In today’s geoadvent post, GSL Education Officer Amy Ball explores the geology beneath the Trafalgar Square Christmas tree…
Norway has presented London with a Christmas tree every year since 1947, in gratitude for Britain’s support during World War II. This tree is famously displayed in Trafalgar Square throughout the festive season, but what lies beneath this well-known Christmas tree? It may not appear to be as dramatic as the Great Glen fault or as impressive as the Giants Causeway but the geology beneath Trafalgar Square and the rest of London can tell its very own Plate Tectonic Story spanning more than 430 million years. London’s geology reveals times of continental collision, folding, faulting, sea level fluctuations and climatic shifts from dry deserts, tropical seas, icy tundra and river flood plains.
The oldest rocks beneath London are known from boreholes drilled over the past 200 years in search of oil, gas, coal and water resources. These basement rocks, approximately 300 m below the surface of the city, consist of folded and steeply-dipping Paleozoic slates, mudstones, limestones and sandstones, which together comprise the London Platform. These sedimentary rocks were deposited during and after the closure of the Iapetus Ocean between Avalonia and Laurasia and they reflect the change in climate from warm, tropical, shallow sea conditions in the Silurian, to semi-arid desert conditions in the Devonian as the continent of Laurussia was formed.
Cretaceous Gault Clays rest directly on top of these much older, unconformable, Devonian sediments. The greenhouse climate and low pole-to-pole thermal gradient of the Cretaceous period led to extremely high sea levels which sunk southern England beneath the water. The Gault Clays therefore represent the shallow muddy-bottomed sea that would have covered southern and eastern England in the early Cretaceous. Fossils including ammonites, belemnites, bivalves, corals, crustaceans and fish bones have all been found in the Gault Clay.
Throughout the Cretaceous period, global temperature and corresponding sea level continued to rise. This lead to the development of a deeper warm, clear, tropical sea across much of Europe and in turn resulted in the deposition of thick layers of chalk, composed largely of coccolithophores (a type of microscopic marine phytoplankton) and calcium carbonate mud.
The Alpine Orogeny occurred predominantly between 65Ma and 2.5Ma, when the African and Eurasian plates collided and closed the Tethys Ocean. The effects of this collision can be seen throughout England as the thick chalk layers deposited in the Cretaceous became folded, faulted and uplifted. The chalk in the London region became folded into a syncline, a downward fold or trough, forming the London Basin.
Overlying the Cretaceous chalk is a thick succession of stiff blue-grey impermeable clay known as the London Clay. The London Clay is about 45 m thick under Trafalgar Square, and formed during the early Eocene (56 – 47 Ma). Animal and plant fossils found in the London Clay such as Nypa palms (currently only found in the swamps and deltas of India), crocodiles, birds, snakes, turtles and sharks indicate that London was under a warm, shallow ocean during the early Eocene, bordered by a lush tropical forest with mangrove swamps, somewhat analogous to Indonesia today.
On top of the London Clay, and making up the final part of our London sedimentary sequence, are loose deposits of sand and gravel up to 10 m in depth. These ‘superficial deposits’ were deposited during the Quaternary period which extends back to ~2.6Ma. During the Quaternary, London endured several Ice Ages and whist never been fully covered by an ice sheet, it would have experienced tundra conditions like modern day Svalbard. During the Anglian stage, which peaked at ~430 Ka, an ice sheet extended as far as north London. This diverted of the ancient River Thames to its present course and has since left behind a complex series of gravels, sands, silts and clays. Abundant fossils have been found in London’s Quaternary deposits and show that animals including hippopotamus, straight tusked elephants, mammoths, narrow nosed rhinoceros, bears, red deer and wild cats would’ve Trafalgar Square and roamed the banks of the Thames in interglacial times 125,000 years ago!
Find out more about London’s Geology and the history of the River Thames in Proffesor Danielle Schreve’s 2011 Geological Society Lecture: The Thames Through Time