The stunning situation of Bamburgh Castle as a defensible complex is no accident, and the entire history of occupants have enjoyed centuries atop a geological formation created hundreds of millions of years before humans.
About 337 million years ago, marine and delta environments formed the limestone (which are in essence fossilized marine creatures) and sandstones/siltstones/mudstones (progressively tinier grains of minerals that have formed sedimentary rocks) of the Alston Formation. The red sandstones of Bamburgh were formed at this time. These are the stones you see at the base of the castle outcrop from the village, as well as used in some of the earlier masonry around the castle. Around 290 million years ago, however, volcanic activity injected molten rock into the sandstone’s layers. These igneous or volcanic rocks became the dolerite that we know now as the Great Whin Sill, perhaps most obvious as the ridge of rock upon which the central part of Hadrian’s Wall is constructed. This stone is extremely hard, but good building material, while the sandstones can often be easily carved. For us, the dolerite is basically the bedrock into which occasionally we see slots for buildings but under which there is no human occupation. The highest point of the entire outcrop is 45m, measured just outside the keep.
At the lowest level of the castle keep, there is a well that was cut through the dolerite and sandstone during the Saxon period. Unfortunately, as of the last tasting in the late 19th century, the water was brackish at best. But we know from an entry for 774 by Symeon of Durham in his Historia Regum: “There is on the west and highest point of the citadel, a well, excavated with extraordinary labour, sweet to drink (dulcis ad potandum) and very pure to the sight (purissimus ad videndum).”
The beach at the base of the castle has been gradually been deposited particularly over the last few centuries, while sites further down the east coast (such as the Lincolnshire waterfront) have seen massive erosion: the beaches are shrinking there, devouring abandoned medieval coastal settlement sites, and growing via sand deposition here along the north east. Some of the dunes at Bamburgh are therefore only 300-400 years old, but other areas of the beach and dune network were first laid in later prehistory (about 2000-3000 years ago).
There was an inlet during the Anglo-Saxon period that allowed ships to come much closer to the castle’s entrance at the northeastern tip of the West Ward. It is also highly likely that during this time the tide actually reached the seaward base of the rock upon which the entire complex was built.