This report details the geology in the northeast of England by analyzing geological events that determined the present-day rock and drift composition, with a particular focus on the Carboniferous onwards. The impact of rock and drift geology on slope stability and methods to improve stability has also been discussed.
The Carboniferous is defined by two phases of deposition, forming the Yoredale Group and Pennine Coal Measures Group, and a period of uplift. In the early Carboniferous, Britain was located at the equator, conditions were semi-arid and there was eustatic sea level rise. Lithospheric stretching occurred due to north-south tension, which caused fault-related differential subsidence, forming graben basins with intervening blocks, raised by low-density granite.
Lower Carboniferous strata formed in shallow marine conditions of the Rheic Ocean via deposition of sediment. This formed horizontal marine limestone composed of clastic detritus with sandstone, siltstone, and dolomite from the north and east, and marine deposits from the south and west. There is a variation in the thickness of Lower Carboniferous strata due to faulting during deposition. The Yoredale facies is a distinctive cyclic succession from the Namurian that is well developed in the Northumberland Trough and Stainmore Trough. Each cyclothem consists of limestone overlain by mudstone, sandstone, seatearth, and coal. They were caused by a sequence of marine transgression followed by shallowing and finally the growth of swamp vegetation. This sequence was followed by the deposition of marine shales, siltstones, and coarse sandstones with cross-bedding called millstone grit. During this period there was more uniform thermal subsidence of blocks and basins.
Throughout the Upper Carboniferous, the northeast was covered by river deltas. This facilitated a swampy environment leading to continued cyclic sedimentation and the burial of plant material which produced the Pennine Coal Measures. These consist of mudstone, siltstone, sandstone, seatearth, and coal, with coal seams making up less than 5% of the total sequence. In the Stephanian, uplift, faulting, and folding occurred due to the Variscan orogeny. Erosion removed large amounts of coal, forming the Permian unconformity, Westphalian beds were only preserved in synclines. Basaltic lava intruded the Carboniferous Limestone to form the Whin Sill, a hard tabular layer of dolerite, with its stratigraphical level changing abruptly via faults and joints.
In the early Permian, Britain lay 10˚ north of the equator; conditions became arid and desert-like and there was oxidation of exposed Carboniferous strata, resulting in a red appearance. Strong winds transported sand, sandstone, and breccia, which were redistributed by flooding into channels between sand dunes. This formed the Yellow Sands Formation of the Rotliegendes Group, this stratum is weakly cemented. There was the transgression of the Zechstein Sea in the late Permian, this formed a shallow lagoon environment where evaporation and deposition occurred. The Marl Slate Formation overlies the Yellow Sands to form part of the first Zechstein cycle. This is overlain by interbedded magnesian limestones and marls. There are seven Zechstein cycles, and they define the marine Permian deposits.
In the early Triassic, Britain drifted 20-30˚ north of the equator, and arid desert conditions were re-established. Sandstone, siltstone, and mudstone sediments were transported via the Budleighensis River and deposited forming the Sherwood Sandstone group. This is overlain by the Mercia Mudstone Group. By the Jurassic, Britain reached the latitude of the present-day Mediterranean, and eustatic sea level rise formed a shallow sea, resulting in the deposition of fossiliferous sediment. The youngest solid strata in the northeast are Jurassic rocks in the North York Moors, subsequent uplift and erosion removed all younger solid strata.
Paleogene dykes are vertical sheets composed of basalt that intruded the Carboniferous coal measures during the Paleogene, magma rose vertically from magma chambers located at Mull. They are distinguished by their east-south-east trend, the two most prominent dykes are the Acklington and Cleveland dykes.
Glacial drift deposits were laid down in varying thicknesses throughout the Quaternary. The climate alternated between cold and more temperate stages, resulting in a complicated deposition pattern. The Albion Glacigenic Group is the oldest quaternary sediment in the northeast, it filled vertical fissures within rocks of the Zechstein Group. During the mid-quaternary, five glacial-interglacial cycles were removed by erosion. The best-known pre-Devensian Till is the Warren House Gill Till formation, a pebbly sandy clay. It contains detritus from Norway, indicating that the till was deposited by ice originating from Scandinavia.
The Caledonia Glacigenic Group includes tills, gravels, sands, silts, and clays that form deposits within the Devensian ice sheet. The northeast comprises deposits from the North Pennine Glacigenic Subgroup, these are dominated by clasts from the Carboniferous. The last glacial maximum occurred in the Late Devensian when ice traveled from the east across the Pennines. The North Sea ice stream advanced and diverted ice from the Pennines. Both streams retreated, with the North Sea ice remaining just offshore restricting drainage and forming lakes, such as the glacial lake wear. A sheet of boulder clay formed across much of lowland Northumberland and Durham. Wear Till is the thickest and most consolidated till, Butterby Till is less compact and susceptible to slope failure.
