3.1 Geology

3.1.1 Geologically the area is significant because it marks the western limit of the typical English lowland landscape characterised by scarps and gently undulating flat topped hills and broad clay valleys. The older formations of Wales and land to the south-west by contrast are characterised by gnarled and rugged scenery all intensely folded and faulted. These older formations are represented, for example, by the Carboniferous Limestone of the Avon Gorge at Clifton which is outside the area and the Mendip Hills, of which only a narrow strip to the far south-west is within the area.

3.1.2 Drift geology of the Quaternary period is represented within the area by alluvium, terrace loams and gravels, and head. Alluvium consists of accumulated river born materials which have been deposited in earlier valley systems. The terrace deposits were laid down at higher levels than the alluvium. The head deposits are locally derived poorly sorted materials from nearby slopes. They can include silty sand, loamy limestone gravel or clayey loams depending on the parent material.

3.1.3 The cover of younger rocks, Triassic Sandstones, and Jurassic Clays (Fuller’s Earth) and Limestones (Oolitic and Lias), largely dictate the characteristics of the geology within Bath and North East Somerset. They are only gently folded or tilted with near horizontal strata which, on the Cotswolds, gives relatively unbroken crestlines. Oolitic Limestone is particularly significant forming the Cotswold Hills. The Oolitic Limestone has been eroded back to the existing scarp slope over geological time revealing the older contorted rocks of the coal measures of the Carboniferous period. In places it survives as outliers such as at the Dundry Hills, The Sleight near Timsbury and Stantonbury.

3.1.4 Lias Clays underlie the Jurassic Limestones. They make a poor foundation and landslips are characteristic for example at Bath and on the Dundry Hill slopes. The Lias Clays lie on top of the Lias Limestone and are exposed in the valley floors of the Cam, Wellow and Newton Brooks. The Lias Limestone gives rise to characteristic plateaux above the underlying Penarth Group clays and shales and Mercia Mudstones of the Triassic period. There is a distinction between the Lower White Lias which was quarried and used for building to the south of the area and the Blue Lias found above which was quarried around Keynsham and yields numerous fossils including the locally characteristic ammonites. The Triassic formations, including the Mercia Mudstones (formerly referred to as Keuper Marls), make up much of the western area such as around the Chew Valley Lake and give rise to the characteristic red soils and generally low relief.

3.1.5 Finally, the oldest formations in the area are the Carboniferous series which through erosion of the younger formations are exposed in two sections of the central area. They are between Hallatrow and Clutton and between Pensford and Compton Dando. The Carboniferous Limestone of the Mendip Hills has been referred to earlier. In the central areas the geology is characterised by sandstones, shales and mudstones banded with coal seams. The coal seams have been economically important to the area and have a strong present day influence on the culture and landscape. Pennant Sandstone which forms part of this series had to be excavated in order to reach the coal measures but was also quarried in its own right as a building material, for example at Temple Cloud.

Table 2 Summary of Geological Succession within Bath and North East Somerset.

3.1.6 This range and diversity not only helps to characterise different parts of the area but also provides features of significant interest and importance including sites which show special geological sequences, sites which show unique ammonite and bivalve fauna and sites with important fossiliferous Pleistocence gravels. There are currently 11 geological sites of national importance which are designated as Sites of Special Scientific Interest (SSSI) and 47 sites of local importance, which are designated as Regionally Important Geological Sites (RIGS).

3.1.7 No reference to the geology of the area would be complete without reference to William Smith, ‘The Father of English Geology’. Whilst engaged in surveying the Somersetshire Coal Canal in 1792-95 he discovered the principle of stratigraphy; the regular succession of rock strata and their fossils. He created the first geological maps of the area and popularised the term Fuller’s Earth to describe the clay between the top of the Inferior Oolite and the base of the Great Oolite.

3.1.8 Reference should also be made to the hot springs at Bath. Although they are not within the area covered by the assessment they have been a major factor in the development and importance of Bath since and even before Roman times. The hot springs and their influence on development can therefore be seen as an important influence on the landscape character of much of the area covered by this assessment.

3.3 Topography

3.3.1 The complexity of the underlying geology and the nature of the drainage pattern are the principal determinants of the varied topography within the area. Man has had a smaller but nevertheless distinctive additional influence on the topography through coal mining activities. These have resulted in some significant man-made “hills” such as at Old Mills just to the west of Midsomer Norton.

3.3.2 The topography is characterised by hills and plateaux dissected by river valleys. However this description suggests a simple topography which is far from the case. The hills range from extensive land masses over 200m in height such as the Cotswolds and Mendips through to small, rounded outliers such as Stantonbury Hill and Barrow Hill which rise out of plateau areas at around 180m in height. The plateaux range from the high and exposed tops of the Cotswolds to the lower and rather more undulating Lias Limestone plateau between Newton St Loe and Clutton which averages100-150m in height. The river valleys range from the gentle, shallow-sided Chew south west of Pensford to the steep sided Wellow and Cam. The River Avon, the dominant factor in the drainage pattern, alternates rather incongruously from dramatic gorge-like stretches such as the Limpley Stoke Valley and Hanham Valley to open flood-plain between Keynsham and Saltford.

3.3.3 The Valleys 
The vast majority of the area lies to the south of the River Avon and here the three principal tributaries of the Avon - the River Chew and the Cam and Wellow Brooks, flow roughly parallel to each other. They have given a pronounced west to east or south-west to north-easterly grain to the topography. Furthest to the west the River Chew rises in the Mendip Hills and then runs across the soft Mercia Mudstones to form a broad valley narrowing down-stream of Pensford where the underlying rocks are harder. The Cam and Wellow Brooks lie close together and are separated from the River Chew by the Lias Limestone plateau. Both the Cam and Wellow Brooks flow through distinctive steepsided valleys as they cut through harder Lias and Oolitic Limestones except in their upper reaches where the rocks are softer. To the north of the River Avon its tributaries cut down into the hard Oolitic Limestone of the Cotswolds to form steep-sided valleys generally running north-south.

3.3.4 The Hills and High Plateaux 
The parts of the Mendips and Cotswolds and, to a lesser extent, Dundry Hill within the area are small parts of rather more extensive hill ranges. Only a very small part of the Mendip Hills lies within the area, a section of steep scarp slope between Ubley and Hinton Blewett, although the prominence of the slope and ridgeline is felt over a much wider area. Slightly more of the Cotswolds lies within the area to the north and east, encircling Bath and again the skyline is prominent from views over a much wider area. Here, however, the scarp slope is less of a feature, whilst the plateau and incised valleys are of greater landscape significance. Dundry Hill is an Oolitic Limestone outlier of the Cotswolds on the south-eastern edge of Bristol, reaching some 210m in height. Again only a very small part of the hill lies within the area to the north-west around Whitchurch, Queen Charlton and Norton Malreward, from where there are some spectacular views over much of the area.

3.3.5 The Rolling Lias Plateaux 
Much of the central area is Lias Limestone plateau. A much smaller and more isolated part of the area at Nempnett Thrubwell is part of another plateau which extends outside the area westwards to Felton and Lulsgate. The large central plateau area is much lower than the Cotswolds and more gently undulating and the plateau is less continuous in nature. It has some distinctive, rounded hills of Oolitic Limestone rising up out of it and is dissected by a series of valleys.