Harrow and Hillingdon Geological Society

Previous Meetings

Flint – a Diverse Resource

Diane Smith

Historically, flint has always had a fascination. It was thought to be something which was dug up or stones which “grow”. In 300BC it was thought to, grow spontaneously or regenerate. Hag stones were flints with a hole in the middle believed to have magic properties for fairies and witches and used as personal protection by such people as fishermen. Another form is pot stones or paramoudre (Irish). These are normally black or grey in colour and can be yellow. They are homogeneous and the colour is controlled by the micro structure. The cortex is part of the flint but can be decorticated. Hertfordshire pudding stone contains flint, some effects of humic acid and siliceous concretions but flint should really only be used for the Cretaceous period. Silica is the mineral from which it is made.

Flint – generic in Australia and USA but should be siliceous cretations.
Primary – whenever they were formed in situ.
Secondary – derived from chalk by erosion, glaciation, etc.
Its forms may be crystalline or amorphous (opal).
The pores are isolated. Chert is cryptocrystalline full of spicules which have dissolved. It is also possible to see diatoms. The flint is 98% silica with pores which are inter-connecting allowing water to move within the stone. Properties: very hard, brittle, sharp edges- knapping, used also for fire making.

There are 3 possible forms:

1. Hot from below which had cooled and crystallised
2. Dark, heated shellfish debris in a cold aqueous solution with precipitate, which depended on the solubility of the silica.
3. Contained marine animals contemporary with the flint. There is silica on the sea bed, and zoophytes on the sea bed changed and became flint. Paramoudres formed around burrows which sink. Silica also comes from sponges and diatoms.

Flint in Britain and Europe

Grime’s Graves, in prehistoric times, flint mines were found in France, Denmark, Holland and Poland. The Blackpatch mine near Worthing dates to 3,000 BC but was disused by the Bronze Age. Fresh flint is like toffee, easy to work initially but becomes harder. Grime’s Graves consisted of 360 pits dated to 2,000 BC, used for producing tools, it was the first industrial site in Britain. It was discovered that right handed people used left antlers for flint knapping. They polished the flint with sandstone as it too was 7 on the Mohr scale. Obsidian was used to make scalpel blades

Knapping in 19th C was done by people known as knackers or crackers who shaped the big nodules. In Brandon British gun flints were made. Silicosis led to early deaths, workers used sponges as masks. Flint was also used for making fire. Calcine flint was used as a whitener and stiffener in pottery or tiles. The Romans put flint on chalk to form their roads and in West Dean there are superb examples of its use in the decoration of buildings.

As always Diane Smith gave a very informative and well illustrated talk. Her enthusiasm is infectious and she obviously has studied many aspects of her subject. My only other feeling was that there was so much information that it possibly went on too long and we could actually have had her return for another lecture at a later date.

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