# What could make an area of land be accessible only at certain times of the year?

I've looked around here a bit, but I haven't managed to find a post that really answers my question (though if there is one, please let me know).

I'm trying to write a story where the two main characters decide to stow away with some travellers who come through their village annually, and I want the travellers to realise that these kids are there when it's too late to turn around and take them back, so the travellers have no choice but to go about their usual annual route before dropping the kids back at the same time next year.

I would like this to be due to some geographical or even season-related reasons and to have the region where the village is located only easily accessible perhaps two or three times a year. So I'm not looking for something like the tides making a certain island only accessible once a day, since that's far too frequent (though it is of course fine if it's a tidal event that somehow only happens a few times a year). Let's also assume that the years and seasons are basically the same as here on Earth.

So, is this possible? And if so, what kind of environment would make it possible for this region to only be easily accessible two or three times a year? (I don't mind if the amount of times the region is accessible in a year is more than two or three, but I'd definitely like it to be no more than four or five, if possible.)

I would also appreciate it if an answer for both how long the region remains accessible (preferably a week or so), and how long it takes for whatever occurs to make the region inaccessible again is given, too.

As a note: It doesn't have to be completely inaccessible for the other parts of the year. Just difficult enough that a band of travellers who are rather slow moving with some carriages and heavy luggage don't think it's worth trying to enter the region outside of those few times a year.

Thanks in advance for any answers given :)

EDIT AFTER CHOOSING AN ANSWER: Thank you everyone who has answered this so far! I have waited about a day and chosen Zxyrra's answer as the best because it's got the most up-votes and for the most part I think it's one of the most useful. Although, there are a number of answers that are similar and I'm thankful to everyone who gave suggestions :)

• A glacier comes to mind: in summer it melts opening a pass through the mountain. Once it freezrs again you need to wait for the next year – algiogia Oct 17 '16 at 10:42
• @algiogia i'm not sure that's how glaciers work, they stay frozen and flow like a river, but slowly. – Whinja Oct 17 '16 at 11:08
• Interesting that so far all of the answers use water - is there nothing else dynamic & cyclical enough? – plast1k Oct 17 '16 at 12:09
• How old are the kids? (ie, could they travel back on their own if everything was well, can they work a bit) and how long after the departure are they discovered? – Matthieu M. Oct 17 '16 at 13:38
• @Whinja: Not necessarily a glacier, but snowpack would definitely work. (A certain scene from the movie Seven Brides For Seven Brothers comes to mind...) – Mason Wheeler Oct 17 '16 at 18:55

There are many ways to change access to an area throughout the year:

Seasonal Extremes:
Typically, the further from a major body of water a location is, the hotter the summers are and the colder the winters are. Proximity to a lot of water sort of averages the seasons' temperatures together as long as you are not close to a pole or the equator. If you have a location in the center of a massive continent that is neither close to the pole or close to the equator, then the summer months would make travel extremely hot - up to deadly, desert temperatures. In the winter months, the snow, ice, and cold would also be extreme, leaving a few weeks in the spring and a few weeks in the autumn to travel to and away from this location.

Ice Needed:
You could make your location similar to an island in the Bering Sea. During the summer months, the sea is liquid. Maybe in your world, it is full of creatures that prevent sailing, or reliable ships that cross this area are expensive. In the winter, the entire Strait freezes, at which point your characters could buy metal spikes for their wagon, or hire a dog team, to cross the new bridge of sea ice to the desired island or northern continent. This would leave several months for travel, but for the majority of the year this route would not work.

Water Needed:
The opposite of the above. If an area up north is frozen for much of the year, but the ice isn't safe to cross, maybe it thaws for several weeks allowing passage.

King Tide:
About once yearly, the alignment of the sun, moon, and Earth produces what's known as a king tide - in which some areas see extremely high tides while others see extremely low. You could use either the high tide or the low tide during this event to access a location:

Low tide - imagine the only route to your town is through the mountains, and the only route through these mountains is a natural pass that looks like this. Now, imagine that this borders the ocean, and that the pass is actually a small saltwater inlet. During all other times of the year, this arch is completely submerged, but, when your planet, moon, and star align perfectly, people use the pass for travel and commerce for several hours. A stretch, yes, but unique.

High tide - finally, your location could be on an island with extremely high cliffs such as these if not much, much taller. Only once during the year, when the planet, moon, and star align, does the tide rise up and allow ships to do business for a few hours.

