All living plants and animals take up carbon from the atmosphere to build it into their tissues, including the very small fraction of 14C. The radioactive decay of 14C is fairly slow and, in any given sample, half of the 14C nuclei will have reverted into nitrogen in 5,500 years. While in the atmosphere this loss is made good constantly by cosmic ray bombardment, the same is, of course, not true for plant or animal tissue that has been buried in the earth. In this the 14C content will decay without being restocked from the atmosphere. In fact, the fraction of 14C in buried samples will diminish by about one per cent every eighty years. Thus it is pos- 160 sible to determine the age of these samples by measuring the 14C proportion in their carbon content.
While the physical basis of carbon-dating is straightforward, its practical application requires very sensitive instruments and is open to errors which have to be avoided. The amount of beta radiation revealing the 14C content is very small indeed and anything that has happened to the sample in the course of centuries, such as waterlogging or prolonged exposure of small samples to the atmosphere, may falsify the results. It is therefore advisable to carry out tests from different samples at the same site whenever possible. Nevertheless, already in its first applications, carbon- dating proved its worth by giving dates from material in the Zoser and Snofru pyramids which closely agreed with the historically accepted ones. The method is therefore immensely valuable for determining the age of pre-Columbian samples for which we have no independent dating whatever.
It was carbon-dating that provided the surprise of the great age of the Gulf civilisation. Samples from the Olmec site at La Venta suggest a flowering of that culture between 800 and 400 BC, many centuries earlier than had previously been suspected. Similar tests have also provided a guide to the mysterious city of Teotihuacan and it appears that the two great pyramids were constructed just before the beginning of the Christian era. Combining the archaeological evidence with carbon-dating, we can begin to reconstruct this early growth of Amerindian civilisation. It seems fairly certain now that the origin of the Maya glyphs and their knowledge of the calendar came from the Olmecs at the Gulf but, while the Maya built steeper and higher pyramids than the cult mound of La Venta, they never passed through the phase of gigantic monuments such as those erected at Teotihuacan. These, the largest structures ever erected in America, provide a close similarity with the Pyramid Age of Egypt.
The forerunner of Teotihuacan in the Valley of Mexico was Cuicuilco, just south of the present university campus of Mexico City. The cult mound is a circular and rather flat step pyramid, with a base diameter of 145 m. but only 20 m. high. Nowadays the flat appearance is further enhanced by the fact that the building was engulfed to a height of nearly 10 m. by lava when the volcano Xitle erupted about 2,000 years ago. This agrees with the type of figurine found there and with the radiocarbon tests which date Cuicuilco at about 400 BC. The mound itself, totalling in its final stage about 7,000 m.3, was built in two phases: the first had two steps, to which a further two were later added. It was evidently abandoned well before the eruption took place.
The material chiefly employed in the construction of this ‘pyramid’ was clay, strengthened in parts with large river boulders. The builders appear to have been aware of the danger of plastic flow to which a clay structure is prone when exposed to rain. Although the mound was extremely flat, it was clearly considered necessary to strengthen the circumference against slip. The clay nucleus is therefore surrounded by a ring-shaped dyke made up of a mixture of clay and stones whose walls were formed by large stones embedded side by side in clay. It appears that mortar was not known, and added strength was obtained by providing the dyke with a series of outer walls, not unlike the buttress walls employed by Imhotep in the construction of the Step Pyramid of Saqqara. Trying to reconstruct the building process, one would suspect that the outer dyke was erected first and that the clay filling was subsequently tipped into the central cavity.
The main reason for assuming this type of construction is the deployment of labour in the most efficient manner. If, as seems likely, a large enough labour force was available, the fastest method of erecting the pyramid was to employ the workers at the periphery of the structure. This would allow the builders to use the maximum number of men simultaneously without their getting into each other’s way.
Steps led up to the platform from both east and west, and the remnants of altars were found both at the height of the fourth and at that of the second, buried, platform. The altars were probably surmounted by roofed buildings which have long since disappeared. The Cuicuilco cult mound is the exact opposite of the Aztec pyramids at the time of the Conquest. We do not know what kind of ritual was enacted at the altars but it is obvious that the flat and low platform cannot have provided a good ‘stage’ for a spectacle, such as was the purpose of the Aztec pyramids.
