Sinkin in Ripon EarthCache

Sinkin in Ripon

GYPSUM (CaSO4 .2H2O) is readily soluble in water and develops karst features much more quickly than does Limestone, this can be up to 100 times faster than Limestone

I have placed this Earth Cache near Ripon to help understand the impact of this Geological feature which over the years has proved to be both mysterious and extremely expensive to both businesses and householders alike, this you will discover on your journey around Ripon. From Hartlepool in the North and curving South West towards Darlington, then due South down to Ripon and finally onto Derby and Nottingham lies one massive belt of Gypsum. The area you will investigation to the East Ripon is associated with the floodplain of the River Ure, and closely neighbouring areas, this area is the most severely affected by subsidence related to gypsum dissolution in this belt of gypsum, however the problem extends over a much larger area. Ripon has the worst gypsum related subsidence in England, at least 30 major collapses have occurred in the last 150 years. The subsidence features, which are associated with the underlying gypsum, are commonly 10-30m in diameter and reach up to 20m in depth. To the east of the city, a collapse sinkhole in the Sherwood Sandstone is 80m in diameter and 30m deep. The highest density of subsidence features is along the sides of the buried Ure valley where the artesian water escapes from the gypsum sequence. However, the apparent density of subsidence hollows is lower on the present river flood plain owing to the hollows being constantly filled with over-bank flood deposits. The Geological sequence beneath Ripon comprises Upper Permian dolomitic limestones, evaporites (two distinct layers of gypsum), mudstones and marls overlain by the Sherwood (Triassic) Sandstone. This sequence is overlain by Quaternary deposits comprising of boulder clay, sands and gravels. The present River Ure is a misfit river whose antecendent, the much larger proto-Ure cut down through the bed rock during the Pleistocene, to a depth of about 30m below the present floodplain. This proto-Ure valley is critical in the development of subsidence around Ripon, Cooper (1998) illustrated 16 sinkhole variations in the subsidence belt at Ripon These variations are caused by a number of variables including; the type of gypsum karst, the nature and thickness of the overlying deposits, presence of solid rock overlying the gypsum and the size of voids in gypsum. To the west of the Ripon, The Edlington Formation lies directly beneath the drift deposits. Therefore the marl unit and drift deposits above gradually subside into the pinnacled karst developing in the gypsum Between the city and the River Ure, the limestone of the Brotherton Formation overlies the Edlington Formation. Here the karst developed comprises caves in the gypsum which have migrated up through the overlying rock to form features known as breccia pipes.

The Ripon Bypass was constructed to the east of Ripon. It crosses the Permian sequence which includes approximately 35m of gypsum in the Edlington Formation (formerly the Middle Marl) and 10m of gypsum in the higher Roxby Formation. These two gypsum sequences rest on two limestone aquifers, the Cadeby Formation (formerly the Lower Magnesian Limestone) and the Brotherton Formation (formerly the Upper Magnesian Limestone) respectively. The limestone dip slopes act as catchment areas and the underground water flows down-dip into the gypsiferous sequences, before escaping into a major buried valley along the line of the Rive Ure (Cooper, 1986, 1995, 1998). Complex cave systems have developed in the gypsum, and artesian, sulphate rich springs are locally present. Because of the thickness of gypsum the caves are large and surface collapses up to 30m across and 20m deep have been recorded. The subsidence is not random, but occurs in a reticulate pattern related to the jointing in the underlying strata (Cooper, 1986) Around Ripon, a significant subsidence occurs approximately every year (Cooper, 1995) The dates of the subsidence events show that some areas are more active than others, especially areas bounding the Ure valley where cave water escapes into the buried valley gravels. The new Ripon Bypass crosses the subsidence belt and the new Ure Bridge is situated in a very active area. The Ripon By Pass and Bridge pass close to several subsidence hollows. Engineering solutions In addition to the standard site investigation that was undertaken for the Ripon Bypass, a desk study of the subsidence features was carried out. This study included an assessment of the likely magnitude and frequency of the subsidence events along the route of the road. Even if a costly investigation of closely spaced boreholes had been undertaken, it is unlikely that any cavities or breccia pipes which were identified could be stabilised. Grouting was impractical due to the large size of the cavities and the fact that filling them was likely to cause accelerated dissolution in the adjacent ground. Because the line of the bridge could not be changed the decision was made to design and construct the bridge with inbuilt protection against future ground subsidence. The philosophy of the approach has been to maintain public safety, while not over-engineering the structure to an excessive degree with the resultant heavy financial costs.

Now the cache and important questions, The Co ords will take you to the start of the Ripon By Pass, you will be standing close to the New Ure Bridge; This will be the starting point for your walk around this area which over the years has seen many dramatic events, I recommend that you follow the way points in numerical order, the route can be done in well under an hour, however this is an interesting walk so be prepared to spend longer than expected, but thats entirely up to you! Please email your findings and log the cache, any logs without emails may be deleted

1; At the Bridge and easy question to get you started, the date the Bridge was built? And who opened it? Can you explain how this bridge is protected?

2; When standing with your back to Albion Terrace look over the wall, Tell me what you can see and what has happened to it? Please be aware the footpaths are narrow around here.

3; You will be at Magdalen's Chapel take a walk around and look closely at the Chapel something has happened to the Chapel, please describe what you see and can you describe why?

4; Number 80 What is going on here?

5; Walk along Princess Street and you will arrive at No’s 19, 20 & 21 Can you explain your observations and the reason for these? Across the way 34 & 35 show similar symptoms

6; Broken Back Terrace, yes it’s gone but can you explain why?

7; The Field, Stand at the wall and look North West across the Field, (The Field is private) What can you see? The Geological Feature that you can see is believed to be still active, can you explain what is happening here?

8; Visit Ure Bank and describe the Geological feature seen here please describe the size and shape that still remains, this is on Private Property (The property is believed to be unoccupied, but I think builders are now working on it) but it can be seen from the Footpath without trespass.