Whats it made of? The pebbles of the Chesil Beach are almost entirely composed of very resistant minerals, mostly quartz and chalcedony (with a hardness of 6 on Moh's Scale, i.e. harder than steel).
The bulk of the beach pebbles, 98%, consists of flint and chert. This comprises grey to brown flint from the Cretaceous Chalk and light bluish grey chert from the Upper Greensand. The dominant flint and chert has been derived from Cretaceous rocks, perhaps by way of Tertiary gravels such as those at Blackdown and Bincombe. At the Portland end, there is some black chert from the Portland Cherty Series, the Portland Stone and the basal Purbeck Formation. In this area some limestone from the local Portland and Purbeck Formations also occurs. Chert from the Portland Roach can be recognised easily because of the large molluscs and particularly the gastropod Aptyxiella portlandica .
But the most curious--and wonderful--thing about the bank is the size-profile is so perfect that experienced fishermen who beach at night can tell their position by the size of the stones.
How did it form? The sea has risen steadily in the Holocene epoch, since the end of the last ice age. This rise is called Flandrian, and gave the British seacoast its intricate drowned topography. It began about 10,000 years ago with a rapid rise that then began to ease off about 8,000 years ago, gradually slowing to the modern slow rise. During this period, ancient beaches and gravels were driven before the waves, washing out the sand and silt, and rounding the pebbles. Chesil Beach is the current accumulation of this material, too heavy to be moved other than before the waves, and representing deposits from land eroded from what is now in the Channel up to 35 or 40 metres below sea level. This is a good reason for the gravel in the beach not being typical of the land near it, and for its peculiar grading.
There is still not complete agreement, but this explanation seems to be the best available. Lyme Bay has a number of scenic and geological features that make a visit rewarding. It extends from the limestone reef of Portland to Start Point in Devon, with cliffs of changing character, from the chalk at the eastern end, lias, quartzite, red Triassic and Permian conglomerates and mudstones, to metamorphic rocks.
The western end includes Babbacombe Bay, Tor Bay and Start Bay. Slapton Beach in Start Bay is a small Chesil Beach, also made of shingle. Lyme Regis features the ammonite, whose fossils weather out of the cliffs together with icthyosaurs.
A sign at Burton Bradstock warns against the crime of taking a few pebbles as souvenirs from the beach. (And yes someone was prosecuted in East Devon for such a crime in 2000, Quote “He was found guilty, conditionally discharged for 12 months and ordered to pay £250 costs! The mayor said "East Devon District Council brought this case because they wanted to make an example of somebody".)
If we assume that the height of the beach is 10 m, its width at the top 10 m and at the base 30 m, its length 30 km, and the average pebble to have a volume of 2 cc, there are about 3-5 x 1012 pebbles in the beach. If everyone in Britain came and took away a handful of pebbles, the beach would be depleted by 1 part in 10,000. So children taking souvenirs would not seem to be a very great danger to the beach. Such stupid officiousness is more than typical in such matters, harming efforts when protection is really necessary, and alienating those who could be supporters. We are very familiar with it in the case of fossils. Fossils weather out continuously, and if not found soon weather away. Those usually found are very common and of very little palaeontological or commercial value, like circulating coins to a collector. Without amateurs, most fossils would be lost, and rare specimens would never be found.
The above is not my quote or my view but taken from another site which can be found here, https://mysite.du.edu/~jcalvert/geol/chesil.htm. I do not condone people removing pebbles from the beach.
To claim this cache you must;
1/ Take three pictures of the bank, one facing west towards Bridport, one facing East towards Portland and one with St Catherines Chapel in the background (you will have to walk along the bank to get the chapel in). All pictures must have your GPS in, (No holiday snaps) No GPS…….No log, and it would be nice if you could be in it too (But not essential).
2/ Name 3 types of pebble that are found on the bank?
3/ Some types of pebble, show conspicuous curved cracks or percussion marks. These are the results of what?
4/ Also email me whether the pebbles get bigger or smaller as you walk west to east.
Be warned, do not put your answers in your logs, email me through my profile. If you do not comply with the requirements of the above, I will simply delete your log. I have asked nicely ;-) Thanks
