Retaining wall blocks are made from a type of light and aggregate concrete. They are considered to be very sturdy and some of the easiest to use.
Although they look very complex they are made using a simple mixture of sand, stone, and other materials similar to these. They are ideal for building low walls in a yard or around the garden. Some will use it for decorative purposes while others will use them for something functional. You can also look for 3d wall blocks via http://www.modulararts.com/blocks/designs.html.
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These blocks come in a variety of styles that will interlock or use mortar to hold each of them together. Each one has been designed to fit perfectly into the other and to create what looks to be a very solid structure without having extra effort or the added price of bringing in mortar.
Each of them has to be flat on every side or simply flat on three sides with only one rounded end. The rounded end might be rough or smooth depending on the design that you wish to have for your yard. The interlocking retaining wall blocks come available in various styles.
These are designed for larger walls or ones that will have large amounts of soil retention are designed with flanges that will secure into the parts of other blocks. To make things more sturdy these will have holes running through them that are aligned also with holes in the block underneath. This allows pins to be run through to keep the blocks in place. This is not a hard process to do.