Guide to Backyard Geology

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.