Pudding at the Castle EarthCache
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The GZ for this Earthcache is on a residential street, please be mindful of this as you seek out this Earthcache.
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The published coordinates are the location for this Earthcache. An Earthcache is a special geological location people can visit to learn about a unique feature of the Earth.
To claim a 'find' all you need to do is provide answers to the questions below by observing the geological location and reading through the cache page.
At the published coordinates you'll find a few boulders by the side of the road that at a glance could be mistaken for lumps of concrete. Please take a closer look, you can admire the boulders from the pavement. What you'll find are examples of Hertfordhire Puddingstone and also Sarcen Stone as the underside of one of the boulders grades into Silcrete.
Hertfordshire Puddingstone
Hertfordshire puddingstone is a conglomerate sedimentary rock composed of rounded flint pebbles cemented together by a younger matrix of silica quartz. The distinctive rock is largely confined to the English county of Hertfordshire but small amounts occur throughout the London Basin. Despite a superficial similarity to concrete it is entirely natural. Like other puddingstones, it derives its name from the manner in which the embedded flints resemble the plums in a Christmas pudding.
The flints were eroded from the surrounding chalk beds some 56 million years ago in the Eocene epoch and were transported by water action to beaches, where they were rounded by wave erosion and graded by size. A lowering of sea levels and general drying during a brief arid period known as the Paleocene–Eocene period. Thermal Maximum drew out silica from surrounding rocks into the water immersing the flint pebbles. Further drying precipitated the silica which hardened around the pebbles, trapping them in the matrix.
The Palaeocene Reading beds consist of mottled and yellow clays and sands, the latter are frequently hardened into masses made up of pebbles in a siliceous cement, known locally as Hertfordshire puddingstone.Examples of Reading Beds outliers occur in what are otherwise chalky areas at St Albans, Ayot Green, Burnham Green, Micklefield Green, Sarratt, and Bedmond. The Reading Beds were laid down about 60 million years ago when the area was a river estuary receiving river sediment from land to the west.
The Neogene is a geologic period starting 23.03 million years ago and ending 2.58 million years ago. The second period in the Cenozoic Era, it follows the Paleogene Period and is succeeded by the Quaternary Period. The Neogene is sub-divided into two epochs, the earlier Miocene and the later Pliocene.
(Bucks Earth Heritage Group - BEHG)
The London Clay is a stiff, blue clay that weathers to brown and rests nearly everywhere upon the Reading beds. It represents the time 55 to 40 million years ago when Hertfordshire was once again under a deeper sea but was near enough to land to receive fine mud deposits.
It may be difficult for people in Hertfordshire to believe, but puddingstone is one of the world’s rarest rocks. It occurs in pieces which vary from a few inches to several feet across.
Puddingstone is a conglomerate of small pebbles bonded together by silicon to form solid rock. When broken, it shows a cross section of pebbles which give it the plum-pudding appearance which accounts for its name. All puddingstone is several million years old and its formation began when small, mainly flint, pebbles were deposited in river beds and then later covered by London clay. This puddingstone mixture was subsequently compressed and bonded together during the Ice Age, becoming strong enough to resist crushing.
When the ice melted, large pieces of this rock were tumbled by the flood water to produce the rounded blocks that are still underneath the local fields today.
Created by the precise conditions prevailing locally during the last Ice Age, nearly all the puddingstone in the world is found in Hertfordshire and most of it lies in the Gade and Bulbourne valleys. Puddingstone has an exceptionally hard surface and its earliest known application was as a Stone Age Quern, which was a primitive mill stone used for grinding corn. Stone Age culture also used circles of puddingstone in its ritual worship and, because of the deliberate incorporation of some pagan beliefs into the Christian faith by Pope Gregory’s missionaries (circa AD 601), it is still possible to find some puddingstone built into the walls of many local churches. Although in some ways an ideal material due to its strength, puddingstone has never been widely used as a building material, simply because of its scarce supply. However there are one or two rare buildings in Hertfordshire built entirely of puddingstone. A small structure in Radlett was unfortunately demolished in the 1970’s but one house survives on the Westbrook Hay estate; the old Ice House at Ashridge is also built entirely of puddingstone.
