Rocks soil and geology on your farm.

As an ‘almost a farmer’ I seem to worry a lot about the weather, the growing season, water and compost but not as much about rocks on my farm.  An yet the geology can tell me so much – not just about the types of soils I have, but also about the history (age) of those soils.

rocks soil and geology on your farm

Plants will find a way!

Rocks soil and geology on your farm

If you scrap back your memory cells to about 8th grade geology (high school in some countries, middle school in others) you will remember that rocks come in three very broad types:

Igneous rocks – are those that formed from the cooling of molten rock either below ground (plutonic rocks like granite or gabbro) or above ground (volcanic rocks like basalt or rhyolite)

Igneous rock Gabbro outcropping in Australia.

Igneous rock Gabbro outcropping in Australia.

Sedimentary rocks –  are those formed from the cementing together of cobbles, sands, silts or mud size particles into layers of rocks.  Most of these are formed in the oceans on the margins of the continents where these materials have have washed from the land in rivers such as sandstone, shale, conglomerate or built up from the remains of animals like limestone.

Layers of sedimentary rocks - sandstones and shales

Layers of sedimentary rocks – sandstones and shales

Metamorphic rocks – these are the squashed and/or cooked (but not melted) remains of any rocks that have been in locations of mountain building, continents colliding or close to huge masses of molten material.  Rocks like schists and marbles, quartzites and gneisses.

Old sea floor squashed and cooked into metamorphic rock

Old sea floor squashed and cooked into metamorphic rock

Any of this ring a bell yet? For most…maybe not.  But that is ok 🙂   It is never too late to learn at least a little about the rocks soil and geology on your farm.

What ever rock type you have on your farm will directly affect the type of soil you have to work with – or at least start with if you are one to add compost and with love and care improve your soil immensely.

Minerals are the key
When any rock forms it is made up of mostly a mix of the major rock forming minerals and a splattering of some of some minor minerals.  The major ones are:


Some of the major rock types that you could have on your farm include:

Rock type Rock Name Minerals
Igneous Granite Quartz Feldspar Mica
Igneous Rhyolite Quartz Feldspar Mica
Igneous Basalt Amphibole Feldspar Pyroxene Olivine
Metamorphic Marble Calcite
Metamorphic Schist Mica Quartz
Metamorphic Gneiss Quartz Feldspar
Sedimentary Sandstone Quartz Feldspar Mica Clays
Sedimentary Shale Clays
Sedimentary Limestone Calcite

Once a rock is exposed to the weather it will start to break down “physically” into smaller grains and “chemically” into different minerals.  The major rock forming minerals break down into the following minerals you will find in your soils:

Original Mineral New Mineral Released Elements
Quartz Quartz
Felspars Clays Potassium Sodium  Calcium
Mica Clays Potassium
Amphibole Clays Iron
Pyroxene Clays Iron
Olivine Clays Iron Magnesium
Calcite & Dolomite Calcite Calcium Magnesium

So, generally speaking,  if your farm is built on granite, you will have soils containing quartz grains and clays that will have good mix of the elements of potassium and sodium. If your farm is on basalt, you will have clay rich soils with loads of iron and magnesium.

Some things do not get better with age

In spite of what grandma tells you, age does not improve everything!   Our planet has been around for 4.5 billion years and is continually recycling its surface through Plate Tectonics, uplift, weathering and erosion.  Some places have managed to avoid some of these changes such as the middle of some continents where the oldest rocks have been discovered (such as in Australia).   Here the soils are very old and deep.  The deep soils are normally very poor in nutrients.  Why?  Cause they have had millions of years of exposure to the weather washing away those elements available for plants.

Uluru (Ayers Rock) is surrounded by very old deep and nutrient poor soils.

Uluru (Ayers Rock) is surrounded by very old deep and nutrient poor soils.

In some places the planets surface was wiped clean down to bedrock in the last glacial period as glaciers moved like bulldozers across the landscape scraping off all the soil from huge areas and dumping it in ‘moraines’ in others.  The new fresh rock surfaces have had only 20,000 or so years to develop new soils such as in Canada and most of northern USA.

And is some other places the rocks are so new that you can still see the heat from them as they cool – such as in Hawaii.   Rocks less than 40 years old here are starting to support plant life.

'Ōhelo plant growing in a 1979 lava flow in Hawaii

‘Ōhelo plant growing in a 1979 lava flow in Hawaii

So what can you do to find out about the rocks soil and geology on your farm?
To get a general idea of what rocks you will find in your area, the best way is to wander around your farm with someone who can identify what you have.   I know this is not always possible – and so getting at least a geological map of your area is a great start.   Your country or state/province will have a geological survey that can provide you a map.  There are also some great online resources.  Below are some great links to help you out:

Sources of information for rocks soil and geology on your farm:

USA – How to find your state geological survey 

USA – Awesome USGS Map interface – enter your zipcode and see your geology map

Canada – geological map

Australia – Shaping of a Nation – a fantastic coffee table book on the geology of Australia

Australia – a good breakdown of the rocks by state

Other Countries – Geology maps of all nations

So what about my farm? 
Like a lot of New England (USA) I have granite as the dominant rock on “Murramarang”…but to make things interesting I am also close to some metamorphic rocks that used to form part of an ancient ocean floor but were squashed and cooked as part of a collision of two huge tectonic plates.   I find some of these as boulders on the farm.  Then just to make it even more interesting, we have some boulders and sands dropped by the passing glaciers that once covered the area that are made up of some exotic rocks from the north.  These are known as “erratics”.   My soil is fairly young in places deep as it is made up of those glacial materials.   The granite has provides lots of quartz and clays as it weathers and so the soil is a loam (sand, clay mix).

So the geology is worth knowing about on my farm.

Some more details on the following rocks and soil :-


Limestone and Marble

Sand and sandstone


Let me know if this is helpful….maybe I have sparked an interest into you finding out more about the rocks soil and geology on your farm.


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About Gaz

I'm an Australian transplanted to rural Maine where I live on a small property with my wife and two youngest children. Life about family, work and trying the make the planet a better place for everyone.
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5 Responses to Rocks soil and geology on your farm.

  1. Great little refresher for folks…thanks for doing that!



  2. Very good article. I absolutely love this site.

    • Gaz Gaz says:

      Thanks 🙂 I will be adding more about “rocks and your farm” soon. I am so glad it is providing helpful information for people.



  3. Kevin Ray Evans says:

    The USDA NRCS has a great online soil-mapping application. On the page below, push the green button. Once you zoom in to your area. Select an area of interest (AOI). On the tab above, select Map. You’ll see the soil name and characteristics.

    PS, don’t click on any of the ads.

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