Click here to download a pdf version of this page Click on any image to open a full-size version in a new window. Hertfordshire Puddingstone is both the county's geological claim to fame, and a most interesting rock. It is an intensely hard mass of flint pebbles fossil beach shingle , cemented by a plain grey sandstone - actually sarsen.
Famously the rock will break through clasts and cement alike, if hit hard enough. Typical external appearance Clean-broken surface It is found mainly in Hertfordshire with further sources in east Buckinghamshire and to a lesser extent west Essex.
Most of it has derived from glacially disturbed Tertiary deposits on the Chilterns 'Plateau Drift' and has been moved from its source either by ice or by man. Pudding stone jewelry, ornaments, garden decorations, and even nightlights made from pudding stones are becoming more and more popular.
Many people also like to collect pudding stones just to admire their unique colors and patterning. These special pudding stones and the hobby of collecting them has been passed down through Michigan families for generations, with families passing down stones as heirlooms and taking children out to look for the unusual rocks. The age has never been proved, but it is thought to be about 50 million years old.
Puddingstones are a natural rock called conglomerate with an unusual history — a story of extremes. In fact, puddingstones are just one type of deposit that is often called sarsen, but that geologists call silcrete.
This type of rock can be either sandy or pebbly. Silcrete by definition is a silica-rich rock i. Puddingstone usually consists of very rounded flint pebbles, with sand filling the gaps and held together with a quartz cement. Take a close look to see, you may need a magnifier.
The sand grains are well sorted nearly all the same size and they have shiny surfaces. These are important clues as to how the rock formed. Glassy grains indicate water transportation the alternative would be wind transport and these sand grains have matt surfaces.
On the fresher surfaces you can see different colours in the flint pebbles — some are brownish, some are orange-brown and others are a little redder.
Some pebbles have a darker coloured ring around the edge. However, as the pebbles begin to weather they get a white crust on them, so the original colour is not easy to see. These rings around the outer edges are caused by exposure to high temperatures, such as being left out in the hot sun for a very long time. Rocks similar to puddingstone form today only in semi-arid regions such as central Australia, and this was the environment of Nettlebed 50 million years ago.
Sudden rainfall carried pebbles and sand in broad river channels. When the water is flowing rapidly the heavier pebbles could be carried with the sand, but as the flow slowed, only sands were transported further on in the channels. The water energy therefore makes the difference between the pure sandy and the pebbly types of sarsen.
The fact that sand was also deposited with the pebbles in the Nettlebed puddingstones testifies to very turbulent water action which just dumped a whole lot of sediment together. The water quickly drains away in these semi-arid environments and the hot winds would then blow quartz dust around, some penetrating between the grains. At night dew moistens the quartz dust encouraging crystals to grow as it dries with the heat the next morning.
First, a network of rapidly flowing streams tumbled red and coffee-brown jasper, funeral-black chert, hematite and quartz in their churning water. Next, the streams deposited the material as sedimentary fill in eroded troughs and as alluvial fans, when the streams reduced their velocity and scattered the colorful pebbles onto mounds of sand Lowey, ; Baumann et al.
Later, intense heat and pressure metamorphosed the matrix of sand into a light-colored, coarse-grained, sugary-textured quartzite that tightly held the pebbles Schaetzl, n. These geological forces formed the puddingstones around 2. Today, geologists recognize these conglomerates as part of the Lorrain Quartzite of the Cobalt Series Door and Eschman, This area is located 65 km 40 miles east of Sault Sainte Marie in Ontario. Puddingstones traveled south during the last ice age with the immense Laurentide Ice Sheet as it flowed at a glacial pace down from Canada.
This ice plucked the puddingstones from the underlying bedrock, carried them hundreds of kilometers, and delivered those rocks to Michigan about 24, years ago. This slowly advancing ice plowed across the landscape for thousands of years until rising temperatures, brought on by a climatic shift, ended their movement in Michigan.
As the glacial ice melted, it deposited glacial till that contained the puddingstones.
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