 The bricks of the segmental arches are made of yellow @-@ red clay , mixed with fine brick fragments . The bricks are rectangular plates , measuring ca . 40 cm × 50 cm ( 15 @.@ 7 in × 19 @.@ 7 in ) and 5 cm ( 2 @.@ 0 in ) thick . They are placed with the shorter side upright facing outwards , so that the total thickness of the arches measures ca . 80 cm . The bricks are bound with grout of lime mortar mixed with brick shards and fine gravel , forming 4 cm ( 1 @.@ 6 in ) -thick joints . The two later semi @-@ circular arches were built with smaller bricks , although surviving bricks from the original arch were re @-@ used at places . The abutment stones are dressed limestone ashlar blocks , and form a sloping surface to support the arches . 
