Three major Karst area's in Italy: in the north, the wider area around Rome and in southeast Italy. View from the Strada Provinciale San Gregorio (south of Tivoli) looking west, towards Rome. The Alban hills seen from the most southeastern part of Roman Vecchia, almost parallel to the ancient Via Latina. One of the many views on the river Tiber in Rome.
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Geology of Rome

Rome is situated on the Tiber River, which follows a structural depression created late in the geologic history of the region, when the land was being pulled apart by movements of the Earth's crust. The river's basin is one of the largest on the narrow Italian peninsula. Most of it's 403-kilometre length runs parallel to the Apennines across Tuscany, Umbria and Lazio before it enters the sea at Ostia. The Tiber drains a huge area, more than 17,000 square kilometres (Heiken, Funiciello & De Rita, 2005:65). The river rises in the Apennines, near Arretium (Speake, 1995:635). This is in modern Emilia-Romagnaan administrative region comprising the two historic regions of Emilia and Romagna.
The key structural feature of the peninsular of Italy is the presence of the Appennines. They run from continental Italy through a length of 1000 km, cover a breadth of between 50 and 100 km, down to Sicily. Less than 20% of the peninsula is lowland (Stoddart, in Rosenstein & Morstein-Marx, 2006:103). The Apennines are structurally complex, made mostly of sedimentary rocks that were deposited in ancient seas, subjected to high temperatures and pressures while deeply buried, consolidated and then thrust up to their present elevation. These rocks are mostly limestone )1 and and dolomite )2. Over time, slightly acidic rainfall cuts into these rocks and dissolves them, creating networks of caves and fissures, known as 'karst' terrain )3. The central Italian Apennines contains karst terrains over an area of about 8,000 square kilometres, and it is calculated that this supports a cumulative groundwater outflow of 220,000 litres of water per second (Heiken, Funiciello & De Rita, 2005:37).
The Tiber enters Rome from the north, then turns southwest towards the Tyrrhenian sea. The hills west of the Tiber are composed of million-year-old marine mudstones and sandstones, giving evidence that once the region was beneath the sea (Heiken, Funiciello & De Rita, 2005:11). Eruptions in volcanic fields located southeast and northwest of Rome created two plateaus that descend towards the Tiber. Flows of ash and gas from volcanic eruptions damned the river with deposits of ash, called tuffs )4, and changed its course. Both of the volcanic fields, the Sabatini to the northwest and the Alban hills to the southeast, played important roles in creating the terrain; plateaus pinching the Tiber floodplain and creating high ground for Rome (Heiken, Funiciello & De Rita, 2005:11). Despite the advantageous location, Rome is still susceptible to flooding due to the large drainage area of the Tiber. The climate from the end of the republic, throughout the years of the Empire, up to perhaps between 800 and 1200 A.D., was warmer and drier than later years. During the wet period between 1310 and 1320 A.D., and the so-called 'little ice age' of 1500 to 1800 A.D., Rome was more susceptible to flooding (Lamb, 1995). This is perhaps a good thing, as repeated natural destruction of the city may have had a large influence on the superstitious Roman mind, providing 'evidence' for the displeasure of the gods, and perhaps the resulting abandonment of the site.
Alban hills
The Alban hills are approximately 50 kilometres in diameter with an elevation of nearly 1000 metres above sea level, and span the coastal plain between the Apennines and the sea. The summit is broad and dominated by a caldera, which has mostly been covered with material from later volcanoes. The slopes were once covered with oak, hazel and maple trees. Archaeological evidence from around the edges of the Nemi and Albano lakes indicate that the area has been occupied since the Bronze Age. Most of the Alban hill's volcanic deposits were produced by pyroclastic flows, which flowed in all directions, including into the area that became Rome (in deposits 5 to 10 metres thick). Much of the stone used to surface the highways near Rome came from these lava flows (Heiken, Funiciello & De Rita, 2005:33). The most common building stone used in Rome from the 6th to 5th centuries BC, 'tufo pisolitico' was quarried from deposits left by eruptions in the Alban hills 600,000 to 300,000 years ago.
Stone and Water
After the conquest of Veii in 396 BC the Romans acquired new territories to the north. There, in the Sabatini volcanic fields, they began to quarry 'tufo Giallo', which replaced the weaker 'tufo pisolitico' as the favoured building stone. The volcanic events that created these tuffs were at least seven in number and occurred about 500,000 years ago. The flows covered an area of about 400 square kilometres (Heiken, Funiciello & De Rita, 2005:44). The history of Rome can be read in the stone used to build her.

The highlands of the Alban hills and the Sabatini volcanoes have a rain catchment area of 5,100 square kilometres, which recharges a number of lakes and the aquifers)5 below the hills and fields. Today, the area provides a cumulative flow of surface and groundwater amounting to 45,000 litres per second. (Heiken, Funiciello & De Rita, 2005:137). The water derived from all these various sources makes Rome the only city of its size in the world that is chiefly supported by groundwater in a sustainable manner.

