Geology
This bracing walk takes you past an interesting stretch of beach
and geological section which is important for research into many
aspects of Pleistocene and Cretaceous geology in the UK. Some of
the youngest chalk in the UK is exposed in the cliff immediately
above the east end of the Overstrand promenade.
The Overstrand cliffs are made up of glacially deformed and
tilted rafts of Lower Maastrichtian Chalk, embedded in glacial till
(known as First Till) and glacial lake clay (Intermediate Beds).
They rest on the Cromer Forest Bed Series which were laid down
under deltaic conditions and in which remains of trees and
mammalian teeth and bones are frequently found. The outcrops
include thin marl bands, about 3 cm thick, usually at the base of
the chalk masses which mark slippage planes formed during
glaciotectonic movements.
Erratic Rocks 1: Walk to the exact Earthcache site
coordinates and turn to face back towards Overstrand (east. From
here you can see two large chalk rafts that have been pushed up by
glacial movements from their base level which at Sheringham (10 km
west ) is only exposed at low tide whilst here the chalk is about
10-20 metres above sea-level. The chalk rafts are 10-15 m thick and
include layers of flints and fossils of sea-urchins (echinoderms)
and bullet-shaped belmenites may be found. Between the two chalk
rafts the intervening Lower Pleistocene sands and gravels are
folded and turned upwards at the upper thrust plane indicating the
horizontal movement was from the northwest (see figure). A shelly
marine sand bed contains teeth of voles, lemming and desman, which
may be recovered by sieving.
Larger components of the gravelly beds in the till and the chalk
masses themselves are known as “Erratics” the word generally
ascribed to large ice- or water-borne blocks which are different
from the bedrock.
Glacial thrusting during the Anglian glaciation has pushed the
clays from right to left in the photograph. When the sea eats into
the toe of the cliff, gravity takes over and the cliff becomes
unstable. To the alarm of the residents on the cliff top properties
above, the slippery clays just slide back the way they came 400,000
years ago.
Erratic Rocks 2: Wooden groynes and revetments
have been constructed to slow the advance of the sea. In 1995
another major initiative was made with the import of large blocks
of hard erosion-resistant rocks barged down from Norway and piled
on the beach. The rock type is Larvikite and close examination will
reveal centimeter scale, rectangular mineral laths (rhombic
phenocrysts of feldspar) whilst examination under a microscope will
show apatite, zircon and olivine. This handsome rock is
characterised by its coarse grain and distinctive feldspars which,
especially in polished surfaces, exhibit a beautiful blue
schillerization. Next time you walk down your high street keep a
look out for blue grey polished building-stones (many banks and
modern buildings have them) and you will probably find the
distinctive Larvikite. Although introduced by man and barge, these
rocks are the second type of erratic you will find at Overstrand
(Lat. N52° 54.921' Long. E001° 21.084' )
Further Information:
Field Guide to the Coastal Geology of North East Norfolk -
Overstrand-Sidestrand by M Warren
Norfolk Musuems; Field Guide NE Norfolk
British Regional Geology East Anglia and adjoining areas. Institue
of Geological Sciences publication
To log this Earthcache you are required to comply with 3
requirements:
:- send Geodirect an estimate of the thickness of the
sediments between the two chalk rafts (think carefully about the
orthogonal distance)
:- send to Geodirect the colour of the Larvikite and
:- Post a photo of the chalk raft location