Tertiary
The Tertiary period began sixty-five million years ago with fire, and ended only two million years ago in ice. It opened with a meteorite about the size of the Tees Valley, slamming into the Earth with unimaginable force near to where Mexico lies today. This catastrophic event dealt the final blow to a now-declining population of dinosaurs, along with other Mesozoic creatures such as ammonites and belemnites. It ended when a large part of the Northern Hemisphere was overwhelmed by advancing glaciers at the onset of the Ice Ages.
No sedimentary rocks of this age exist locally today, but the Tees Valley’s only igneous rock, the Cleveland Dyke, formed during the Tertiary period some fifty-eight million years ago. Intense volcanic activity along the west coast of Scotland ended with the earth’s crust being stretched as the Atlantic Ocean grew between the continents of North America and Europe. Magma was injected into fissures deep under the surface, which, on cooling, formed dykes that can today be found at, or close to, the surface. One of these extends all the way from the Isle of Mull to Teesside. The magma formed a durable, blue-grey rock known as whinstone much used for road metal and cobbles. It was extensively quarried and mined between 1869 and the 1930s at Cliff Rigg, near Great Ayton, as well as Preston-on-Tees, Ingleby Barwick, and numerous places on the North York Moors.
The Tertiary period also saw evolution of human ancestors including Homo habilis, the first tool user, around three-and-a-half million years ago. True humans, like you or I, did not appear until a mere one hundred-and-fifty thousand years ago, and geology as a science is said not have really got going until the late 1700s. However, could it be that their expertise in the use of simple stone tools made these primitive ancestors the world’s first real geologists?