Dykes of Tertiary age have been identified by modelling offshore marine and aeromagnetic data and onshore aeromagnetic data. Dykes have been found in the North Sea off NE England, and in the Irish Sea. Published aeromagnetic maps have also been used to locate non-outcropping Tertiary dykes in the Scottish Borders. Many of the offshore anomalies are caused by multiple dykes that cannot be separately resolved. All dykes retain the dominant NW-SE trend of the known Tertiary dykes although this trend approaches E-W near the east coast of England. Under the North Sea the trend swings back and becomes increasingly southerly trending. The dykes may be the result of NW-SE compressional or NE-SW tensional stress with deep crustal weaknesses causing changes in trend.