World-Building #3


Siting Mountains, Rivers, Valleys, and All That

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These are so closely related they cannot be untangled. If your story demands that a mountain be exactly here then there are things local rivers will do, and if your river must flow this direction, you are going to have to place high country to allow this.

Mountains, especially chains, are formed mostly by continental collision. Spread a towel on the table. Put your palms flat on the towel a couple of spans apart. Slowly slide your hands together. See? Mountain building! It's a lot like that when tectonic plates come together. "Tectonic plates" are vast rafts of solid rock that float on the molten magma, moving infinitesimal amounts each year into new configurations. Some plates carry above-water continental masses; some are sea-floor plates. They are the basis of the continental drift theory of tectonic development, also known as plate tectonics, which has worked out better than any of its predecessor theories.

Mountains that are remnants of ancient collision zones will be lower, rounded and worn away by erosion, like the Appalachian Mountains. Present collision zones will have relatively sharp and rugged chains, like the Rocky Mountains and Andes cordilleras (cordillera = chain of chains), where the American plates are hitting the Pacific plates. The speed with which the continents are colliding may be just enough to keep up with erosion, a little less, or may even build mountains faster than they can be worn down. The Himalayas are like this: Mount Everest is a trifle taller every year.

The second way mountains form is by volcanic activity. This is actually a minor note. Vulcanism occurs only in a relatively few places. Most common is where a sea-floor plate is sliding under a continental plate. The leading edge of the sea-floor plate not only is melted back into magma as it goes deep enough, but provides a path for magma, molten rock, to reach higher into the lithosphere, the rocky crust that we call "solid earth." Only under these circumstances can the magma find its way to cracks in the pressure shattered, thinner leading edge of the continental plate, and rise to the surface. Our prime example here is the Ring of Fire around the Pacific, an ocean which is slowly shrinking. Its floor plates are being subducted under all the surrounding continents. However, if you check, there are other "lines of fire" all around the Old World. These coincide with the earthquake zones.

The second volcanic zone is where two plates are moving away from each other. In this case, the tiny gap between plates allows the magma to rise. The most dramatic example of this is the Mid-Atlantic Ridge and other submarine expansion ridges. However, these can be found in continents if two plates are moving apart. Such areas are marked by "rift valleys," valleys created by a giant split in the earth, moving faster than wind and water-borne sediment can fill them up. The Great Rift Valley of Africa is a classic, though there are smaller. The Gulf of Baja and the Central Valley of California are one rift valley, the lower end of which has subsided enough to be under water. You will find vulcanism in Africa follows a neat line along the Great Rift. However, if you pay close attention, you will find there is much, much less volcanism along a continental rift than along a subduction zone, and vastly less than along an oceanic ridge.

This is less humbug to work out for an imaginary world than you may be thinking. Simply, you say, "I need big mountains here, so I declare these areas plates in collision. I need a mid-continental volcano here, so I'll put a rift valley nearby." You're the demiurge: the plates are wherever you like. The point to look out for here is that you can't have a perfect volcanic cone in Appalachian-round mountains, or amid otherwise non-volcanic features. Also, volcanism runs in strings and lines; it is not isolated, one mountain here, another two thousand miles away. There may be gaps in a string of hundreds of miles, but not thousands.

There is a third type of volcanic mountain, but it forms only over thin spots in an oceanic plate, and is pretty rare. If this "hot-spot volcano" rises high enough, it becomes an island. The Pacific is scattered with them. Again, they occur in strings, like the Hawai'ian Islands.

Once you know where the really high peaks are, you can figure that next to them are the foothills, then the highlands. Continents tend to be higher in the middle and lower at the edges -- going down to sea level, you know. So the seaward side of a mountain chain may slope more quickly than the inland side, where the highlands may form a massif or plateau hundreds of miles across. The lowest spots in any continent will be a water feature, or a former water feature. Many desert low spots are dried beds of prehistoric lakes, like the Salt Flats in Utah.

In earthquake zones you can have the occasional irregular hill or hole in the terrain. The high level of faulting leaves the ground fractured in a relatively small patchwork. Blocks created by faulting are under great sideways pressure. Depending on their shape, this can result in emerging fault blocks and subsiding fault blocks. A classic emerging fault block hill is Palos Verdes Hill (which would be called a mountain in some parts of the world) on the Los Angeles Harbor, or the nearby Signal Hill across the harbor in Long Beach. These rise out of the the flat plain inside a ring of "real" mountains in a high earthquake zone. Subsiding fault blocks normally result in lakes or sudden coves or harbors. Not far from Palos Verdes is Bixby Slough (now Harbor Lake).

