Creating realistic terrain involves more than simply sculpting out a feature and plonking it on the landscape. To truly create a believable landscape, the process of natural mountain formation has to be understood else Mountain ranges may be formed in places that simply make no sense, or worse, disrupt the sense of locations around them.

Mountains are best looked at as huge ripple lines or wrinkle lines in a world's surface. They exist where the land bunches up and raises, in long lines called ranges. Several mountain ranges next to each other are called chains.

The height of a mountain is typically measured in metres above the level the sea normally reaches - not to be confused with tides. As the mountain rises, the atmosphere around it becomes less and less dense, able to hold less and less water, and becoming colder and colder. The net effect of this is mountains usually have lots of snow and ice on them, on the side facing into the prevailing wind. The side sheltered from the wind also has some snow and ice, but not much as most fell as it was pushed out of the colder, thinner air. The side away from the wind is usually dry, and a long rain shadow stretches out.

Making Mountains out of Molehills

It is an interesting quirk with mountains that the taller the mountain, usually the younger it is. Over hundreds of millennia, tall mountains are worn down by wind and rain, carved by ice, and mud. Therefore, really tall mountains are far more likely to be over an active fault in the crust than short, stubby ones.

Mountains are made on planets where like the Earth; the crust of the planet floats on a heavily pressured molten core. If your intended world does not have a hot, molten core, it is unlikely mountain ranges would have formed.

A planetary crust is not one solid mass, and is broken up into several large plates, which move about as the currents in the liquid beneath them ebb and flow. Sometimes the plates grind together, sometimes they are pulled apart. These changes happen slowly, over millions of years, and it's when they grind together that mountains can form.

Fold Mountains

When two plates collide, one of the plates slides under the other. The one that is pushed up, wrinkles and folds, contorting as it twists and bends, pushing up high, thick mountains full of warped layers of rock. This often exposes layers that had previously been buried miles under ground, and in this way, often exposing fossils millions of years old, by forcing them up to the surface in jagged, uneven lines.

Block Mountains

Sometimes when cracks or faults appear in the crust, the plates are not forced together. Instead, one side or another may actually sink downwards, creating a 'mountain' which in reality has not pushed upwards at all, simply the cliff line of where the land used to be. Even so, these blocks can be several kilometres high on Earth.

Alternatively, a block of land can be pushed up, sliding against fault lines either side, to create a tableau mountain, rising as a single pillar - until the wind and rain get at it and carve it, of course.

Weather, Water and Wind

All mountain ranges start out as blocks of rock, misshapen or otherwise. Cracks and chasms appear as the rock splits and breaks forming the basic outlines of future mountains, but they lack so many of the features the nooks, crannies, caves, and slopes of the mountains you might expect. This is because mountain landscapes are carved, worn away by the heat of the sun, and the power of wind and water.

The process known as weathering, erodes the rock, claiming the loose stones and softer rocks first, leaving the harder rock peaks less affected. Over tens of millions of years, even the highest mountains will be worn down by weathering to the same height as the land around them.

Heat from sunlight warms exposed rocks during daylight hours - this process will be much quicker with multiple suns. The heat makes the rocks expand. When the sun(s) set, the night air rapidly cools the rock and it shrinks. Over time, the stress this puts on the rock, can make them crack apart. Water then seeps into the cracks, and, if this freezes, the ice that forms will push the rock apart. At the same time, the wind is whipping up stones, and loose pebbles and bashing them at exposed rock faces with a sanding motion. Over time the cliffs collapse as the wind wears them away, and contribute more loose material for the next face.

Running Water

Most rivers start in mountains as the excessive snow, and sometimes glacier output is an excellent source of water. Failing that, a river can start from a spring - a place where a higher, porous rock meets a lower laying non-porous rock strata, and the groundwater flowing from higher ground, bursts forth and flows above ground. This is how rivers start in hilly terrain, and they always start partway down a mountain slope, never at the top, but always where two differing rock strata meet.

The steep river is not one single river, but many tributaries, each flowing a separate path until they slowly join up, one after another, the river getting wider each time. Each tributary flows fast enough to pick up loose stones, and dirt. Over time, the effect of the rocks banging on the river floor, and the water lifting particles out, carves a channel out of the rock.

These channels are always V shaped, because the water always flows fastest in the middle. Over time it wears down into the rock, until the sides collapse in, and the river carries away the loose material - a V shaped depression forms with the river in the middle.

Waterfalls

Waterfalls often occur in mountains, and are spectacular forces of erosion. As with the springs, waterfalls happen where two different rock strata meet.

Waterfalls are usually found where the fast river flows over a layer of hard rock, then a layer of soft rock. The soft rock is worn away far faster than the hard rock, until a straight drop is achieved, where the original terrain would only call for a slope. Over time the harder rock too is worn away, but the softer rock is worn away faster, and deeper, creating a greater and greater drop, with an overhanging hard rocky crust, with littered pieces of the harder rock buried in the floor of the waterfall, where the old cleft has fallen.

Rapids

Just the same as with waterfalls, rapids require a mixture of hard and soft rocks. The softer rock wears away first, leaving outcroppings of hard rock forming almost a mini-delta, high up in the mountains or hills, forcing the water to flow in sub-channels round them, creating conflicting currents, each of which wears the rock away at different angles, slowly expanding the rapids sideways.

Rivers of Ice

Glaciers are rivers of ice. They start high up in the mountains, and flow down the mountainsides extremely slowly, but with far more power than liquid water. A glacier is made as snow builds up and up on a mountainside. Every time new snow falls, its weight compacts the snow under it. Eventually, the old snow hardens into something halfway between snow and ice - slush called névé or hard snow forms. The great weight of névé forms a hollow in the side of the mountain. As new snow continues to fall, the névé pulls away from the upper edge of the hollow, or cirque, and begins to erode the lower edge, to form a crevasse. As it moves, the névé, free of the weight of the snow on top of it, hardens into ice, and begins to flow steadily downhill. Moving anything from a few centimetres to a few metres each day, the sluggish glacier is an unstoppable juggernaut, and rips out rocks in its path, carrying the larger lumps with it, smoothing out everything else into a U shaped valley.

As with other rivers, many tributaries often join together to form one large glacier, maybe as much as a mile across, carving out and smoothing down a U shape valley. The only jagged rocks left will be above the glacier level. Rocks carried by the glacier are worked out to the sides until they are deposited at the edge of the valley in long lines called moraines. Occasional rocks which manage to stay in the flow, are carried to the glacier's final melting point lower down, called the snout. There, great piles of heaped, smoothed rocks, called a terminal moraine, mark the end of the glacier. Even if the glacier eventually stops flowing, these piles will remain as monuments, as will the great, U-shaped valleys.