All evidence indicates that Teotihuacan is somewhat later than Cuicuilco, but not very much later. It appears that the great Pyramid of the Sun was the earliest edifice of note at this site. The area covered by it is almost exactly the same as that of the Khufu pyramid but its original height was only about half that of its Egyptian counterpart. This, of course, means that the Pyramid of the Sun has just less than half the volume of the Khufu pyramid and rather less than half its weight. Khufu’s architects had to quarry and pile up about 6| million tons of limestone for his monument whereas the men who built the Pyramid of the Sun had only to provide 20 million tons of stone and earth. The effort required at Teotihuacan was probably a third of that which was devoted to one of the large Giza pyramids but even so a recent estimate by Stierlin that the Pyramid of the Sun could have been built by 3,000 workers in thirty years seems unrealistic.
However, before attempting to correct this estimate, we must describe the construction of the two large pyramids at Teotihuacan. Neither of them seems to have any internal chambers or corridors; our knowledge is entirely based on surface examination and on the archaeological tunnels which have been driven into them. The first archaeological work on the Pyramid of the Sun was carried out by Leopoldo Batres at the beginning of this century and was inspired by President Porfirio Diaz. The occasion was the centenary of Mexico’s liberation from Spanish colonial status and its foundation as a sovereign state in 1810. Batres set to work with great energy and a large labour force. He cleared the pyramid of the vegetation covering it and then tried to lay bare its real surface. In his enthusiasm he unfortunately went too far, removing what had been left of the original outer layer of stone, mortar and plaster from the entire north, east and south faces. The result was disastrous. The underlying adobe surface started to dissolve in heavy rains and began to exhibit plastic flow which threatened to destroy the whole edifice. It was saved only by the very high viscosity of the dissolving clay which rendered the disintegration slow enough to be arrested by hurried remedial measures.
The three exposed faces of the monument were given a new skin of stones and cement, so that today we see the original surface only on the frontal western side which Batres had left largely untouched. The result of his precipitate action is that the pyramid is now somewhat smaller than originally and that all its original surface features have disappeared. However, by peeling off about 7 m. of the outer layer he laid free a number of stone walls which stand out from the building like fins and which had the purpose of holding the original surface in place.
Two tunnels have been driven through the Pyramid of the Sun. The first, dug by Gamio in 1917, enters the monument in the middle of the east face and goes through to its centre. The second tunnel constructed in 1933 by Eduardo Noguera enters at the centre of the western face and meets the Gamio tunnel. Both tunnels are roughly horizontal and run close to the base of the pyramid. These investigations yielded a number of very important results. First and foremost, no sign of an earlier pyramid was found on which a later structure was superimposed. This means that the Pyramid of the Sun was erected in one operation to its present size. Secondly, there are no internal strengthening features and the only means of holding the great mound together is an outer skin of between 15 and 20 m. thickness made up of adobe and stones. The possibility cannot be excluded that, higher up in the edifice, fin walls of stone were introduced, of which the visible fins laid bare by Batres are part. Such fin walls to hold the loose fill were certainly used in the slightly later Pyramid of the Moon; however, there is no indication at ground level of their existence in the older pyramid. The fill visible in the tunnel walls consists 164 of clay, stones, gravel and various types of soil containing figurines and potsherds.
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| Mexican Pyramids |
It was carbon-dating that provided the surprise of the great age of the Gulf civilisation. Samples from the Olmec site at La Venta suggest a flowering of that culture between 800 and 400 BC, many centuries earlier than had previously been suspected. Similar tests have also provided a guide to the mysterious city of Teotihuacan and it appears that the two great pyramids were constructed just before the beginning of the Christian era. Combining the archaeological evidence with carbon-dating, we can begin to reconstruct this early growth of Amerindian civilisation. It seems fairly certain now that the origin of the Maya glyphs and their knowledge of the calendar came from the Olmecs at the Gulf but, while the Maya built steeper and higher pyramids than the cult mound of La Venta, they never passed through the phase of gigantic monuments such as those erected at Teotihuacan. These, the largest structures ever erected in America, provide a close similarity with the Pyramid Age of Egypt.
The forerunner of Teotihuacan in the Valley of Mexico was Cuicuilco, just south of the present university campus of Mexico City. The cult mound is a circular and rather flat step pyramid, with a base diameter of 145 m. but only 20 m. high. Nowadays the flat appearance is further enhanced by the fact that the building was engulfed to a height of nearly 10 m. by lava when the volcano Xitle erupted about 2,000 years ago. This agrees with the type of figurine found there and with the radiocarbon tests which date Cuicuilco at about 400 BC. The mound itself, totalling in its final stage about 7,000 m.3, was built in two phases: the first had two steps, to which a further two were later added. It was evidently abandoned well before the eruption took place.
