IANICULUM
the long ridge on the right bank of the Tiber, running almost due north from a point opposite the Aventine to what is now called Monte Mario (Mart. iv. 64; Dionys. ix. 14. 8), a distance of about 5 kilometres. This was in the AGER VATICANUS (q.v.), and was sometimes called Mons Vaticanus (Hor. Carm. i. 20. 7). It is separated from the plateau behind by a long depression, and is itself not entirely continuous, being partially broken on the south, west and north-west of the Vatican by natural and artificial valleys (BC 1892, 288). The term Ianiculum is now limited to the part of the ridge immediately opposite the city, from the point where it approaches within 100 metres of the river near S. Spirito southwards, a distance of 2 kilometres. The highest point of the ridge in its larger sense is the northern end, Monte Mario, 146 metres above sea-level, and the highest point within the line of the Aurelian wall is west of the present church of S. Pietro in Montorio, 69 metres. At the porta Aurelia (porta S. Pancrazio) it is about 82 metres high, and a short distance farther west about 81. The average height of the ridge above the campus Martius is 60-70 metres (cf. Cic. de leg. agr. i. 16; ii. 74). This ridge is a marine formation belonging to the Older Pliocene period, and consisting mainly of a bluish grey marl, much used for making bricks and pottery, and of yellow sea sand, of great value for building purposes.The name was usually explained by the ancients as meaning' the city of Janus' (Serv. Aen. viii. 357; Varro ap. August. civ. Dei vii. 4; Solin. ii. 5; Macrob. i. 7. 19; Ov. Fast. i. 245); sometimes, apparently, as the 'gate' (Fest. 104). The connection between the hill and Janus was doubtless due to the presence here of a cult of the god, who was afterwards explained as an early king of the district (cf. WR 103 ff.). No trace of this cult existed in historical times, but it may be inferred from that of FONS or FONTUS (q.v.), the reputed son of Janus. According to Pliny (NH iii. 68), the original name of this settlement was Antipolis (v. PAGUS IANICULENSIS).
Ancus Martius was said to have fortified the Janiculum in order that it might not be occupied by a hostile force (Liv. i. 33 ; Dionys. iii. 45), and during the republic a guard was always posted on the hill while the comitia centuriata was meeting in the campus Martius (Liv. xxxix. 1 5; Cass. Dio xxxvii. 28); but there is no evidence of any fortification until the completion of the first permanent bridge over the Tiber, the pons Aemilius, in 142 B.C. Whatever was built then was probably at the top of the ridge, near the porta Aurelia in the line of the later wall of Aurelian, which was brought up to this point from the river for this very reason (Richter, 51, 120). It was the first point of attack for Marius and Cinna in the Civil Wars (Liv. Ep. 80; Appian, BC i. 67; Flor. iii. 21, 23).
For a discussion of the derivation and meaning of Janiculum and of the hill and its fortifications, see Richter, Die Befestigung des Ianiculums, Berlin 1882 ; Elter, Vaticanum, RhM 1891, I I 1-38; Mayerhofer, Gesch.-topographische Studien, Munich 1887, 7-21 ; RE ix. 691 ; Jord. i. I. 197, 242-243; HJ 623-5; Nissen, Landeskunde ii. 489-490; DE iv. 3-5.