GEOLOGY SOUTH OF THE WEAR

"SUNDERLAND'S SECRET GEOLOGY" 17th Sept 2011 Leader: Andy Lane

Photo's can be accessed by clicking here.

As Andy Lane, our leader said, this field excursion took us to see some of the less well-known geological sights of the Sunderland area. That they are less well-known does not mean that they are not spectacular!

Overview: Towards the end of Carboniferous times, about 290 Ma., Northern England underwent uplift followed by a long period of erosion of about 30 million years. It is estimated that up to 500 metres of Upper Carboniferous rocks were removed. The land subsequently became a low lying undulating plain situated just above the equator, the climate at the time undergoing hot and arid conditions.

It was under these conditions that desert sands, the Yellow Sands Formation, were laid down. Their age is uncertain but they are thought to be Late Early Permian, about 270 Ma. The unconformity between Upper Carboniferous rocks and overlying Yellow Sands, well exposed in the cliffs at Tynemouth Priory, therefore represents a gap of c.30 Ma.

About 260 million years ago, a sea which invaded an area stretching from roughly the eastern side of Britain across Germany to Poland resulted in the deposition of the Marl Slate followed by a succession of dolomitic limestones. Periodic evaporation of the sea, due to it being almost land locked, resulted in 4 or 5 cyclical sequence of  limestones and evaporites such as salt, anhydrite and potash.

About 245 Ma., at the beginning of Triassic times, the land in the Durham area became a sabkha type environment, i.e. flat and very salty areas of sand and silt lying at almost sea level. Around 200 Ma., with the opening of the Tethys Ocean, the sea once again invaded the area and heralded the beginning of Jurassic times.

Locality 1Field House Farm (NGR 355506) This is a now a part working, part landfill site. It displays perhaps the best section of the Lower Permian Yellow Sands in the region, and details of massive desert/dune structure and the nature of the sand were clear to see. Above the sands was the patchy development of the Marl Slate (the oldest formation of the Late Permian and famous for its fossil fish, followed by beds of the Raisby Formation (oldest of the Magnesian Limestones) well up the quarry face.

Locality 2 Houghton Hillside Cemetery. This is an abandoned but historic burial ground, currently being restored by the site's Friends, cf. their website: http://www.theoldcem.co.uk/page1.php
From a geological point of view, it is unique in that the cemetery was established in a quarry! Bedding and texture  of the Raisby Formation are well displayed. Partial replacement back to calcium carbonate can be seen giving the rock its distinctive appearance. A fault was observed which seemed to have caused some subsidence in the nearby A690 road, indicating that the fault could still be active, probably due to post mining subsidence. (See photo's.). The opportunity was also taken of engaging in some "Graveyard Geology".

Locality 3 Hendon Promenade NGR 412549 .  Here, a section of the Concretionary Limestone Formation (part of the Upper Magnesian Limestone) was examined. The section showed a vast range of alteration structures caused by early Tertiary(?) collapse and dissolution processes which altered the rock beyond recognition. Small scale faulting was also visible, probably tectonic in origin but may have been enhanced by mining subsidence.

The party then walked southwards towards Salterfen Point to admire the 250Ma Permian/Devensian unconformity, viz. Devensian sediments consisting of about 3 metres of boulder clay overlying the Concretionary Limestone Formation. Above the boulder clay was a considerable thickness of cross bedded sand, possibly caused by meltwater outwash from glacial Lake Wear.

Locality 4 Tunstall Hill NGR 396544. The hill forms part of the famous Permian barrier reef type (Ford Formation - Middle Magnesian Limestone) which runs in a north-south direction. "From its northerly outcrop at Down Hill, the reef can be traced southwards through the western suburbs of  Sunderland into County Durham, where it is known to extend a short distance south-west of Hartlepool, a total distance of around 32km (Hollingworth and Pettigrew, 1988)." (From Limestone Landscapes - a geodiversity audit and action plan for the Durham Magnesian Limestone Plateau, Geology and Landscape, England Programme. Open Report OR/09/007, BGS & NERC).
From the top of the hill an idea of the ancient palaeogeography was well demonstrated. Houghton Hope to the south of hill was observed. It is interpreted to be a melt water channel from glacial Lake Wear, maybe one of the channels which carried outwash sand to Salterfen Point.

This last locality brought the field trip to its end. Our leader was warmly thanked by everyone for a most interesting and informative day.

Photo's can be accessed by clicking here.

(Any errors in this report are entirely the responsibility of the web editor.)

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