The coat or arms of Plasencia de Butrón, with a nao (a seagoing vessel) over the waves, with castles in bow and stern, bowsprit, tops in the main and fore masts, lateen rigging in the mizzen mast. Above the ship there is an 8 point star, and two swords showing the contribution of Plasencia to the armies of the Lord of Bizkaia. The fore and main masts have flags waving in the wind, showing the ownership of Plasencia and Bizkaia.
View of Plentzia in 1865. Litography of J. E. Delmas.
Little evidence remains in the district from eras prior to mediaeval times. Just a few tools and utensils from the Palaeolithic era and Roman coins, probably related with trading activities. Plentzia, originally called Plasentia de Butrón, was founded on the old settlement of Gaminiz in 1266 by Don Lope de Haro and was awarded the Charter of Logroño in 1299 by Don Diego Lope de Haro. From the 11th century onwards it is mentioned in treaties, directions and on maps. It is interesting to observe the different names that the authors of these, mainly French and Dutch, attribute to the town. In its origins it was a sailing and fishing village, and in its founding document it was attributed an area for whale fishing. Later, its seafarers worked in cabotage and the transport of iron ore in the Bay of Biscay, with most boats being registered for scallop fishing and deep-sea sailing. From the 19th century onwards, owing to the Wars of the Convention with England and the Carlists, practically the entire fleet registered in Plentzia disappeared. Sailors from Plentzia, enrolled on boats from other ports, were especially active in trading with Andalusia, England, the Caribbean and the Philippines.
All aspects of navigation were governed by the Seafarers’ Association of Plentzia, called San Pedro, which brought together sailors from Barrika, Sopelana, Lemoiz and Gorliz. Its oldest regulations date back to 1524, and it lasted until 1870. Varied documentation pertaining to it is exhibited, such as registration certificates, sea lists, tickets, etc. It was the Association that promoted the Plentzia Sailing School, which had been operating from time immemorial and existed until 1933. Also during the 20th century the Customs, the District Court and the Marine Assistance Board were to disappear.
Owing to the excellent quality and quantity of wood from the area’s forests, shipbuilding yards had been in existence since the Town’s beginnings, in the place where today there is a square named after them, as appears in the print by Delmas from the mid 19th century. They disappeared following the Second Carlist War, in the last third of the 19th century. The estuary has played a decisive role in marine development, and it was a subject sung of by the poet Basterra, whilst its course can be seen on various maps. Problems of water depth made worse by the construction of exterior docks in the early 20th century, the channelling of the estuary and the construction of the Clinic Maritine main road have spoiled and varied the old topographical profile.
   