3+8 THE ROMAN CAMPAGNA 



would form for a time a barrier to the erosion which 

 must have been comparatively rapid among the soft 

 overlying tuffs. The river, ponded back into a lake- 

 like expansion in front of the mouth of the Anio, 

 was made to sweep in a wide curve into the softer 

 Pliocene strata of the Vatican ridge on the right 

 bank. The travertine extends through the Parioli 

 ridge and round the base of the Pincian Hill. A 

 similar rock emerges in the precipitous bank on 

 the west side of the Aventine, and farther south 

 at the Monte Delia Creta, opposite the Magliana 

 bridge. Even where this resisting stone disappears, 

 its place has often been taken by a variety of tufF 

 much more compact than the usual rock of the dis- 

 trict. It is the presence of these more durable kinds 

 of stone that has curbed the erosive progress of the 

 river on its left bank, through the area on which the 

 city stands. Where these barriers to its action were 

 locally absent, the river was able to scoop out bays 

 and recesses among the softer parts of the tuffs. The 

 Capitoline, Palatine, Aventine and Celian hills have 

 survived as more or less isolated eminences, owing to 

 their fortunate possession of a more obdurate stone than 

 the' loose granular tuff of the surrounding Campagna. 

 They once rose as islands out of the flood plain, 

 and their steep sides, such as the Tarpeian rock and 

 the cliffs that surrounded the original Roma Quadrata, 

 owed their precipitousness mainly to the scour of the 

 Tiber as it swept past their base. 



Hardly less attractive would be the task of decipher- 

 ing the history of the rough, craggy ridges and broad, 

 smooth plateaux which form such distinctive features 