If an area used to be an earthquake zone, some of these emerging fault block hills may be left as worn down stubs in otherwise flat plains.

Undersea volcanoes that build their cones high enough emerge from the water as small islands. This goes on in the Mediterranean and Caribbean, but especially in the North Atlantic Ridge up by Greenland.

The Wet Way

The number one point to remember -- the oft-forgotten "Duh!" -- is that water moves by gravity, which means it flows downhill at all times. Water can never flow over a hill or up a rise. To do that artificially requires the use of locks (look up any encyclopedia article on canals, especially the Panama Canal, for how locks work). A river cannot pass over mountains: it must flow through them in a deep gorge. Remember that when you look at a real world map of a river in mountainous country: it is always following the path of least resistance, always from one spot to the next lowest.

In nature, the water will flow around the hill, or if it is a whole ridge, the water will puddle up until the body of water formed is deep enough to start spilling over the lowest part of the ridge. Eventually, the moving water will start eroding a channel in that low part, cutting it ever lower, unless it is very tough rock. The water may only be able to cut down to a tough layer, and the escape stream will still pour from a lake.

On the other hand, the bed of the lake will cease to erode when it is formed. While it was a stream bed, cut by moving water, it might have eroded, but once the force of the stream is slowed in the holding waters, it ceases to cut. A lake with a very fast current may carry all the debris and dirt deposited in it along the outlet stream, but in most cases the load of sediment carried by in-coming streams is dropped, so that the lake slowly becomes more shallow, and therefore broader. Eventually, the rising lake bed and the down-cut channel may meet, and the lake ceases to exist, becoming just a wide spot in the river. At this point, the soft sediment bottom is easy for the river to cut in a deep, narrow channel. Geology is a constant state of evolution from one form to another.

Many, many features in the landscape are cut by moving water. For example, the earnest layman may think that the shapes of desert rocks are cut by sand blown by the wind. Never. Every one of them is water-cut, including the natural arches. If you compare, you will find the only other place rocks are cut in arches like this is along sea coasts or the courses of rivers.

Water doesn't fall often in a desert and may only run occasionally in some stream beds, but all those landforms like Monument Valley were either cut when the area was wetter (in some cases along giant ice-age lakes) or are being slowly bitten out by the annual gully-washer. The Grand Canyon of the Colorado is entirely water-cut, in the midst of a thorough-going desert. This applies on other planets, too: the Grand Canyon of Mars shows all the dendritic form of a water-cut river valley. At some time up there, it ran really wet, even if ten million years ago.

So in desert areas you will have dry lakes and dry stream beds. They may have been dry since ancient times, or they may be only presently intermittent. A storm in the mountains can send a ten-foot deep flash-flood down a dry stream bed, or a near-empty watercourse gully, neatly wiping out any party camped in it.

Excepting rift valleys and folds between mountains, all other valleys are river valleys -- that is, cut by moving streams, even if the "valley" is only a ten-foot wide gully. On soft ground, streams merge in what is called a dendritic or tree-like pattern. On hard rock, they may be forced into angular patterns, following the fractures of the stone beds where it is easier for the water to chip off a path.

So now you have mountains uplifting, with slopes down to the sea and gentler ones between chains or between the arms of chains. Then, from the crags down to the sea, you have rivers and lakes.

Uh, not always. The idea that if you follow any stream down you will eventually reach the sea has killed a certain number of explorers, hikers, and lost people in general.

Water is not bright. It is not forward-looking. It doesn't check the map and say, "I can reach the ocean by turning left here." All water knows is the next inch of travel. It will always follow, not only the slope, but the steepest slope it touches. Sometimes those slopes lead to inland basins, not the sea. Nothing feeds out of them.

Very large blind basins are the likes of the Caspian Sea, Lake Baikal, the Great Salt Lake of Utah, and the Dead Sea. The last is much saltier than sea water; centuries of incoming water bearing mineral salts, followed by evaporation of the diluting water off the surface of the Sea, have resulted in extreme salinization. The Caspian is also becoming more brackish at the present time.

The Great Salt Lake is only the remnant of a far vaster prehistoric lake, one of many that dotted the Southwest. When the pluvial rains and glaciers that fed them ceased, and the climate warmed as well, they slowly evaporated, like any lake that loses all its feeders. Only the likes of the Great Salt Lake and Lake Mono remain. If you see the Salt Flats in Utah, you are looking at the dried-out bed of a giant lake that became as salty as the Dead Sea, then saltier, then left only its bones of salt behind.

Your world may have blind-basin lakes in any of these stages. The size seems to depend less on local rainfall than on the overall climatic stage, swinging from ice-age to warm.

