RISING TIDES
The moon, the sun and the spinning of the Earth all pull on the vast briny mass of the world’s oceans to create tides, and a fascinating, ever-changing shore for walkers to explore. But did you know…
The forces that move oceans shift land too, although you won’t feel it move underfoot as you stroll along. Known as an earth or crustal tide, the ‘elastic deformation’ of the ground rarely exceeds 12 inches, but fascinates volcanologists who believe it triggers eruptions.
Some places have a single high and low tide each day (called diurnal) but most of Britain sees two (semi-diurnal).
In most spots, high tides occur roughly every 12 hours and 25 minutes, but low water isn’t always at the halfway point. The bathymetry (contours) of the seabed affects ebb and flow, so check local tide times before a walk on the shore. Take the east coast towns of Cromer and Aldeburgh: separated by just 60 miles but with high tides many hours apart.
The first known British tide table was written by John Wallingford, a 13th-century Abbot of St Albans.
Tide heights vary with the movements of moon and sun. The biggest range – the highest high water and lowest low – is called a spring tide. It comes from leap up (not the season) and happens just after full and new moons, when the sun, moon and Earth align. The highest spring tides of all occur around the equinoxes. The smallest tidal range coincides with the moon’s first and third quarters. It’s called a neap tide, from the AngloSaxon meaning without power.
The Severn Estuary has the second-biggest tidal range in the world: the water rises as much as 50 feet between low tide and high. As the sea races into the river’s narrowing channel – the Severn is five miles wide at Avonmouth, less than 100 yards by Minsterworth – it can push a wave upstream that tops six feet tall. Known as a bore, it’s most impressive on high spring tides around the equinoxes and while people do try to surf the wave, we’d suggest a walk along the banks at Minsterworth.*
The shore between high and low water is called the intertidal zone and is an extreme habitat: sometimes submerged in icy sea, sometimes exposed to baking sun. As well as mussels, cockles, and seaweeds, the receding brine can reveal dinosaur trails and prehistoric woodlands. Low-tide walkers can spot the footprints of iguanadons at Hanover Point* on the Isle of Wight and of ornithopods at Staffin Bay on Skye. At Borth* on the Ceredigion Coast, a petrified forest – the stumps of trees that died 4500 years ago – appears at very low water.
The tides reveal, but can also destroy, ancient heritage around the shore of Britain. CITiZAN (Coastal and Intertidal Zone Archaeological Network) encourages volunteers to record archaeological discoveries before they’re washed away by the waves. Explore their interactive maps to get the most from your next coast walk or join as a surveyor. See www.citizan.org.uk.