Geoscience Reference
In-Depth Information
cliff line that is only reached by storms during high tides. It seems likely that this cliff
line was created by storm action relatively recently, after the sea rose to within about
10 m of its present level some 6,000 years ago. However, as along many other stretches
of the coast of Southern England, initial erosion may have taken place some 130,000
years ago during the Ipswichian, when the sea was last as high as it is today.
Landscape B: The Somerset Levels and Moors and the Mid-Somerset Hills
The flatness of this Landscape is one of its most remarkable features. In Southern Eng-
land, the only other flat area of comparable size is the Fens of East Anglia (Area 15).
Both the Fens and the Somerset Levels owe their flatness to the presence of easily
eroded mudstone bedrock in areas that have experienced repeated invasions and re-
treats of the sea during the last 2 million years. These repeated invasions have resul-
ted in large flat surfaces of eroded bedrock covered by young, surface-blanket sedi-
ments, deposited by low-lying rivers along the coasts or washed inland by the sea. In
the Somerset Levels, the flat areas are underlain by the soft mudstones of the lower Li-
as (Early Jurassic), which are often covered by a surface blanket of Quaternary muds,
formed during the late stages of the Flandrian sea-level rise. Some areas, locally called
moors, are peat-rich and were formed in wetland landscapes.
The hills, although low, often present abrupt slopes overlooking the flat lands.
Brent Knoll (
b1
) is a striking conical hill rising over 100 m above the surrounding
Levels, and best visible to people travelling southwards along the M5 motorway. It
consists of flat layers of Early Jurassic mudstones capped by a small patch of Middle
Jurassic limestone and a prehistoric fort. Brent Knoll owes its existence to its cap of
limestone, which is a more resistant bedrock layer than the mudstones underneath it.
Erosion of the slopes of the Knoll was active both during times of warm climate and
high sea level, and during cold episodes, when freeze-thaw action led to slumping of
bedrock material and soil down-slope to produce the conical form we see today.
To the east, the Wedmore to Wookey ridge (
b2
and
b3
) is another distinctive fea-
ture of the northern part of the Somerset Levels. The zigzag plan shape of the ridge
follows the pattern of a number of faults that cut limestones of Late Triassic and Early
Jurassic age. This is another example of deformation (in this case faulting) that has
taken place since the Early Jurassic, changing the pattern of resistant bedrock and res-
ulting in a distinctive present-day geometry in the scenery.
The stretch of coastline bordering Landscape
B
has a low-lying hinterland be-
cause it marks the area in which the main Severn Valley is joined by the River Parrett
(
b4
; Fig. 158). The Parrett is one of the main tributaries of the Severn, bringing water
and sediment from much of Somerset. At the maximum of the last cold episode of the
Ice Age, some 20,000 years ago, the entire Bristol Channel was dry land at least as far
