GeoLogic

Geological hammers are all well and good, but there will inevitably be occasions that they aren't quite enough by themselves.

For those occasions where a little more welly or finesse is required, it's always good to have a chisel or two stowed in your rucksack.

As far as the geologist is concerned, there are two types of chisel worth carrying; each is best suited to splitting different rock types. The first one we’ll deal with is a pry bar. This is typically very fine for its whole length, and is designed to split fissile, finely bedded mudstones and shales. These rocks, in the field, have the appearance of a pile of wet newspapers – there are lots of thin ‘leaves’ which may be very difficult to tease apart. A pry bar can get in between the leaves and split the rock along its planes of weakness – conveniently, the weakest planes are usually those with fossils on! The length and thinness of the pry-bar means it is possible to gently prise the layers apart without damaging any fossils within, or shattering the rock. Our model has a double point to maximise the pressure while minimising the risk of crumbling.

The second type of chisel is the splitting chisel. This is intended to split rocks of all types with a little more violence. Like the pry bar, you are wasting your time if you try to split a rock other than along its planes of weakness. In most fossiliferous rocks, you will be able to make out ‘bedding surfaces’ – that is, layers in the rocks representing ancient sea-floors or beaches, upon which fossils and sediments would eventually accumulate. These layers constitute the planes of weakness, and fossils will generally lie flat along them. You should align your chisel alongside these rocks before applying force.

What sort of hammer is best for chiseling? While a geological hammer will do the job, a lump hammer is much more comfortable to use, and allows a more efficient and comfortable application of force. Its larger head makes it easier to transmit power to the chisel, reducing the number of blows required. (It also makes it harder to miss the chisel – although hand guards can still save the knuckles a few bruises on wet days!) Its shorter handle also makes it easier to use the hammer in tight corners.

The force you will require depends on the hardness of the rock you are attacking – softer rocks demand more rapid, gentle blows to avoid crumbling or unwanted fissure – you want to keep the split to the bedding plane. More firmly cemented rocks may need rather more aggression with a nice large chisel! The size of chisel used is proportional to the size of the rock being split, and its strength - a wider chisel will make it more likely that the whole rock will split, but soft rocks may not like the wide angle of its point. The best compromise for general use, if you only take one chisel with you, is probably our standard splitting chisel, and through bitter experience we always use a hand guard these days! But if you are likely to encounter finer slate-like rocks, it's definitely worth picking up a pry bar too, especially as they are light in the rucksack.