Thread: stone ID

Great looking rock Adam. At first glance I thought it was one of the J-nats.

Alex, just curious, I googled indurated to find that it meant hardened. What might have indurated the sandstone to the depth of 125 feet ? The first thing that comes to my mind is glaciers shifting and work hardening the rock. Anything to that ? Is the cause known ?

The main cause for induration is compression in one form of another, glacial movement, contact with harder strata or simply because lower strata have more weight on them than the upper layers. Heat can also turn certain rocks into whetstones or hones. Weathering too, can play a part in this process, but in many cases weathering will destroy stone rather than hardening it, like the novacuite of Carp River.

There are examples of hones produced by the various methods of compression to be found in the literature. Slate from Hestercombe in England and Novaculite from Bald's Head, Maine are hardened by contact with Syenitic Dykes and Quartz dykes respectively. At Hestercombe, the induration turns the ordinary slate into honestones, but at Bald's Head, it is the material farthest away from the dykes which is most desirable, as the novaculite in contact with the quartz has too splintery a fracture to be of much use.

In Wales, there are many areas where the slate has been metamorphosed into honestones due to contact with greenstone. The greenstone initially compresses, and ultimately intrudes into, the slate forming a spotted rock known as Snakestone. This is very common in Caernarvonshire and several localities were exploited as a source of honestones.

Examples of hones that are produced by virtue of being found at an increased depth include Hindostan stones, where the lower layers are preferred due to being harder and less friable; and also hones that were found at a very great depth when digging a well in Hampstead, England.

Baked shales and baked mudstones producing hones are found in Ayr, the TOS, and at Enterkine Quarry in Tarbolton, respectively. There is also a baked and compressed shale that was used for whetstones in Teesdale, England.

Another factor that can cause hardening and metamorphosis of other rocks into hones is the presence of iron ore. There were certain grades of Hindostan that contained considerable quantities of iron and these were preferred because they were less friable than the regular sandstone used to make the Hindostan whetstones. There is another honestone from Indiana, known as the Fera Hone-stone, that was very highly regarded. This was a clay-slate that contained iron. Unfortunately, this type of hone was only produced commercially by one quarry, and therefore supply was very limited.

Also, many different types of hones were processed after extraction by either boiling in oil or baking at very high temperatures. Examples of the former include: Turkey Hones, Chapel Hill Novaculite from North Carolina and the Devil's Punch Bowl Whetstone from the Mangerton Mountains in Ireland. Stones that were baked to improve their performance include the Norway Ragstones, the Kentucky Caron Hone and a hone from Virginia about which I have been unable to find out anything more.

Incidentally, the sandstone from the Arbuckle Mountains, was so hard that it causes saprks to fly when struck with steel.

Hope this is of at least some help or interest to you.

Kindest regards,
Alex

Originally Posted by A_S

Hope this is of at least some help or interest to you.

Extremely so. Thanks !