MENAI STRAIT, a channel of the Irish sea, separating Anglesey from Carnarvonshire, north Wales, extending 14 m. from Beaumaris to Abermenai, and varying in breadth from 200 yd. to 2 M. It is famous for the suspension and tubular bridges which cross it. The suspension bridge carries the Holyhead road from Bangor. It was designed by T. Telford, begun in 1819 and completed in 1826. The length of roadway between the piers is 55o ft., total length i,000 ft. and height above the spring tide high-water level is ioo ft. The tubular bridge, which carries the L.M.S. railway, was finished in 185o. The bridge, 1,841 ft. 5 in. long, is supported by a central (Britannia) tower and two side towers. Here the channel is about ',1o° ft. wide, and divided in the middle by the Britannia rock, bare at low water. The tide generally rises 20 ft., with great velocity. There are 'or ft. be tween the sea at high tide and the bridge roadway bottom. The limestone used is from Penmon, 4 m. from Beaumaris. The en gineer of the tubular bridge was Robert Stephenson, assisted by Sir William Fairbairn and Eaton Hodgkinson.
The origin of the Menai strait has been much discussed (see E. Greenly, "Geology of Anglesey," Mem. Geol. Survey, 1919). The channel consists of an eastern and a western reach, having a parallel direction from south-west to north-east, and being joined by a middle reach, 2 m. long, having a north to south direction. The two longer arms are typical valleys, belonging to a system of parallel valleys upon the Menaian platform. In pre-glacial times the strait did not exist, but its location was marked by three rivers, one flowing north-east, a small one flowing south and joining a larger one flowing south-west. In late glacial times the western and eastern reaches came into existence, but the water of the latter was dammed back by the Ogwen glacier from the main land, thus forming a long glacial lake which overflowed into what is now the middle reach, following the line of the south ward flowing river, the valley of which it quickly deepened and thus emptied the lake. Post-glacial subsidence and scouring by the tides have completed the work.