivalent to 25-30 km of overlying crust. Coronas like these can be seen with the unaided eye in most exposures of olivine metagabbro. (See Plate 2 for places where olivine metagabbro appears at the surface.) (B) shows another type of corona that forms in olivine metagabbros. Here, the two core crystals of ilmenite (black) reacted with plagioclase feldspar to form coronas of hornblende, biotite (black mica), and red garnet (white in photo).
Figure 4.11. These photos are two views of unusually large Adirondack garnets. (A) shows garnets surrounded by rims of the mineral hornblende. The rock is olivine metagabbro. These garnets are found along Wall Street, near I-87, east of Chestertown, Warren County. (B) is a closeup of a single garnet from the Barton Mine at Gore Mountain, Warren County. New York State's garnet mines are world famous, and garnet is the official State mineral.
Figure 4.12. These two photos illustrate the kinds of dramatic effects of deformation and metamorphism that occurred during the Grenville Orogeny. The contorted layers in (B), found in the Adirondacks, once looked like the flat layers shown in (A), younger limestone beds of Ordovician age. The limestone beds are found near the edge of the Adirondack region. Their gentle tilt was caused by the rising of the Adirondack dome (Figure 4.23). The white rock in (B) is coarse-grained marble; it was once fine-grained limestone and dolostone. The contorted dark layers are calcsilicate rock; they were once unbroken, parallel layers of impure dolostone.
Figure 4.13. This photo shows complexly folded rock layers in the northwest Adirondacks. The thin layers are impure quartzite and calcsilicate rock. These layers were originally flat-lying.
Figure 4.14. These photos show dramatic folding in Adirondack rocks. The severely crumpled rocks in (A) are alternating layers of marble (light) and calcsilicate rock (dark). The rock in (B) is granitic gneiss (light) with a layer of amphibolite (dark).
Figure 4.15. These two photos show foliation in Adirondack rocks. Foliation refers to layer-like structures that form when a rock is deformed. (A) is a garnet-bearing gneiss. (The vertical channels are drill holes that were used in blasting this road cut.) (B) is calcsilicate rock.
Figure 4.16. This photo shows lineations-streaks of minerals that form in rock when it is severely flattened and stretched. The lineations are ribbon-like bands of quartz; they show the stretching direction. The rock is granitic gneiss.
Figure 4.17. This photo shows an Adirondack mylonite. Mylonites are formed as minerals recrystallize in a ductile shear zone. This process makes the mineral grains in the rock much smaller. The large grains are made of the mineral feldspar. Their shapes tell us the directions of the deforming forces. The "tails" on the upper left and lower right of these grains point in the direction of movement (as shown by the arrows). The streaks in the rock are foliation (Figure 4.15).
Figure 4.18. (A) shows a small fault in the Adirondacks. (B) shows breccia in another fault in the Adirondacks. Large, angular fragments of gneiss are enclosed in finer grained, crushed and shattered rock of the fault zone.
Figure 4.19. This photo shows a well exposed fracture zone at Split Rock Fall near Elizabethtown. Although the rock has shattered in place, it did not move along the zone. This fact makes a fracture zone different from a fault.
Figure 4.20. This cliff contains widely spaced joints. joints are fractures that looks like neat slices through the rock. The rock has not moved along the joints as it does along faults. The . joints in this outcrop are vertical. The horizontal lines are foliation (Figure 4.15).
Figure 4.21. These three photos show dikes in the Adirondack region. These dikes formed when magma was pushed up from below and hardened. The dike in (A) is made of pegmatite, a very coarse-grained igneous rock, cutting across olivine metagabbro. The dike in (B) is the igneous rock diabase cutting across marble. The cracks in the dike formed when the magma hardened and shrank. The dike in (C) is diabase cutting across metanorthosite.
Figure 4.22. The rock in the lower part of this picture is gneiss. The layers are vertical and the rock has foliation (Figure 4.15). The gneiss ends abruptly; on top of it is a horizontal layer of pebble conglomerate. As we continue to move upward, the conglomerate becomes finer grained until it eventually become quartz sandstone. (The vertical line in the sandstone is a drill hole that was used during the blasting of this road cut between Ticonderoga and Port Henry.)
This picture tells only part of the story. The gneiss is a folded metamorphic rock that formed deep within the crust. A long period of erosion uncovered the gneiss. Then the land was submerged beneath a shallow sea. The conglomerate and sandstone were deposited on top of the gneiss in that sea.
Rare fossils in the sandstone tell us that it is Cambrian-a little more than 500 million years old. Radiometric dating tells us that the gneiss is at least 1.1 billion years old. That means that almost 600 million years of geologic history are lost in the time gap between the two rock units. The surface that separates them and represents the time gap is called an unconformity.
Figure 4.23. These drawings show three stages in the uplift of the Adirondack dome. (A) represents the situation 10-20 million years ago. The region is flat, with layers of sedimentary rock covering the contorted, metamorphosed basement rock. In (B), uplift has created a dome shape. Running water, in a radial pattern, begins to wear away the sedimentary layers. (C), representing the present, shows the basement rock exposed, surrounded by eroded sedimentary rock. The escarpment of sedimentary rocks is grossly exaggerated to illustrate the concept of upturned sedimentary rocks surrounding the dome.