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Ironbridge Gorge

The geological trail across Shropshire continues with a glimpse into the impact of glacial ice, or rather its melting.

The River Severn runs across the county from west to east. Around Ironbridge it runs in a deep gorge which has been cut through the rock.

The Gorge was cut by the erosive action of a river flowing beneath a glacial ice sheet that had moved south from where the Irish Sea is today across the North Shropshire plain, about 20,000 years ago.

 

The valley sides rise steeply from 40 m at river level to over 140 m on the plateau above. The rapid downcutting by this subglacial river steepened the valley sides so much that they became unstable. Landslides are still occurring to this day.


Ironbridge Gorge photograph taken in 1974 (BGS photo number P211697).

 



Ironbridge Gorge map (Top); Hillshade relief map (bottom) taken from NEXTMap Britain elevation data from Intermap Technologies.
© BGS NERC 2013 http://www.bgs.ac.uk/landslides/IronbridgeGorge.html

 

Two of the larger landslides in the Gorge are Jackfield and Lloyd's Coppice, also described in the separate GeoTrail guide.

The Jackfield landslide

In 1952, a landslide occurred at the village of Jackfield, Shropshire, on the River Severn just over 2 km downstream of the Iron Bridge, destroying several houses and causing major dislocations in a railway and road. The following quotation is taken from the famous civil engineer and soil mechanic, Alec Skempton, when he gave the Fourth Rankine Lecture:

"It is possible that previous landslides may have taken place along at least a part of the present slip surface, but the slope must have been more or less stable for a long time before 1950, when warnings of instability were observed in the form of a broken water main serving cottages near the river bank. Towards the end of 1951 further movement was noted, and by February 1952 the road was becoming dangerous. During the next month or two the landslide developed alarmingly. Six houses were completely broken up, gas mains had to be relaid above ground, the railway could be maintained only by daily adjustments to the track and a minor road along the river had to be closed to traffic. By this time the maximum downhill displacement totalled 60 ft . The strata, consisting of very stiff clays and mudstone, alternating with marl-breccia and occasional coal seams, dip gently in a south-easterly direction with the strike running roughly parallel to the section of the landslide. The slide, however, was confined wholly within the zone of weathered, fissured clay extending to a depth of 20 ft to 25 ft below the surface. The slip surface ran parallel to the slope (which is inclined at 10º), at an average depth of 18 ft. The length of the sliding mass, measured up the slope, amounted to about 550 ft and in the winter 1952-53 ground-water level reached the surface at a number of points, although on average it was located at a depth of 2 ft".

In 1984, further ground movement occurred to the west of the 1952 area of landslide. Salthouse Road was carried into the river and was replaced by a wooden slatted roadway constructed along the line of the former railway.

 

The Lloyd's Coppice landslide

Across the river from Jackfield lies Lloyd's Coppice, woodland on ground that is actively moving as a series of rotational and translational slides. Crags of sandstone have moved down from their outcrop near the top of the hill, bringing with them the overlying thick till. Geological faults, ground water, surface water, the nature of the geology, construction and mining are all associated with the instability of this slope.

The backscarp of this landslide was photographed in 1924 by the BGS:


 

Buildings and roads have been affected by the landslide, sometimes dramatically (the greenhouse is vertical!):


 

Considerable investigative work has recently been undertaken by Telford and Wrekin Council on this landslide to develop appropriate management plans to anticipate and mitigate the effect on the land usage.

 

 

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