Biscay (b?s?k?) Bay of, an eastern arm of the Atlantic, extending from Ushant Island to Cape Ortegal, and flanked on the east by France and on the south by Spain. It is about 350 miles long by 300 miles in width. Violent circular currents, as well as storms, make the navigation difficult. The Spanish coast is rocky, but the French coast is low and sandy. The depth varies from 20 to 200 fathoms. The principal ports on the French coast are Bayonne, Bordeaux, Nantes, Rochefort, La Rochelle and Brest; those on the Spanish coast are Santander, San Sebastian and Bilboa. The Rivers Loire and Garonne flow into the bay.