• High tide, traditionally, lets water clear shoals, as opposed to reaching docks. Making docks that dip another few feet down isn't hard: clearing shoals is. – Yakk Oct 17 '16 at 13:18
• "Proximity to a lot of water sort of averages the seasons' temperatures together as long as you are not close to a pole or the equator." As a Minnesotan, I'm hesitant to agree with this. – schil227 Oct 17 '16 at 17:32
• @schil227 However you are also relatively far north which has climate impacts of its own – Zxyrra Oct 17 '16 at 17:46
• For the first example, think of the Appalachian Trail on the US east coast. It runs from Georgia to Maine, and if you're going to thru-hike it from south to north there is only a small window of time in which you can start if you're going to avoid getting caught up to by bad weather. If you miss that window you would probably be fine hiking for a long time, but you would be virtually assured of bad weather catching you before you finish. Just make the weather as extreme as you like to suit your story. – Jeff Oct 18 '16 at 17:33
• The Great Lakes do moderate the temperature of at least some coastal parts of Minnesota, AFAIK. That doesn't mean it's nice - just that it would be even worse otherwise. (Interior Siberia is significantly worse climatically, though it's also on a larger continent.) OTOH, some other parts of the Midwest get hit by a "lake effect" that means more snow because of the Great Lakes. – cometaryorbit Oct 19 '16 at 5:07

Winter

Harsh winters came to mind first being from North America. The best part about winter is that it can either enable or prevent travel to a certain location.

A. If your village is separated from the outside world by a mountain pass big enough to have snow caps in the summer, you can bet anything it would be suicide to attempt traversing during the winter months (see Donner Pass & the Donner Party).

B. Areas up near the arctic are only available to modern trucks when the sea water, large rivers, lakes, and just the ground itself freeze solid, forming ice roads. Obviously You could raft or kayak across but if you have enough supplies or people, the expanse is large enough or the terrain perforated enough, then you would need a caravan and the path to be frozen.

Deserts

If you need to cross a vast expanse of desert, you'd best do it if you know you can carry enough water or get some along the way. Desert areas can have ephemeral rivers, which only flow when there are significant rains. You have a ton of flexiblity here. There are examples in Australia that only flow for a week when they get hit by just the right rogue monsoon, which may take years or decades to happen. You can also have semi-predictable wet and dry seasons. Others might flow seasonsly and for a bit longer as snow melts from near by mountains during the spring months.

• Keep in mind that once a pass becomes important enough, it will get kept open year-round. (Donner Pass is an example of this, with both I-80 and the railroad lines running across it...) – Shalvenay Oct 17 '16 at 11:43
• @Shalvenay: But we're not talking modern tech levels here, are we? (If so, we just put the kids on a plane back home :-)) Though traversing the Sierra in winter is suicide only if you're woefully unprepared to deal with winter. See e.g. the story of Snowshoe Thompson, who spent many winters carrying mail across the Sierra a bit south of Donner Pass. – jamesqf Oct 17 '16 at 17:03
• @jamesqf Sure, but I'd classify a band of traders/travelers who were weren't planning on roughing it in the first place with what OP states as slow carriages, luggage, and a few 8 year old tag-alongs to be "woefully unprepared" enough to avoid the risk. – plast1k Oct 17 '16 at 17:29
• @plast1k: But the OP says the travellers do the trip annually, so they should be prepared for a heavy early snowfall (as the Donner Party wasn't). Of course, the fact that they could, if necessary, make it through the snow-covered pass doesn't mean they want to go back (with all the financial loss &c that would incur) just to take a couple of bratty kids back home. So they maybe send a letter via the local Snowshoe Thompson: "Sorry, your kids stowed away with us. We'll drop them off next year." – jamesqf Oct 17 '16 at 22:10
• @jamesqf Good point - and sounds like the making of a good story :) – plast1k Oct 17 '16 at 22:26

Disease. Specifically insect-borne disease. The rainy season arrives, the lowland swamps have a population explosion of mosquitoes, midges, tsetse flies or the equivalent. These insects carry some nasty disease to which humans (or their domestic animals) are susceptible. Fatal or just debilitating is up to you. Maybe not fatal but with a side effect such as horrible smallpox type scarring, or deafness/blindness or impotence. So everyone just avoids the lowlands at 'fever times'.

You can have more than one rainy season a year - for instance, Kenya has 'long rains' and 'short rains'Kenya climate.

The travellers would also know that just as they are heading south to avoid the lowland swamps, the kids' community will be heading north to also put some distance between them and the insects.

There is a small town Verhneimbatsk - You can see it on the map. I think, since 17 century this town was trading outposts - natives (called kets) were trading furs with Russian merchants from city of Krasnoyarsk.

There are only 3 reliable ways to get to this town -

1. by air, postal helicopter 2 times per week,
2. by Yenisei river, using boats from middle of May to middle of October.
3. by Yenisei river, using snowmobile or sledges with dogs from the end of October to end to April (but using dog sledges can be quite dangerous and hard for trade).

And, probably not very reliable way, use really big team of lumberjacks to make road through taiga forest.

On the second half of october the winter starts, and river is covered in ice in few days.