Rock similar to the sand matrix of Hertfordshire Puddingstone, and with similar silica cement, but lacking the pebbles, occurs further west in Southern England, and is called ‘Sarsen’ stone. It is rare to find both examples together such as here in Castle Hill. Sarsen Stone was used in part of the construction of Stonehenge.
Sarcen Stone
Sarsen stones are sandstone blocks found in quantity in the United Kingdom on Salisbury Plain, the Marlborough Downs, in Kent, and in smaller quantities in Berkshire, Essex, Oxfordshire, Hertfordshire, Dorset and Hampshire. Sarcen stones are the post-glacial remains of a cap of Cenozoic silcrete that once covered much of southern England – a dense, hard rock created from sand bound by a silica cement, making it a kind of silicified sandstone. Tough sarsen stones and puddingstones found in these areas are thought to have originated from a now-vanished layer of the Reading Beds within the Lambeth group.
(Bucks Earth Heritage Group - BEHG)
The word "sarsen" is a shortening of "Saracen stone" which arose in the Wiltshire dialect. "Saracen" was a common name for Muslims, and came by extension to be used for anything regarded as non-Christian, whether Muslim, pagan Celtic, or other.
The builders of Stonehenge used these stones for the heelstone and sarsen circle uprights. Avebury and many other megalithic monuments in southern England are also built with sarsen stones.
Where did the sarsens in Berkhampsted really originate? It is often said locally that they are “erratic” boulders carried by the ice sheets from some distance away. Whilst a few may have been moved very short distances by ice, most are definitely local in origin and were part of the “solid” geology beneath Berkhamstead. They show no signs of the typical scratches that would normally be visible on “erratics”. Had they been dragged and ground along under the ice meeting sharp flints and other rocks in the clay such scratches should be visible, even though the sarsen is very hard.
Fire and in later times explosives were sometimes employed to break the stone into pieces of a suitable size for use in construction. Sarsen is not an ideal building material, however. William Stukeley wrote that sarsen is "always moist and dewy in winter which proves damp and unwholesome, and rots the furniture". In the case of Avebury, the investors who backed a scheme to recycle the stone were bankrupted when the houses they built proved to be unsaleable and also prone to burning down. However, despite these problems, sarsen remained highly prized for its durability, being a favoured material for steps and kerb stones.
The Questions
To log this Earthcache you need to visit the location then answer the following questions.
Please send your answers to us using the link to our username above. Please do not post your answers in your log. Please note that we may delete any logs where your answers have not been received.
Observe the boulders - you’ll see examples of Puddingstone and Sarsen stone:
1. After reading the cache page and examining the boulders, which do you think are the Puddingstone examples?
2. By looking at the stones, why do you think the Puddingstone is named as such?
3. Explain what is absent in the example of Sarcen Stone that the Puddingstone has.
4. How many of each stones are there?
5. From reading the text above and from observing the boulders; describe what you think the stones feel like, based on your findings (and reading the information on the cache page) do you think the stone is igneous, sedimentary or metamorphic rock?
6. In which geological period were the stones formed? (you can find the answer to this one if you read the cache page carefully.)
7. Just for fun, it would be nice if you can upload a photo of yourself or GPS at GZ (not compulsory).
Update: October 2016
We've been made aware that a plaque has now been installed at GZ by the Hertfordshire Geological Society. Many thanks to IBSearching for letting us know and supplying us with a photo of the plaque.
Additional Hints
(Decrypt)
Cyrnfr abgr gur cnexvat erfgevpgvbaf va gur nern 'Zbaqnl - Sevqnl 11.00 - abba'.