CULTURAL GEOLOGY OF ROME
ROME, THE ETERNAL CITY AND ITS SEVEN HILLS
An introduction to the geological origin of the Seven Hills of Rome for Travelers.
Rome, “La Cita’ Eterna” is the capital of Italy, the third largest city in Europe and the largest metropolitan area physically dominated by volcanoes in Europe. It might be accurately called “La citta’ dei volcani” (the city of volcanoes). Rome’s 500 square mile metropolitan area, In the Lazio region lies in the floodplain of the Tiber River, only 16 miles from the Mediterranean. The City occupies a coastal plain underlain by both intrusive and extrusive volcanics. Volcanoes dominate its skyline. To the north of the City, lie the Sabatini Volcanic Complex and to the south, the Alban Hills Volcanic Complex. The Apennine Range a four thousand foot tall, faulted and folded mountain chain, is found to the east of the City. The rich volcanic soil, volcanic landforms, and associated igneous rocks,—as well as occasional violent volcanic eruptions— have had enormous impact on the City’s development and its history.
Rome, a city continuously occupied for nearly three thousand years, has its history intimately tied to seven low hills within the ancient city center. The famous “Seven Hills of Rome” have played an important part in the founding, development, and history of the City. Interestingly, we will see here below how these famous landforms (“la collina”) are part of the volcanic history of the region. Their seven names are legendary:the Capitoline, Palatine, Aventine, Caelian, Quirinal, Viminal, and Esquiline Hills. These Seven Hills played an important role in the early history, myths, culture and development of the City.
The Palatine Hill was the site of the legendary founding of Roman City in 753 BC by the mythical and eponymous Romulus and Remus. While the Capitoline Hill served as the first political and religious center of the Republic. Its first “capital” where we get our word for such a place. The Capitoline was where the most revered religious temple— the Temple of Jupiter was sited. (https://en.wikipedia.org/wiki/Temple_of_Jupiter_Optimus_Maximus). Today, the magnificent white marble National Monument to Victor Emmanuel II and the Tomb of the Unknown Soldier grace the north slope of this, the smallest of the hills. (See https://en.wikipedia.org/wiki/Victor_Emmanuel_II_Monument)
After almost three thousand years of human activity and commercial development, the hills, now densely occupied by famous buildings and well vegetated parks, are not so easily detected. Even in antiquity, they were only modest hills of perhaps 100 feet (30 meters) above the Tiber floodplain in the Field of Mars. For modern visitors , the Hills seem to rise only imperceptibly above the surrounding areas, such as the Campus Martius which is only about 30-40 feet (10-12 meters) above sea level.
Rome’s Seven Hills, like many other features in this part of Italy, are the result of volcanic eruptions of the distant past. For Rome these ancient volcanic events originated in the Alban Volcanic Hills, only 25 miles south of Rome . See: https://en.wikipedia.org/wiki/Alban_Hills )
The Alban Hills are a complex feature of old and new volcanoes, the crests of which can be seen along the southern skyline off Rome from many parts of the City. The Hills form as a series of 3,000 foot peaks visible from Rome. The Alban Hills complex first erupted violently about 600 thousand years ago. Eruptions continued intermittently until the last major eruption of about 20,000 years ago. They are still considered “active” volcanoes and are monitored closely as a “threat” to the City. .
Geologists believe that that last great eruption of twenty thousand years ago may have been very similar to the 79 AD Mount Vesuvius event. Vesuvius, near present day Naples, erupted violently and buried Pompeii in “pyroclastic flows” (https://en.wikipedia.,org/wiki/Pyroclastic_flow), a mixture of fine volcanic ash, pebble and gravel sized volcanic particles called “lapilli”. That ancient city was buried to a depth of almost 30 feet.
In very similar circumstances, twenty thousand years ago (20,000 YA) eruptions from Alban Hills poured similar volcanic debris down upon the ancient site of Rome. Geologists have reconstructed flows of pyroclastics which sped down the slopes of the Alban volcanoes into the then unpopulated valley of the Tiber River to partly fill that area with as much as 100 feet (30 m) of volcanic ash and pyroclastics (fine ash, and rock fragments of pebble and cobble size). These deposits hardened into a volcanic rock called “tuff” or tufa.
After thousands of years, erosion, flooding and mass wasting much of this rocky volcanic fill, perhaps formed into a “plateau like” landform, was partially dissected and eroded away by tributary streams of the Tiber River. After this long period of erosion, only isolated remnants of the volcanic flows remained by the 8th C BC when the area was settled by a Latin tribe. By this time much of the volcanic fill had been washed away by streams and other processes and had taken on the form of seven low hills.
These volcanic deposits from past eruptions were transformed into the iconic “Seven Hills” of Rome which were to play so important a part in the early history of the City and the Republic.
The original hills were no doubt somewhat higher, and were probably vegetated with native trees and shrubs such as the olive, myrtle, laurel, oleander, mastic, Aleppo pine, and cork oak.
Thus Rome as an early settlement of of local Latin tribes began among the partly eroded deposits of a 20,000 year old volcanic eruption. They may have chosen the site for its access to fresh water, hill tops were more secure from threat and safe from flooding, and the general geographic setting afforded protection from invasion. The “hills were partly surrounded on the east and northeast by the meandering Tiber River. To the south, lie the Enea Hills, and to the southeast, the Alban volcanoes rise up a thousand feet, while the 4,000 foot Apennines dominate the region to the north and east.
