Aggregates are as useful as they are decorative and perform a multitude of functions in the garden. They can improve drainage, are perfect for filling gaps between different surfaces and when used with a membrane are low maintenance. They can be used to form pathways, driveways and cover the edges of ponds giving a gentle slope to allow animals easy access to water.
We get aggregates from rocks. They can be naturally formed by weathering and erosion such as pebbles on the beach, or mined and crushed to a specific size. What gives different rocks their characteristics like colour and hardness, are the minerals that they are formed from and the minerals themselves are made up of elements. They all originate from the earth's crust and there are three types, igneous, sedimentary, and metamorphic.
Formed when magma or lava from volcanoes cools. Examples include basalt and granite. Most igneous rock is very hard. Some of the most spectacular rock formations on our planet are made of igneous rock; in Britain we see granite shapes called tors in south-western locations like Bodmin Moor, Dartmoor and Land's End
Formed over millions of years when sediments (tiny pieces of rocks and animal skeletons) are pressed together at the bottom of seas and rivers. Examples include sandstone, coal and chalk. Some sedimentary rocks contain fossils (bones or shells of living things that were buried long ago and have turned to stone).
Formed when other rocks are changed due to heat or pressure. Examples include slate and marble. Metamorphic rocks are very hard but can be damaged by acids like acid rain (on buildings) or even lemon juice (on worktops!).
Pebbles & Cobbles
Pebbles and rocks distinguish themselves by size and colour in the aggregates world, rather than by their chemical makeup. They are generally sorted by size first. Pebbles are larger than granules which are around 2 - 4mm, but smaller than cobbles weighing in at around 65 - 250mm in diameter.