Backyard geology is an important hobby that has small beginnings and can have large personnel consequences. As a small boy I was interested in rocks and minerals and 20 years later I graduated as a geologist, and later still qualified as a gemologist. Today geology is but one of my many interests.
I grew up in New Zealand, a country that has a varied landscape of snowy mountains and active volcanoes. Both the North and South Islands were the scene of gold rushes in the 19th century, and even today their are several large open cut gold mines operating. It was the lure of finding gold and becoming rich that first attracted my interest in geology.
But what can you learn about geology in your backyard? A school boy is limited to walking or venturing forth by bicycle into the countryside. Plus there is the need to read many books about the subject and teach yourself to identify the common rocks and minerals.
In the old days my family did all the cooking on an ancient black cast-iron stove using coal as a fuel. Every few weeks the coal man would drive around town delivering bags of coal. These he humped to our house and tipped the coal into a bin. The house was heated by an open coal fire place, and it was my job to clean out and rebuild a fire each day which we sat around in the evening reading books, as television was not invented then.
The lumps of coal were fascinating to me. They came from the Huntley coal field located south of Auckland, and was of Oligocene Age (about 40 million years old). I found fossil fern fronds and other plant debris, kauri gum (amber) nodules, seams of pyrite or “fools gold” (I was not fooled), calcite crystals and quartz pebbles, and so my rock and mineral collection began!
From this simple start there is much that can be learned. How do you tell calcite from quartz when it forms a white vein through a rock? Well, you can scratch calcite with your pocket knife but not so quartz. Using a simple hand lens to observe the scratch, if there is one, you will see that calcite breaks up easily by fracturing along its rhombohedral cleavage planes. Also, the white powder will release bubbles of carbon dioxide gas if placed in vinegar. Quartz is much harder, tougher and inert as it has no cleavage, but it does exhibit a conchoidal fracture.
How do we tell “fools gold” from real gold? Here too the physical properties of hardness and crystal form are important. Sometimes one finds a rock showing a few specks of “gold”, but is it really gold? Gold is soft and pyrite hard but the specks are too small to test with a pocket knife. Usually you can tell the difference using a hand lens. Pyrite often shows crystal faces of the cube and pyritohedral form and so reflects the light more in one way than another. Turn your specimen various ways in the light and see if any flat faces brightly reflect the light. Gold specks do not do this as they rarely show crystal form in a quartz matrix or as “gold dust” when panning alluvium. It is useful to have a quartz specimen with real gold specks to keep for comparison purposes.
Still not convinced? Well, you can use a pestle and mortar and crush the rock to powder and use a gold pan and water to separate the light minerals from the heavies and do more tests. Now you have what is known as a pan concentrate which is very useful for prospecting purposes. Do you now still have a residue of yellow “gold” particles and what do they look like when viewed with a hand lens, or better still, with a binocular microscope? Once you have seen gold grains through a microscope (and gloated over them) you will never be fooled by “fools gold”.
This problem of correctly identifying gold from pyrite, or chalcopyrite, is exemplified in the children’s classic literature and it has annoyed hell out of me for a lifetime. I am a great fan and collector of Arthur Ransome’s children’s books which were written in the 1930’s. The scene is the Lake District in England and most of his books describe the adventures of a competing group of children, about their little sailing boats on a lake, and with their activities of camping, fishing and prospecting of the adjacent fells.
Arthur Ransome was a journalist, yachtsman, angler, and lover of the English Lake District. His most famous book is “Swallows and Amazons”, however, being a geologist, my favorite is “Pigeon Post”, which describes the children’s prospecting activities for gold in the fells above the lake. Oh’dear, I wish they had studied their backyard geology before venturing forth. They crushed a lot of promising ore, panned it off and smelted a large amount of what was chalcopyrite and were surprised to find no gold! I have even thought that “Pigeon Post” should be rewritten by a geologist or modern prospector, but it is a good story and I reread it every few years.
So much for gold. What about finding fossils and gemstones in and around your backyard? That is a challenge. By backyard, I mean within a radius of 25 kms of where you live, which is accessible by bicycle if you are a schoolboy or girl. I grew up in Palmerston North, on the North Island of New Zealand. The city is located on alluvial flood plains and the nearby range of hills (1000 meters) is of uplifted and folded unfossiliferous greywackes of Palaeozoic Age, and rather boring. However, draped over them at places and around the edges are more recent (Pliocene) sandstones and limestones which were of great interest to me.
I found many fossils of mollusks, brachiopods, gastropods, cephalopods and you name it. Many secret caverns in the limestone did I find which I shared with my best school friends. We explored these caves by candle light and shuddered when we saw clinging to the roof huge wetas waving their feelers at us. Supposedly wetas are quite harmless but, like spiders, I don’t like them.
Gemstones are more difficult to find in your backyard, unless you have a binocular microscope. When you pan off a heavy residue you have a jeweler’s window in miniature but that is another story. Backyard geology is a very exciting hobby. Once you have come to grips with your backyard and understand it then you are ready to explore the whole world.