Today let me propose you a nice map useful to understand what really is the geology of the soil of Rome. What I always tell to my tourists is that the Eternal City was able to develop for many reasons, and one of them was its territory, the hills and the river, and other natural and geological characteristics. 

On this picture, at the foreground, there are the famous seven hills: the Palatine, Capitoline, Quirinal, Viminal, Esquiline, Caelian, and Aventine hill (the most distant from the center). Some of them are still easily visible, such as Capitoline Hill (reachable thanks to the Cordonata staircase designed by Michelangelo), and the Palatine hill. Others, however, after the many centuries that have passed, are sometimes hidden and it is difficult to find or see a slope (the Viminal hill, for example, is completely covering with buildings like the palace of the Ministry of the Interior). The beauty of Rome, however, is that its territory, hills included, was formed and modeled by two active volcanoes, that were thousands and thousands of years old. Good for us that they are not active anymore.

In fact the craters today are occupied by two lakes: Lake Albano (corresponding to the old Volcano of Albano, south of Rome), and of the Lake of Bracciano (corresponding to the ancient Sabatino Volcano, to the north). These two powers of nature changed the soil of Rome, making it full of different materials. For example the first miles of the famous Appian Way were built just above a volcanic plateau, reusing it to make a perfect straight road. But in general the ancient volcanos made the fortune for the Romans of the past,  who could take materials as the tuff or volcanic rocks. They became raw materials, in some degree, which were used to build roads or monuments. Basically if the Eternal City is crossed by numerous hills, it is also true that rests on a volcanic plateau, that made the fortune of the Romans themselves, considering the large amount of material suitable for architectural works useful to the roman community.

But another important geological element for Rome is the water and the Tiber, the river that crosses the city. Imagine how thousand of years ago there were several rivulets, streams and creeks which fell down from the high hills, coming from natural springs. They reached the city but wiith their presence the entire territory changed. Think about the modern Via Labicana, the basically is above an ancient pit that runs below the modern city. This pit crosses partly below the Colosseum, especially its south side. And it is the reason why this side, over the centuries, fell down! Due to the ground that was made fragile by this underground water. Imagine also how here, milliions of years ago, there was the sea: studying the Monte Mario, on of the small mountains that are around the city centre, the experts found several ancient fossilized marine organisms. To conclude this little trip of geology, to say that some of the Hills of Rome are not real hills but, sometimes, a cluster of hills! For example, the Campidoglio actually looks like two small hills divided by a narrow valley. The name of these hills are: Arx (where today stands the Basilica of Santa Maria in Ara Coeli) and the Capitolium (where stands the Palazzo Senatorio), while the narrow valley between the two is called the Asylum. Here, apparently, there was also a brake and a source of water, both made sacred and dedicated to the gods. One way to understand how Nature, to which the Romans relied, was often made sacred, emphasizing the importance of those resources, geological if we want to (just think of the presence of the river Tiber, with all its advantages in terms of economic and not only), that made Rome the Caput Mundi.