A phallic symbol is carved on one of the stones in one of the lowest courses of masonry of the north wing of the bridge abutment
NY9170 : Remains of Chesters Roman Bridge. The ditch here (old river channel) is often flooded and filled with leaves which can hide the carved stone
NY9170 : Hadrian's Wall and Chesters bridge abutment (2). The symbol represented good fortune and protection against evil. Here, it is likely that the symbol was carved to guard the boundary demarcated by the river. There is another carved stone of the same form in the Bath House at Chesters Fort on the north side of the River from the current location
NY9170 : The Bath House, Chesters Fort, Hadrian's Wall. and on a stone at Willowford Roman Bridge
NY6266 : Roman Bridge remains at Willowford
The Romans celebrated Liberalia (17 March) which was the festival of Liber Pater and his consort Libera, with sacrifices, processions, ribald and gauche songs, and masks which were hung on trees. The processional featured a large phallus which the devotees carried throughout the countryside to bring the blessing of fertility to the land and the people. The procession and the phallus were meant also to protect the crops from evil. At the end of the procession, a virtuous and respected matron placed a wreath upon the phallus.
Link
There is clearly another form sculpted to the left of the phallus which is little mentioned in the literature. The English Heritage reference, Hadrian's Wall Bridges, by Bidwell and Holbrooke (1989) describe this as an "ovoid, featureless object which appears to be incompletely sculpted. Both are executed in high relief on the face of the north wing of the eastern abutment of the bridge. The work is rough with heavy chisel marks similar to those dressing the abutment's ashlars. One possibility is that the ovoid object may be a stylised vulva, but in view of the extreme rarity of depictions of this in Roman art perhaps it is more likely to be an Evil Eye being 'attacked' by the phallus. A comparable scene is visible on a block from Chesters fort where a phallus in relief points towards an elliptical object".
There is a similar carved symbol within Chesters Roman Fort on the opposite side of the river
NY9170 : Courtyard of Headquarters Building, Chesters Roman Fort
Chesters Roman Bridge The Roman Bridge at Chesters has been described as the most remarkable feature on the whole line of Hadrian's Wall.
The east abutment of the bridge, which can be accessed on a footpath from a gate on the south side of the road bridge at Chollerford, was part of a large road bridge of 58m length built in about AD 160 to carry the Military Way (the road accompanying Hadrian’s Wall) over the river North Tyne. The abutment, from which the easternmost of four arches sprang, incorporates the pier of an earlier and much smaller bridge which was part of the original construction of Hadrian’s Wall. The later bridge continued in use until the end of the Roman period and was demolished in the AD 670s to provide building materials for St Wilfrid’s church at Hexham.
The course of the river, at the point where it was bridged, has considerably altered since Roman times. The east bank now lies approximately 15m west of the position it occupied then and this shift in its course has resulted in the east abutment of bridge 2 being engulfed in silt.
English Heritage - Chesters Bridge Abutment: Link
Hadrian's Wall Bridges by Bidwell & Holbrook, English Heritage (1989): Link