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).
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 flowsand the main 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.