Shown below is a representation of the rocks underlying the Tees Valley and Darlington districts in Northeast England. The diagram depicts the relative thickness of the different systems. Absolute ages are given alongside. Click on the different areas to view more details about a particular time period.

Geological column showing the relative thickness and absolute ages of rocks underlying the Tees Valley and Darlington.

1. Introduction
2. Sherwood Sandstone Group
3. Mercia Mudstone Group
4. Penarth Group

Introduction

Column showing divisions in the Triassic System of the Tees Valley.

During the Triassic the geography of North-western Europe differed greatly to that existing today. At this time, all of the landmasses of the Earth had agglomorated into a single continent named Pangaea. An episode of mountain building, known as the Variscan Orogeny, locally raised parts of the Earth’s crust to form a mountain chain extending between North Devon and The Wash. The so-called Mercia Highland was a fundamental influence, controlling the nature of the rocks deposited during the Triassic.

Triassic deposits have their type locality in Germany and Austria where thick marine deposits indicate the presence of an epicontinental sea in the German Basin with poor connections with the southerly Tethys Sea. Deposits in the UK are generally continental sabhka – a hot arid coastal environment – or shallow water deposits. Rain falling on the Mercia Highlands washed eroded debris down the slopes of the mountains onto a broad flat tropical plain with a shallow sea to the east.

The Lower Triassic deposits found in the Tees Valley do not correlate well with the German type locality and hence have been redefined to suite the local representatives of these rock units. Here the episode is represented by the Sherwood Sandstone Group, a suite of yellow and red sandstones deposited primarily in shallow water. The Middle Triassic, represented by the Muschelkalk of Germany, is missing in Britain as an episode of erosion ensued, represented by an unconformity know as the Hardegsen Disconformity which extends into the modern North Sea Basin. The Upper Triassic locally is represented by the Mercia Mudstone Group, a suite of red-brown and blue-grey mudstones with occasional halite and gypsum deposits. All of these deposits are unfossiliferous demonstrating the harsh conditions locally at this time.

Triassic rocks crop out at very few localities in the Tees Valley making any exposure all the more valuable. The Triassic succession is mostly obscured by the Tees Estuary on the coast, and thick glacial deposits inland. However, an example of Sherwood Sandstone can be seen at Foxton Bridge, and exposures of Mercia Mudstone can be viewed alongside the River Leven downstream of Hutton Rudby.

Sherwood Sandstone Group

During the Late Permian and Early Triassic the Tees Valley constituted an arid coastal plain. It was subject to rapid deposition of sediment carried by flash floods descending from the Mercia Highlands, an upland area extending from North Devon eastwards through Norfolk. Adjacent to the base of the uplands, thick pebble beds developed when descending water dumped the heavier material it carried as it flowed onto flatter ground. Lighter particles of sediment, sand and mud, were spread in great fans away from the uplands across the flat plain, and it is these that make up the Sherwood Sandstone Group in our region. Alternating beds of yellow or red sandstone and thin mudstones exhibiting desiccation-cracks typify the rock unit, which underwent various episodes of both deposition and erosion. It has a distinct lack of fossils, partly because of the harsh environment but also because life on earth appeared to be still recovering in numbers from the Permian Mass Extinction. An erosion surface (unconformity) marks the top of the Sherwood Sandstone Group.

Mercia Mudstone Group

The next episode in the area’s development saw a shallow sea advance across the coastal plain as an intermittent marine connection opened to the southeast. Circulation of water within this sea was restricted and it frequently cut off from the main water body. When it was isolated there was evaporation which then concentrated the dissolved salts. Saline lagoons, pools of hot mud, and glittering beds of salt developed. As a result, the Mercia Mudstone Group is comprised of beds of dark red-brown or blue-grey mudstone, frequently mottled. The Mercia Mudstone Group is a product of fine-grained material washed or blown into the sea from the flat plain. The mudstones alternate with occasional evaporite beds including halite, gypsum, and anhydrite deposited from evaporating sea-water. The uppermost beds in this sequence have been dubbed the Tea Green Marls.

Penarth Group

The close of the Triassic locally saw the Tethys Sea readvance across the saline Triassic plain. The Penarth Group of rocks were deposited over a peroid of c.4 million years. The highly poisonous saline surface, over which the sea advanced, combined with restricted circulation of its waters, produced considerable variations in salinity and oxygen content at first. The lack of oxygen formed the black sulphurous shales typical of the Westbury Beds in the lower part. In this layer a bone bed indicates mass mortality of many creatures, this is believed to have been caused by algal blooms.

As the sea deepened marine creatures of many kinds began to flourish. Consequently, higher in the Penarth Group succession, and hence later in time, lie the brown and green mudstones, known as the Cotham Beds, containing the fossils of shelly creatures, indicating life beginning to establish itself locally in the improving waters.

« Permian                               Lower Jurassic »

©2011 Tees Valley RIGS Group.

1. Site Description
2. Site Map
3. Site Assessment
4. Surveyors

Grid Reference NZ 459 096
BGS Sheet 33
OS Sheet 93
Forwarded as RIGS 30/09/2003

Site Description

Site Status SNCI
Description of Geodiversity River-cliff exposure revealing the contact between the Sherwood sandstone and the overlying Mercia mudstone. The Sherwood sandstone forms a washed platform and waterfall.
Literature References

Site Map

Site Assessment

Surveyors

Andrew Carter, John Waring

1. Site Description
2. Site Map
3. Site Assessment
4. Surveyors

Grid Reference NZ 526 304
BGS Sheet 27
OS Sheet 93
Forwarded as RIGS No designation

Site Description

Site Status None
Description of Geodiversity Small exposure revealing a wave washed scar consisting of Sherwood sandstone
Literature References The Geology of the Country Between Durham and West Hartlepool – Dennis Smith 1967

Site Map

Site Assessment

Surveyors

Andrew Carter