This page provides a glossary of geological terms to be found on the TVRIGS website and elsewhere. It is an evolving document and will grow as the site expands.

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«Glossary(L-S)

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Around 58 million years ago, as the Atlantic oceanic basin formed, adjacent areas of crust became stretched and weaknesses could be exploited by molten material (magma) being forced into the crust by pressure from below. This magma cooled very quickly surrounded by local rocks and became the Cleveland Dyke.

Stretching for c.350 miles between Mull in Western Scotland and the Tees Valley and North Yorkshire the hot magma cooled to form a dark blue-grey, finely crystalline rock referred to by geologists, more correctly, as dolerite. Dolerite is chemically similar to basalt, the major difference being that basalt is erupted at the Earth’s surface, whereas dolerite solidifies within the Earth’s crust.

Following removal of the overlying strata by erosion, primarily through glaciation, the dyke was exposed at the Earth’s surface. In the west of our region it can be traced crossing the river at Preston-on-Tees, but perhaps its most notable feature occurs near Great Ayton where the more durable rock making up the dyke, and softer Jurassic strata into which it is intruded, exhibit a phenomenon known as differential erosion. The softer sedimentary rock is preferentially removed by erosion leaving the harder whinstone to form a bold ridge called Langbaurgh Ridge.

The geater hardness of whinstone relative to sedimentary rock makes it ideal for use road-stone and cobbles, and it was for this purpose that Leeds City Council leased land around Great Ayton, where the ridge is best developed, in 1869. Large quantities of the rock were quarried at Cliff Rigg, as well as elsewhere along the length of the dyke, for example at Preston-on-Tees, Ingleby Barwick, and at a variety of locations on the North York Moors. The now-abandoned workings today form an unmistakeable scar on the landscape, though the former quarry’s remains allow geologists to study the effects of metamorphism, i.e. the baking of the surrounding sedimentary rock when the hot magma was injected.

1. Introduction
2. Tertiary Rock in the Tees Valley

Introduction

The Tertiary Period began 65 million years ago with fire, and ended a little over 2.5 million years ago in ice. It opened with a meteorite, around 10km across, slamming into the Earth with unimaginable force at the Chicxulub impact site close to Mexico. This catastrophic event marks the close of the preceding Cretaceous Period and dealt the final blow to an already-declining population of dinosaurs, along with other Mesozoic creatures such as ammonites and belemnites. After a period in excess of 62 million years, the Tertiary came to a close when a large part of both planetary hemispheres were overwhelmed by advancing glaciers marking the onset of the succeeding Quaternary Period.

This shaded relief image of Mexico's Yucatan Peninsula show a subtle, but unmistakable, indication of the Chicxulub impact crater. Most scientists now agree that this impact was the cause of the Cretatious-Tertiary Extinction, the event 65 million years ago that marked the sudden extinction of the dinosaurs as well as the majority of life then on Earth.

Tertiary Rock in the Tees Valley

No sedimentary rocks of Tertiary age exist within the Tees Valley today, but the district’s single example of a native igneous rock, the Cleveland Dyke, was emplaced during the Tertiary Period some 58 million years ago. Intense volcanic activity along the west coast of Scotland caused the Earth’s crust to become stretched as the Atlantic Ocean grew between the continents of North America and Europe. Molten rock (magma) was injected into fissures deep beneath the surface. On cooling, emplaced magma formed dykes which can today be found at, or close to, the surface. One of these extends all the way from the Isle of Mull, through Teesside, and terminates on the North York Moors. A distance of 260 miles.

The magma formed a durable, blue-grey rock, a type of dolerite known coloquially 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 under leases with Leeds City Council, as well as at Preston-on-Tees, Ingleby Barwick, and numerous sites across the North York Moors.

An idealised cutaway showing the working of whinstone by both quarry and mine.

The Tertiary period also saw evolution of our human ancestors including Homo habilis (Handy Man), the first tool user, a species that wandered the African plains between 1.9 million to 1.6 million years ago. Homo sapiens (Wise or Knowing Man), like you and I, did not appear until a mere 200,000 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 production and use of intricate stone tools qualify our primitive ancestors to be looked upon as the world’s first real geologists?

« Middle Jurassic                             Quaternary »

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