What’s the difference between minus & screened/clean rock?

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When a builder wants to use crushed rock on his property, he wants either one of two types:

• Crushed rock that has rock pieces but also has dust and fine particles in it.
• Crushed rock that is just the rock pieces, without any of the dust.

Crushed rock with sandy, dust-like particles (or as we like to call them ‘fines’) is called ‘minus’.

Crushed rock without the fines, is called ‘clean or screened’. It has been cleaned and has had all particles removed. This type of rock is usually more expensive because it requires more processing, and it also requires lots of water, to effectively ‘clean off’ all the fines.

Why is it called ‘minus’?

Because ‘marketing’! The opposite of clean rock would be dirty rock, and no one wants to buy a material called ‘dirty rock’! Also, because crush rocked is measured in inches, and there is no way you could measure the dust particles, you could consider fines as ‘negative’ or ‘less than’ when compared to an inch. 

How is it made?

Crushed rock (also known as gravel) is made by blast-mining. Basically, a geological survey is done to see what type of rock is in a mountain (or hill), and if desirable, miners will drill holes, place dynamite in said holes, and then blow off a small section of the mountain.

The explosion breaks off a lot of rock, in very large pieces. These pieces are then taken, placed in a crusher, and then crushed. The resulting crushed rock then gets filtered into various sizes, such as 1/4 inch, 1 1/4 inch, 2 inch etc. Then, at this point the ‘minus’ rock would be ready to go, but the rock that will be turned into ‘cleaned’ rock would need one more step to wash off all the dust.

What is minus and clean rock used for?

Clean rock is often used for aesthetic purposes, when replacing turf, or mulch in making a ‘xeriscape’ (low water consumption) landscape. If you want to have large patches of space on your landscape that will not be walked on and won’t have turf, you’ll want to use crushed clean rock. Scatter some boulders, and some low-water native plants and you’ll have a proper xeriscaped space.

Clean rock is also used to surround drainage areas and for water filtration, as it still lets water flow through, but does effectively slow it down. Retention basins often used crushed clean rock.

Clean rock can also be used in conjunction with minus when building driveways. The clean is the bottom layer, to create stability, and then with minus rock as a top layer, to fill in all the cracks and provide a smoother driving surface. You can make a driveway with JUST the clean rock, but it might be a bit bumpy, and hard on your tires.

Minus can be used by itself for hiking trails and garden paths. These surfaces only need to bare the weight of foot traffic, so it is not essential that they are reinforced with large clean rocks as a foundation (but you certainly can if you want, it just costs more money but does end up lasting longer). Keep in mind, when using minus as a walkway, the dust on these will track on your shoes, therefore home owners should keep that in mind if using them in their backyard.

Where should you get your rocks?

Your landscape design will probably require some crushed rock and boulders! At Kalamazoo we’ve got what you need, and the most gorgeous colors in the area. Visit our catalog to see for yourself!