 As far as can be determined , between arches 2 to 21 , the outer facing of the superstructure consists of four layers of brick , followed by layers of rough stonemasonry bound with mortar . In contrast , between arches 22 and 26 , as well as in both ramps on either end , the facing consists of ashlar blocks . The two repaired arches 27a and <unk> are differentiated from the earlier work through the use of smaller unworked stones and the incorporation of irregularly placed brick tiles . In arch 26 , the lower side of the arch still features a projecting bearing used for the placement of the falsework . 