The Tiber arises in the northern Apennines from springs on Mount Fumaiolo and flows south through the central Italian Apennines passing through Perugia on its way to Rome. Its course is controlled in this southerly direction by a prominent, elongate geologic fault in the mountains called a “graben”. (https://en.wikipedia.org/w/index.php?title=Graben&oldid=1153732450). In In th evicinity of Rome the Tiber River turns sharply west, passes around the Seven Hills and continues on for 16 miles to enter the Tyrrhenian Sea at Ostia.(See: https://en.wikipedia.org/wiki/Tiber ).
During the geologic process which formed the Apennines east of Rome some 23 MYA, a portion of the Earth’s crust was subducted (settled) into the hot mantle. The sinking process tended to“pull” the descending crust downward and caused the slab of descending crust to “stretch”. The results of this stress were faults or cracks in the slab of descending crust. This resulted in the formation of a form of “dropped block fault” or graben—literally a “trench” into which the Tiber River naturally followed. The stretching and faulting of the earth’s crust produced this near-straight-line graben or trench resulting in the near linear course of the Tiber River in this area..
Just north and east of Rome, the River leaves the graben and turns sharply southwest where it is joined by its tributary, from the east, the Anio River. From there, the river flows southwest through Rome, past its ancient port city of Ostia. Over the near 3000 years since the founding of the City and the accumulation of seiment at the mouth of the River, the port of Ostia is now several miles inland from where it originally entered the Tyrrhenian Sea.
The Tiber was an important source of fresh water for the early Romans as well as a means of essential transport and trade for its later occupants. And as not above it also served as a protective barrier during the early history of the City.
In its lower reaches the Tiber is a meandering stream. (See wikipedia “meander” https://en.wikipedia.org/wiki/Meander). Such twisting and turning of the stream bed or “meandering pattern” is characteristic of a very gently sloping river-bed and slow current.
This may be the result of the fact that over the last 10,000 to 12,000 years, sea level has been slowly rising as a result of post glacial warming. As sea level rises it, in effect, reduces the slope of a river. As slope decreases, the force and speed of the flow of water in the river bed is reduced. Rivers with slow currents cannot erode their beds effectively, and begin a process of lateral erosion, or widening their flood plains by eroding and sediment deposition of their banks. This process causes the stream bed to move laterally across its flood plain in a twisting circuitous pattern called “meandering”.
This characteristic meandering flow of the Tiber served as an advantage to the early Romans, since this curving course of the River partially enclosed the Seven Hills and became an important aspect of the area’s security from invasion. ( See: https://en.wikipedia.org/wiki/Meander ).
Early inhabitants had the dual advantages of protection from invasion, as well as an abundant fresh water supply. Furthermore, the seven hills “hilltop” occupation made attack by enemies from low ground more difficult and added this element of protection from attack.
The Tiber was subject to regular seasonal flooding. This no doubt increased the fertility of the meadows and swampy terrane between the Seven Hills. In addition to the fact that the soils in the low lying areas were naturally rich in minerals as a result of their volcanic origin, but the annual floods of the Tiber added to that fertility by deposits silt and organic materials each year which improved agriculture. On the other hand, the hills protected residences and structures of early settlers from these seasonal floods.
The volcanic origin of the Seven Hills had other advantages as well. As noted above the Seven Hills and the entire region is underlain with volcanic deposits from past eruptions. The local volcanic bedrock known as “tuff” besides contributing to the fecundity of the soils also happens to be a useful and easily mined and shaped construction stone (See:https://en.wikipedia.org/wiki/Tuff )
Tuff is an igneous rock composed of volcanic ash and coarser volcanic derived particles welded together into a soft gray or brown rock. The local gray-brown variety in Rome is known as “Cappellacio Tuff”. This rock was deposited on the slopes of the Alban Hills and also in areas around Rome. It formed the bed rock of the Seven Hills during the eruptive stages of the Alban volcanic complex . Ancient Romans and Etruscans extensively quarried this Cappellacio Tuff as a widely available, easily quarried, useful building stone. There are extensive quarries at various sites in the Alban Hills.
The oldest walls in Rome, are the Servian Walls (built in the 4th C BC). The walls are often claimed to have been constructed as a protective barrier after the attack Rome by Gauls in 390 BC. But it seems that is uncertain. The original wall was built sometime in the sixth century BC (500s BC) by the Servius the King of Rome at the time, using the local gray or light brown available Cappellacio tuff as a building stone. A section of this ancient wall is still extant and visible just outside of the central train station in Rome. Tuff is relatively soft and can be easily cut and shaped. The Romans cut the Cappellacio tuff as well as many other types of rock into isometric rectangular blocks for a type of construction called “ashlar construction” widely used in many places in Rome. Keep your eyes open for rock walls with roughly shaped stone blocks—not—cemented together.
But after the Gaul attack of 390 BC the existing rock walls were considered insufficient and were added to or replaced using a harder more resistant yellowish stone called Grotta Oscura tuff. Grotta Oscura became available after the 390s BC after the Romans had defeated the Etruscan City of Veii. The Romans simply had the Veiians deconstruct the protective wall around the defeated city of Veii and forced the Etruscans to transport the blocks to Rome. (See: https://en.wikipedia.org/wiki/Servian_Wall ). Later they also quarried the Grotta Oscura tuff from Etruscan quarries as well.
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