As well, streams may peter out in nowhere, and never even reach a lake. They may feed a bog, or disappear through the soil into completely non-spectacular underground channels.

Where Fresh Water Comes From

It all starts as rain. This falls and runs down hill, always seeking the path of least resistance. Most obviously, this creates freshets and rivulets that combine into streams that combine into rivers that go down to the ocean or blind basins. That's surface water.

Sometimes that path involves soaking into the soil or porous stone. This creates a local "water table" -- a level of underground water. When you drill a well to get water, you are drilling down to the water table. Water table height can vary seasonally, so that well level drops in the dry season, even with no increase of use. Since the dry season is when you need your well water most, lots of wells tapping the water table can reduce its height drastically, more so than it would drop naturally. As an area overbuilds, wells have to be deepened.

In some cases, the ground dips below the level of the water table. At this point a seep pond or slough (pronounced like slew; rhymes with through, not rough or bough) forms as the water comes out of the soil. A slough may have no entry or exit streams, only perhaps seasonal run-off from the banks around it, but still remain full all year.

Where do oases and other water in the desert come from? In some cases wells are dug hundreds of feet deep to a distant water table. In others, especially natural outbreaks of water, you are dealing with "aquifers" -- a porous layer of stone sandwiched between impervious layers of some different stone, the aquacludes. Somewhere far away, perhaps in the mountains, one end of the aquifer is exposed to ground water or surface water, and sucks it down its length. It dips and rises, since most rock layers are curved by pressures of mountain-building. Where it rises, it may break surface, and this lower exposed end emits the water acquired in the mountains. Next time you're using the hose outside, turn the water pressure low and hold the nozzle pointing up, but below the level of the faucet. Out the water bubbles by force of gravity and the slight pressure of water behind. That's how aquifers work.

Of course, aquifers emerge wherever the ground has broken or worn down to them. This doesn't have to be in deserts. Plenty of well-watered places have natural springs, as these outbreaks of groundwater are called. There's a lovely one bubbling up in a vest-pocket park in Kalihi.

If the blocking layer over the aquifer is only cracked, not worn away, water may shoot up under pressure, like a pinhole in your hose, causing it to surface unnaturally. This creates natural artesian fountains. If enough soil blocks the emerging water, you can still dig down to it, or even through the hard layer to the aquifer, to get an artesian well that stays full higher than the local water table.

Of course, after a certain primitive level of technology, people can dig or drill down to aquifers and create their own artificial springs and wells. Say, Bronze Age, certainly early Iron Age. It does not require big steam drills, just picks, shovels, and long ropes.

As this 1851 encyclopedia engraving shows, artesian fountains will not shoot up any higher than their source at the open end of the rock bed. This is important if you are relying on artesian pressure to feed water up a pipe into the upper stories of a building, or a raised garden terrace. They also only fountain when broken through from the upper end to the lowest point. The farther side of the curve will only provide artesian wells (d). Next to that, (e) shows a slough fed by the aquifer.

Water can also emerge from rocks when a stream comes out of hidden caverns. Most cave systems are cut by water (caves are a whole world to themselves). This is tricky for public health reasons. A true spring has pure waters, filtered by miles of passing through stone. A cave spring can be full of germs of decay from a hole into the cave system being used as a garbage dump by the locals. In France, identifying true springs and cave springs is a matter of some concern for water supply. Most "cave systems" are not something you can crawl around in: they are just channels cut underground by moving water, like natural drain pipes. Tracing a stream that dives underground and emerges later may require dyes, not a hard-hat expedition.

For world-building, this means you really can site waterholes wherever you like in basins surrounded by mountains. Just declare the presence of an aquifer. Because most aquifers are layers of stone, this means springs will break out in a line along this exposed edge, kind of parallel to the mountains, letting you have a string of oases, like that which supported the Classical Saharan Garamantes horse-chariot culture. You can have "poisonous" wells or springs anywhere, not just in volcanic zones, because the water comes through a deposit of arsenic or cadmium on its way to the surface, or just because there's a regular supply of dead animals in a cave on the other side of the ridge where this cave spring first goes underground. You can always cause thirst or save from it, by order of world-building god.

THE ETERNAL CITY: Do-It-Yourself Political Geography


or, There's Alien, and Then There's Silly

STORMY WEATHER: Siting Jungles and Deserts


Siting Mountains, Rivers, Valleys, and All That

THE ETERNAL CITY: Do-It-Yourself Political Geography

Caves, Caverns, and the World of Eternal Night.

Some Basic Minerology for Planet Building