Chert, a microcrystalline form of quartz, commonly forms by silicification of limestone. One limestone in which this has happened extensively is the Chalk, the body of Cretaceous fine-grained limestone that forms the famous white cliffs of Dover. The Chalk underlies a <-shaped swath of England that stretches west from Dover and Beachy Head to Warminster and from there northeast to Norwich. |
As the Chalk weathers, the erosion-resistant nodules of chert (or "Flints") like those below remain at the earth surface as a residue. They thus provide a readily available, if difficult to manage, building stone. Note that in the example above from Stow-cum-Quy (east of Cambridge), as well as in another example from Great Sampford, the chert nodules are never used to form corners, but only fill walls between corners made of other stone.
Note in the image below that all of the exposed surfaces of the chert nodules are freshly broken. For comparison, many of the nodules at Great Sampford are unbroken, so that weathered surfaces are exposed. Stone masons commonly go to the trouble to arrange freshly broken faces outward, providing the smooth sharp-edged surfaces abundant in the image below.
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