This Series features tumbled stones from Ward Beach, Marlborough. These are part of a selection that I am leaving with Ronnie and Ian who run the Flaxbourne Motels in Ward. All five Parts in this Series are set out in the Index here. This Part presents a number of Ward Beach stones with noticeable veins as well as some stones with dendrites.
A “vein” is an infilled crack. When rock comes under pressure and stress, it fractures, and cracks of different sizes open up. This allows hydrothermal fluids (such as super-heated water), carrying dissolved minerals, to enter the gaps in the rock. Over time, the dissolved minerals precipitate out of the hot water. Crystals, such as quartz, grow within the cracks and fill them. Often, as the stresses on the rock change their orientation over time, veins will crosscut and even overlie each other. For useful information on this process, see the section on “Veins” in Geology Fundamentals and the entry on “Veins (Geology)” in Wikipedia.
Some tumbled Ward Beach stones with significant veins – all of these stones are relatively soft, most of them being limestone:
Some stones have cross-angled veins and others are quite fragmented so that the veins are quite complex:
“Dendrites” are quite a different phenomenon from veins. The word “dendrite” is from the Greek word for “tree”, referring to its branching. In geology, dendrites are thin, branching crystals, often of some variety of manganese oxide (or iron), that grow over a surface in a rock or mineral – see Mindat.org. They are found in cracks or along bedding planes. They often form lines which branch out across a stone. Dendrites are often given as examples of “pseudofossils”, natural objects that may be mistaken for fossils (of a plant leaf, for example) – see the section on “Dendrites and Manganese Forms” in Pseudofossils.
Some Ward Beach stones with dendrites:
The next Post in this Series features Ward Beach stones containing a mix of soft and hard material, especially limestone and flint.
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