During the Holocene, soils that were weak and erodible at the beginning of the era, gradually gained stability due to the establishment of vegetation and woodland.
The landscape in the northeast is defined by the geological events described in this report, slopes are present in the area due to uplift, faulting, erosion, and deposition. Examples include the Pennines and Cheviot Hills, formed during the formation of Pangaea in which there was uplift, extensive erosion has since removed much of the deposited sediment, which altered the height and geometry of the slopes.
The northeast is mainly composed of sedimentary rock intruded with igneous rock such as Whin Sill and Paleogene dykes. The majority of the rock surface is mantled by unconsolidated drift deposits. The rock and drift deposits have varying strengths due to their formation and the sediment they are derived from.
The main factors that cause slope failure are the angle of the slope, self-weight, and cohesive strength of materials on the slope. Small changes in the shear strength of materials due to the presence of discontinuities on the slope can result in significant changes in the safe height or angle of a slope. Slopes fail in various ways, including wedge, planar, toppling, and circular failure, but this failure can only occur if there is a feasible failure mechanism. The spacing of discontinuities defines the size of blocks such as the joints seen in sandstone in Durham and the orientation of discontinuities is defined by the dip and dip direction. When failure planes caused by discontinuities are parallel or near parallel to the slope, stability is compromised, and planar failure can occur if there is daylight, for example at outcrops. If multiple discontinuities occur, the angle of inclination is larger than the angle of friction and the line of intersection daylights the slope, wedge failure can occur.
Discontinuities in rock and drift materials affect their strength, even those with high characteristic strengths. Glacial Till often behaves comparably to sedimentary rock due to extensive compression between glacial ice. Discontinuities include fracturing, joints, bedding planes, and faults. Bedding planes are common in the northeast and were observed in sandstone in Durham. There are also numerous faults due to lithospheric stretching in the Carboniferous. Joints were observed in sandstone in Durham, these define the block size and reduce the strength of the material from very strong (capable of withstanding up to 250MPa compressive loading) to being at risk of slope failure. Faults and joints in rock with low permeability can create preferential pathways for water, which often impacts rock strength of higher permeability rock, such as joints in the Whin Sill that is interlayered with higher permeability rock.
Water impacts the strength of rock and drift deposits. Rocks such as limestone and dolostone with significant strength and low permeability can be weakened during folding and uplift as their bedded composition is altered resulting in a higher permeability, which can a reduction in shear strength. Water pressure has the greatest impact on rocks, sands, and gravels, however, mudstones and shales are greatly impacted by moisture content. Unconsolidated sediments are often weak when dry or saturated, they can exhibit failure via rotational slip. They tend to be stronger when moist because the water surface tension holds grains together, however, if the sediment is saturated, the water pressure pushes sediment apart and reduces inter-particle friction. Water also increases loading on the slope which can cause failure if friction resistance is exceeded.
Weathering and erosion can compromise rock strength. Igneous rocks such as basalt have high strength, however, weathering breaks them down, making them susceptible to failure due to reduced shear strength. Cyclic sedimentation patterns such as coal measures are formed of multiple sediments, these materials erode at different rates. In Durham, anthracite lies beneath sandstone and the coal is more erodible, hence if erosion continues, the slope is susceptible to toppling due to its self-weight.
It is important to ensure that slopes are stable to avoid failure; civil engineers must ensure that the ground surface is capable of withstanding loading from new structures. There are many techniques used to stabilize rock and drift deposits. Often tensioned rock bolts or cables are installed at an angle to the plane to increase the normal force on the plane of weakness, which increases the frictional resistance between the base of the block and the plane preventing sliding. Another method is to apply shotcrete to the slope, this provides a protective layer that prevents erosion and weathering of the rocks beneath, which would reduce their shear strength, it also helps to prevent loose debris from falling by filling discontinuities in the surface. Adequate drainage systems to reduce water pressure are necessary for slopes to prevent water content from impacting the rock strength failing, holes can also be drilled into the rock slope to relieve water pressure. Alternative solutions for avoiding slip of bedded slopes and other discontinuities (a point of weakness) are to insert sheet piling or retaining walls to prevent daylighting. It is also possible to reduce the slope angle via excavation, this ensures that the failure plane and slope angle are not parallel. It can also reduce loading on the slope, which improves stability by reducing the force between discontinuities. Soil stability can also be improved by grouting to improve cohesion.
In summary, the northeast comprises slopes of sedimentary and igneous rock mantled with drift deposits, the strength of the material, orientation of discontinuities, angle of slope, and loading impact slope stability. Small changes in conditions can alter slope stability, so measures must be put in place to prevent failure from occurring.