Rock Trail :  
 
The Thornham Geology Rock Trail            for children

1. Quartzite
is a rock mainly composed of sand grains, that, during its later history has been subjected to forces that caused its grains to bond more tightly and excatpe between them (pores) to fill with silica cement. The sand may have accumulated on a beach, in a river system or on coastal or desert dunes, and when lightly cemented formed a rock layer of sandstone. In time, pressure from depth of burial or chemical change through percoloating silica-rich water formed the much harder and less porous quartzite. Isolated quartizitic sandstone "sarsens" from the Eocen are locally common in south-east England. This specimen was found here in Thornham.

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2. Shale
When mud carried by rivers settles in lakes and shallow seas it can accumulate in great thinknesses. The thicker the deposit the greater the weight of the deposit of the overburden on the deeper layers and the more it is compressed. The minute particles of clay which make up the mud are platy in shape and tend to settle flat upon each other, a condition reinforced by over burden pressure. Bonding continues as water is expelled, mudstone grading into SHALE and eventually slate as pressure becomes extreme. The parallel orientation of the mineral plates creates planes of weakness allowing slates to part thinly. Shale of this type is typical of Wales not East Anglia.

3. Granite
Beneath the layers of accumulated sediment (the "sedimentary rocks") occur rocks whose components whatever their origin are crystalline minerals of various kinds. Under the great continents these crystalline rocks are most often found to be GRANITE. In granites three types of mineral are visible, pale grey-white or pink feldspar in blocky shapes, glassy quartz and flat, shiny mica which can be pale to black in colour. The feldspars are silicates of potassium and calcuim; the micas are silicates of potassium and aluminium. Outcrops of granite are common because of uplift and erosion of the covering rocks. These are "tips of the iceberg" because because beneath them granite bodies ("batholiths") are huge in size and depth (kilometres ). Granites do not outcrop in east anglia but famous examples occur in south west England (Dartmoor), north-west England (Shap Fell) and Scotland (aberdeen).
4. Gneiss
An often ancient rock that has been subject to such pressures and temperatures that its original sandy appearence and mineral composition has altered, and thus termed a metamorphic rock. Original sandy and clayey layers may still be discernable as light and dark streaks, but these will often be recrystallized, compressed and contorted. Different types of GNEISS are distinguished by the particular components of their original state and by the nature of forces to which they have been later subject. Gneiss does not appear in East Anglia unless as a glacial erratic.

5.Chalk & Flint
Chalk is a white compact limestone composed of 95% calcium carbonate, and of a seemingly uniform nature. Detailed examination reveals a wide variety of composition, ranging from a soft type made up of remains of innumerable calcite plates and spheres of planktonic algae ("coccospheres"), to harder varieties of chalk with larger fragments and whole fossils. Chalk muds were laid down in a deep sea (up to 300 metres) extending over much of the earth, and in a considerably hotter climate than today's. In East Anglia, at least 270 metres of chalk accumulated during a period that lasted from between 95 and 65 million years ago. Marine fossils are plentiful in the chalk. while algal remains dominate fossils of other larger organisms such as sea-urchins, belemnites, bivalves and some ammonities are common.

Flint is composed almost entirely of silica (Si02). It is by far the most common pebble-forming stone to be found on beaches and in river gravel in southern England. Flints weather out from the softer chalk within which they form layers of weirdly shaped "kidneys" at many levels. A freshly mined flint will have a white outer patina but will be grey to black when chipped. Most flints in East Anglia originate from layers in cretaceous chalk which is older than 65 million years. Its ready availability and sharp glassy fracture proved an ideal resource for early man's too -making industries.

6. Limestone & Coralline Crag
Limestone varies greatly depending upon the presence, amount and nature of the fossils that are present within it. The limestone we know as chalk appears fine-grained because the fossils it is make up of are so tiny. Shelly limestones can be quite coarsely textured when the fossils are larger such as molluscs, corals and "sea-lilies" (crinoids) as in this carboniferous limestone, typical of northern England. Their colour may vary from dark to light depending upon how organic matter and clay they may contain. Sand may make up so much of the so-called Coralline Crag of East Anglia that this is not a limestone but a shelly "arenite".