Figure D.11 shows a collage of satellite images of western Italy from a height of eighty kilometres. Rome can be seen slightly left and down of centre. To the top left (northeast) is Lake Sabatinus, known today as Lake Bracciano. The bottom right (southeast) shows the Alban hills with Lake Albanus, known today as Lake Albano. The Tiber can be followed for most of its course.
As a result of the structure of the land and its location, much of Rome was once under water. The 'Forum Romanum', the 'velabrum', the 'Campus Martius' and other valleys were once almost impassable marshes and pools of water. As Ovid put it (Fast. 6.401): 'Hic, ubi nunc Fora sunt, udae tenuere paludes'. Dionysius (2.50) speaks of the site of the forum having formerly been a marshy thicket owing to the depressed nature of the ground. The draining of these valleys was effected by means of the 'Cloacae', which were amongst the first important architectural works of Rome)6 . As Varro says ('Lin. Lat.' 5.149): 'lacum Curtium in locum palustrem, qui tum fuit in Foro, antequam cloacae factae sunt'. Moreover, the hills and ridges of Rome were once more numerous and abrupt than they are. At an early period, when each hill was crowned by a separate village and surrounded by hostile tribes, the inhabitants naturally wanted to increase the steepness of the cliffs to make their villages more difficult for enemies to access. In later years, when these various villages were united into a single city and surrounded by a wall, this became inconvenient. The tendency became, especially in Imperial times, to get rid of all the features that tended to break the city into separate parts. Tops of hills were levelled, whole ridges cut away and gentle slopes formed where there once were abrupt cliffs. The levelling of the 'Velia' and the excavation of the site for Trajan's Forum are instances of this (Middleton, 1892a:4).
ad spem veterem
As the Tiber leaves Rome the slope of the riverbed decreases and the flow is placid as the river approaches the sea. This is an important factor in the economic and military success of Rome, making it possible to establish ports near the city and thus ship men, materials and goods upriver to the city (Heiken, Funiciello & De Rita, 2005:11). It is perhaps of importance to consider the 'Porta Praenestina', or Porta Maggiore as it is called today, because of its importance to the aqueduct system in ancient Rome and as one of the best surviving parts of that system. See Figure D.12 for a satellite image of the modern Porta Maggiore; remains of the aqueduct system can be clearly seen. Frontinus called this entire area the 'ad spem veterem' because of its proximity to an old temple of Hope. The 'Porta Praenestina' was the highest point on the eastern side of Rome, and was thus selected by the engineers of the aqueducts from the upper valley of the Anio and from the Alban Hills as the point at which the water channels should enter the city, so that as little pressure as possible was lost. It was therefore the meeting point of eight or nine aqueducts and as many roads, and therefore one of the most important topographical centres of the ancient city (Ashby, 1970:128). Three of the aqueducts were at ground level or below, so nothing can be seen of them. These were the Anio Vetus, Alexandrina and the Appia. The channels of the Claudia and Anio Novus arrived on tall arches, the latter running atop the former.The location of 'Porta Praenestina' can be seen on Map D.4. This Porta consists of a double arch of the Aqua Claudia and Anio Novus that Claudius built to take the new aqueduct over the Via Praenestina and Via Labicana just beyond their point of divergence. The arches are at an angle to each other and built of blocks of travertine with heavy rustication. The whole is 32m high by 24m wide by 6.2m deep (see Figure D.21. In the central pier there is a small arch, now almost entirely buried. Above this and to either side of the main arches are narrow arches framed with an engaged Corinthian order and pedimented entablatures. The attic is divided into three fasciae, each of which bears an inscription relative to the building or repair of the aqueducts (CIL 1256-1258) (Richardson, 1992:307). The inscriptions can be seen quite clearly in the engraving by Piranesi (see Figure D.22) The 'Porta Praenestina' was incorporated into the Aurelian Wall, and Honorius changed it considerably (Platner & Ashby, 1965:412). The Aurelian Wall still linked to the travertine aqueduct arches is also responsible for preserving short sections of the other three elevated aqueducts that entered Rome here, the Marcia, Tepula and Julia. The branch aqueduct 'Arcus Neroniani', built by Nero, begins at the 'Porta Praenestina' (Aicher, 1995:53). See Chapter 3.5 for information on the inscriptions found here.
On the right bank of the Tiber, especially in the area of the Janiculum and Vatican Hills, are extensive remains of an ancient beach, conspicuous in parts by its fine golden sand and deposits of pure greyish-white clay. At a few places, especially on the Aventine and Pincian Hills, under-strata of Travertine crop out. The conditions under which the tufa hills were formed have been various, as can be seen by the examination of rock at various places. The volcanic ashes and sand, of which tufa is composed, appear in parts to lie just as they were showered down from the crater. In this case, the tufa shows little or no sign of stratification and consists wholly of igneous products. In parts time and pressure have bound together these 'scoriae' into soft and friable rock. In other places they still lie in loose and sandy beds, which can be dug out with a spade. Other masses of tufa show signs of having been deposited in water or else washed away from their first resting place and redeposited elsewhere with visible marks of stratification. This is shown by water-worn pebbles and chips of limestone rock which form a conglomerate, bound together by volcanic ash into a sort of natural cement. On the Palatine Hill there is evidence of extremely hot ash falling on a thick forest. The burning wood of this forest, partly smothered in ashes, has been converted into charcoal, large lumps of which are embedded in the tufa rock. In some places charred branches of trees can be easily distinguished. The so-called 'Walls of Romulus' and some of the other prehistoric buildings on the Palatine were built of this conglomerate of tufa and charcoal. A perfect section of a branch of a tree is visible in the face of one of the massive tufa blocks on the north side of the 'Scalae Caci' (Middleton, 1892a:8).

From the thesis of Evan J. Dembskey on The aqueducts of Ancient Rome

)1 Mostly calcium carbonate (CaCO3), with traces of other elements (Blyth & de Freitas, 1986:124).
)2 A magnesium-calcium carbonate (CaMg(CO3)2), a non-silicate mineral (Blyth & de Freitas, 1986:83).
)3 Named after the Karst area of Istria in the former Yugoslavia Serbia and Montenegro which has this characteristic terrain (Blyth & de Freitas, 1986:32).
)4 See chapter 4.9.1 for further discussion of this useful material.
)5 Water-bearing, permeable deposits.
)6 While originally designed to drain the marshes, it is estimated that by the the imperial period 5000 kg of city waste was being drained through it every day into the Tiber (Gowers, 1995:25)