ANGLESEY or ANGLESEA, an insular northern county of Wales, area about 276sq.m., is cut off from the adjoining main land of Carnarvonshire by the shallow Menai strait. The island differs strikingly from most other parts of Wales in the lowness of the land—the highest points being 7 2 of t., on Holy island in the north-west, and 494ft. on Parys mountain in the north-east. It is a much worn down ancient highland, the worn ribs of which run as lines of moorland between slightly lower lines of marsh in a north-easterly to south-westerly direction. Three distinct lines of very ancient rock (pre-Cambrian schists and slates), now worn down almost to sea level, follow the general north-easterly to south-westerly trend. The north-western section includes Holy head and Llanfaethlu, the central Aberffraw and Trefdraeth, and the south-eastern Newborough and Pentraeth. In the region be tween the central and north-west lowland just described, we have slightly higher ground, composed of granite on its southern side, and much folded Ordovician slates and grits on the north. The latter fan out to the north-east of the island, where volcanic intru sions have given rich veins of copper at Parys mountain. Between the central and southern pre-Cambrian peneplains the slightly raised ground is mostly composed of carboniferous limestone. Similar patches of limestone outcrop on the extreme south of the island, forming the northern shore of the Menai strait. The latter is just another of these lower north-easterly to south westerly lines that has been submerged, and is more picturesque, if less wild, than the island marshes lying parallel to it. There are abundant evidences of glaciation, and much boulder clay and drift sand cover the older rocks. Patches of blown sand occur on the south-west coast.
The importance of Anglesey in prehistoric times is clearly in dicated by the large number of megalithic monuments (chiefly dolmens, called in Wales cromlechs, and menhirs), that cover the island, especially in the south and west (see E. N. Baynes, "The Megalithic Remains of Anglesey," Cymmrod. Soc. Trans. 1 9 1 o- I I, pp. 3–g 1) . Thus Anglesey, together with many of the western promontories of Britain, France and Spain, seems to have been an important station on the coastwise and transpeninsular routes be tween Spain and Ireland (see PEMBROKESHIRE). A group of mediaeval chapels in the neighbourhood of Holyhead suggests the continual use of this route along lines similar to that at St. David's (q.v.) in mediaeval times. There are indications that a similar maritime trade reached Anglesey in the late bronze age. Finds of exotic bronzes, a gold bracelet and jet beads have occurred in the centre of the island and near the coast at Holyhead and Beaumaris. Occupational evidences of some shortly pre-Roman period are to be found in the Hut circles at Ty Mawr on Holyhead mountain. Agricola is known to have conquered and settled Anglesey about A.D. 78. The island was known to the Romans as Mona, and recorded by them as the home of the Druids. Domestic implements of native workmanship are found at Pen-y-Bone in connection with Roman coins of the 1st and end centuries, as well as Samian ware and cinerary urns of Roman origin. In the turbulent post-Roman centuries Anglesey seems to have been the prey of raiders from Ireland, Man, Scotland, England, and Wales. The Scandinavian raids, and the conquest of the island by the Saxons, under Egbert, and their ultimate expulsion seem to be the outstanding features. The raids did not cease with the arrival of the Normans, as the fatal expedition of Hugh of Chester in xog8 shows. They realized that although they could retain the lowlands of South Wales and the Border, Anglesey and Powys could not be retained without serious risk of disaster. Consequently the island retained a measure of independence throughout the early middle ages, and at Aberffraw was located a capital of the Welsh princes of Gwynedd. The large expanses of low-lying ground (so rare in Wales) and the small rainfall, made the island one of the chief corn producing regions of the principality, and thereby earned it its name of "Mon Mam Cymru" ("Anglesey the mother [nourisher] of Wales") . The county was finally subdued by Edward I., who built an important castle at Beaumaris in I295-96 and declared Aberffraw the capital of the island. Old traditions and customs, enriched by the age-long contacts with Ireland, lingered long in Anglesey, and gave a peculiar interest and charm to the mediaeval descriptions of the court at Aberffraw. Penmon priory, 4km. from Beaumaris, was the most important monastic house, founded on an earlier site in the early part of the x3th century.
For centuries after the Edwardian conquest, the remoteness of Anglesey from the English plain made it the centre for those who plotted against the Crown. It became important for its sea faring and fisheries. Its people were deeply attached to the royalist cause in the 1 7 th century. The development of Holyhead as a packet station for Ireland is the central feature in modern Anglesey, and provides an interesting example of how a route famous from prehistoric days has lived on under modern condi tions. It was a regular station in the reign of William III., and was selected by the Government in 1801 as an Irish Packet station and as the terminus of Telford's great road (1815-19) from Shrewsbury, which crossed the island from south-east to north west. A fine harbour was built by the London and North Western railway in 188o, and since then most of the Irish traffic to England has passed through Anglesey. Copper mining at Parys mountain in the north-east of the county was important between 1762-185o. Other minerals, including coal, have been worked in small quanti ties at different times in the island. Agriculture, sheep rearing and coastal fishing provide occupation for the bulk of the popu lation at the present time. The county is connected with the mainland by two bridges over the Menai strait, viz., Telford's suspension bridge (1819-26) the largest in the kingdom, built at the zenith of the stage coach traffic, and Stephenson's tubular railway bridge, built in 185o.
The L.M.S. railway (Chester and Holyhead branch) crosses Anglesey from Llanfairpwllgwyngyll to Caerwen and Holyhead (Gaergybi), also from Gaerwen to Amlwch. The population of the county in 1931 was 49,025. There is no parliamentary borough, but one member is returned for the county. It is in the north-west circuit, and assizes are held at Beaumaris, the only municipal borough, pop. (1931) 1,708. Amlwch (2,561), Holy head (10,707), Llangefni (1,782), and Menai Bridge (Pont y Borth) (1,675) are urban districts. The island is in the see of Bangor, and there are 678 parishes.
See E. Breese, Kalendar of Gwynedd (1873) , and The History of Powys Fadog; E. Greenly, The Geology of Anglesey, vols. i. and ii., and Memoir of Geological Survey (1919) .