If you're going to try building a cairn you'll need a lot of special 'cairn' rocks. Hopefully you'll find this type of rock all around where you're planning to build, both on the surface and under the ground. If not you will have to adopt a different rock material and a new strategy.
The terrain where cairn rocks prefer to gather is usually rugged uneven ground, full of outcroppings and glacial till. Cairn Terrain Rocks (or 'Cairn Terriers' as they're commonly called) are great for building free-standing mounded dry stone monuments. They are small to medium sized rugged rocks which love to do any job that you set them to do. They seem to like to huddle together in groups nestled on top of one another. Their shape is usually somewhat triangular, with smaller rear sections and larger faces. This makes them fit well in rounded structures. Take care not to upset them while you are building with them as they can often be quite aggressive and hurt you, or more likely fall out with each other.
Cairn Terriers must be put in their place at an early stage so that they don't try to pull some quick moves on you. The toughness that makes them suited to aggressively fastening on to other rocks can frustrate you when you try to use them in different combinations with less suitable shaped material. Cairn Terriers can be stubborn and dominant (they want to be the boss) and will make you have to prove that you are the one doing the building. Your walling must be absolutely consistent and you must 'stand your ground' with them. Make them sit up and mark well the place that you have decided they should be.