Triassic
The Triassic Period started around 225 million years ago. It marks the extinction of creatures such as trilobites, the continued development of reptiles, and first appearance of small mammals in the fossil record. This change in fauna marks an important transition between Palaeozoic (ancient life) and Mesozoic (middle life) Eras of life on Earth.
Sherwood Sandstone Group
During the Late Permian and Early Triassic this area was an arid coastal plain. It was subject to rapid deposition of sediment carried by flash floods flowing from the Mercia Highlands (in the Midlands) to the south. Adjacent to the base of the uplands, thick pebble beds developed as the 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 sandstones and thin mudstones 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 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 Formation is comprised of beds of dark brown or blue mudstone, frequently mottled. The Mercia Mudstone Formation was formed from the fine-grained material washed or blown into the sea from the flat plain. The mudstones alternate with occasional evaporites including halite, gypsum, and anhydrite.
Penarth Group
The sea readvanced. The Penarth Group was deposited over about 30 million years. The highly poisonous salt 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 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 brown and green mudstones containing the fossils of shelly creatures. Indicating life beginning to establish itself locally in the improving waters.
Triassic rocks crop out at very few localities in the Tees Valley making any exposure all the more valuable. However, an example of Sherwood Sandstone can be seen at Little Scar, Seaton Carew when the tide is out, and Mercia Mudstone can be found along the River Leven downstream of Hutton Rudby.