The material chiefly employed in the construction of this ‘pyramid’ was clay, strengthened in parts with large river boulders. The builders appear to have been aware of the danger of plastic flow to which a clay structure is prone when exposed to rain. Although the mound was extremely flat, it was clearly considered necessary to strengthen the circumference against slip. The clay nucleus is therefore surrounded by a ring-shaped dyke made up of a mixture of clay and stones whose walls were formed by large stones embedded side by side in clay. It appears that mortar was not known, and added strength was obtained by providing the dyke with a series of outer walls, not unlike the buttress walls employed by Imhotep in the construction of the Step Pyramid of Saqqara. Trying to reconstruct the building process, one would suspect that the outer dyke was erected first and that the clay filling was subsequently tipped into the central cavity.
The main reason for assuming this type of construction is the deployment of labour in the most efficient manner. If, as seems likely, a large enough labour force was available, the fastest method of erecting the pyramid was to employ the workers at the periphery of the structure. This would allow the builders to use the maximum number of men simultaneously without their getting into each other’s way.
Steps led up to the platform from both east and west, and the remnants of altars were found both at the height of the fourth and at that of the second, buried, platform. The altars were probably surmounted by roofed buildings which have long since disappeared. The Cuicuilco cult mound is the exact opposite of the Aztec pyramids at the time of the Conquest. We do not know what kind of ritual was enacted at the altars but it is obvious that the flat and low platform cannot have provided a good ‘stage’ for a spectacle, such as was the purpose of the Aztec pyramids.
All evidence indicates that Teotihuacan is somewhat later than Cuicuilco, but not very much later. It appears that the great Pyramid of the Sun was the earliest edifice of note at this site. The area covered by it is almost exactly the same as that of the Khufu pyramid but its original height was only about half that of its Egyptian counterpart. This, of course, means that the Pyramid of the Sun has just less than half the volume of the Khufu pyramid and rather less than half its weight. Khufu’s architects had to quarry and pile up about 6| million tons of limestone for his monument whereas the men who built the Pyramid of the Sun had only to provide 20 million tons of stone and earth. The effort required at Teotihuacan was probably a third of that which was devoted to one of the large Giza pyramids but even so a recent estimate by Stierlin that the Pyramid of the Sun could have been built by 3,000 workers in thirty years seems unrealistic.
However, before attempting to correct this estimate, we must describe the construction of the two large pyramids at Teotihuacan. Neither of them seems to have any internal chambers or corridors; our knowledge is entirely based on surface examination and on the archaeological tunnels which have been driven into them. The first archaeological work on the Pyramid of the Sun was carried out by Leopoldo Batres at the beginning of this century and was inspired by President Porfirio Diaz. The occasion was the centenary of Mexico’s liberation from Spanish colonial status and its foundation as a sovereign state in 1810. Batres set to work with great energy and a large labour force. He cleared the pyramid of the vegetation covering it and then tried to lay bare its real surface. In his enthusiasm he unfortunately went too far, removing what had been left of the original outer layer of stone, mortar and plaster from the entire north, east and south faces. The result was disastrous. The underlying adobe surface started to dissolve in heavy rains and began to exhibit plastic flow which threatened to destroy the whole edifice. It was saved only by the very high viscosity of the dissolving clay which rendered the disintegration slow enough to be arrested by hurried remedial measures.
The three exposed faces of the monument were given a new skin of stones and cement, so that today we see the original surface only on the frontal western side which Batres had left largely untouched. The result of his precipitate action is that the pyramid is now somewhat smaller than originally and that all its original surface features have disappeared. However, by peeling off about 7 m. of the outer layer he laid free a number of stone walls which stand out from the building like fins and which had the purpose of holding the original surface in place.
Two tunnels have been driven through the Pyramid of the Sun. The first, dug by Gamio in 1917, enters the monument in the middle of the east face and goes through to its centre. The second tunnel constructed in 1933 by Eduardo Noguera enters at the centre of the western face and meets the Gamio tunnel. Both tunnels are roughly horizontal and run close to the base of the pyramid. These investigations yielded a number of very important results. First and foremost, no sign of an earlier pyramid was found on which a later structure was superimposed. This means that the Pyramid of the Sun was erected in one operation to its present size. Secondly, there are no internal strengthening features and the only means of holding the great mound together is an outer skin of between 15 and 20 m. thickness made up of adobe and stones. The possibility cannot be excluded that, higher up in the edifice, fin walls of stone were introduced, of which the visible fins laid bare by Batres are part. Such fin walls to hold the loose fill were certainly used in the slightly later Pyramid of the Moon; however, there is no indication at ground level of their existence in the older pyramid. The fill visible in the tunnel walls consists 164 of clay, stones, gravel and various types of soil containing figurines and potsherds.
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