So, the situation can be like this (i assume it is pre XX century level of tech) - in the middle of October, traders came by boat to this city, exchange wares for furs, than they sail in hurry to the next town, because river is covering in ice. All travelers were busy by setting sails, using sticks and paddles to repel ice from boat. And, in this hurry, they were unable to see, that few children managed to hide between sacks of goods.

UPD: also it is worth notice, that end of September is usually very rainy, and river surface is raised to ~ 1 meter. Maybe there is a lot of stones on the bottom of the river, and they can break the boat. And, with elevated river surface after rains, sailing is more save.

UPD2. Answer for @Yakk's question: The weather can be quite unstable - the ice can start appearing on every day from 5th to 20th of October, and usually it takes few days for river to be covered in ice. The mecrhants was not aware of the exact departure time. By careful weather observation, everyone knows, that if there was 3 days in a row with strong wind from North, than ice will start appearing, but if there was only 2 days in row - no ice. And merchants used this knowledge to plan departure.

Why i think it was acceptable risk - The current speed is about 8km/hour. And merchants were travelling down the stream. So, if they were acting properly, they can easily travel more than 300 km until river is covered in ice, it is enough. And, in 300 km there can be a major town, where they can trade goods too and pass the winter.

• If your departure is tight with the start of winter, even if turning around is possible it will not be worth the risk of being stranded. – Yakk Oct 17 '16 at 13:19
• Excellent that you mentioned a specific existing town that is actually close enough to what OP wants to merit further research. – Mad Physicist Oct 18 '16 at 13:38

I really like the animals idea, and in a similar vein the passage of an area could be restricted by plants.

There are a myriad of real-world poisonous plants that can cause irritation, pain or even death just by humans coming into contact with their leaves.

It would not be a stretch to believe that one of these plants blooms at certain times of the year, perhaps within a forest that can only be crossed during winter/autumn when all of the leaves have died. Then when spring comes around, the forest becomes inaccessible again.

Of course, having a forest that makes a village entirely inaccessible for half of the year is unlikely, so perhaps it just makes it extremely inconvenient for the two villagers to return home.

Another reason not to return could be due to economics.

If the travelers cannot afford to travel back to the village, as they don't have enough food to account for the extra time spent heading back, or have perishable goods to trade at their destination, they may decide that it would cost more to take the villagers back than it would to let the two stowaways stay with them. Just because the travelers can physically head back to the village, doesn't mean they would be willing to.

If the only requirement is that the travelers cannot just send them back on their own, as they would refuse to escort them back, then this allows a greater number of reasons for them being unable to travel back alone until the larger group returns to the village. This could be anything from highwaymen that target smaller groups of people to animals that only ignore the larger groups of humans.

### Spring Floods

As the snow in the mountains and hills melts, small creeks become raging rivers. Sure, one could camp out until the waters calm down, but the caravan might not have enough food to wait that long.

### Water Hole Capacity

In a dry area, the caravan uses water holes which take some time to replenish after a large group went through and took all the water. So a caravan can never turn back, only go forward. (Of course that would mean two different caravans would be disaster for both.)

As Zxyrra stated in their answer, there are many ways to build an inaccessible area. Here's just another idea I haven't seen mentioned yet.

Animals

You could have your piece of land be inhabited by dangerous predators. Navigating through the area without encountering these predators may be possible but will become more difficult the larger the traveling group, especially if also traveling with wagons.

In winter, these creatures go into hibernation, allowing for a safe passage.

• Or the place is on the annual migration path of swarms of something like fire ants. – Anton Sherwood Oct 18 '16 at 6:52
• Army ants. In this case, hypothetical monster driver ants which migrate "there and back" between two regions. en.wikipedia.org/wiki/Dorylus – RonJohn May 17 '17 at 2:12

## Mountain pass

It's reasonable to have places only reachable through a single mountain pass.

It's reasonable for that pass to be traversable only part of the year.

For a real life example, maybe it's illustrative to look at the remote Georgian region of Tusheti, which is still currently accessible only for part of the year unless you use a helicopter - there's a single road leading to it, it leads over a 2900m / 9300ft pass, and the road is not considered traversable in winter.

From the pictures e.g. http://unusualplaces.org/one-of-the-most-treacherous-roads-in-georgia-the-road-to-tusheti/ it seems rather obvious that if it was snowed over, waiting a few months would seem a reasonable option.

• Also the north rim of the grand canyon is only accessible May through November as snow makes the roads impassible. – cybernard Apr 19 '17 at 22:27

Either the land is surrounded by water and swamp for much of the year but it dries out enough to cross in summer, or it's surrounded by water all year but it freezes over enough to cross in winter. Take your pick.

• Man might invent means to cross water and - for lack of a better word - call it ship. – Ghanima Oct 17 '16 at 11:00
• @Ghanima But according to the question it doesn't have to be completely inaccessible, just inaccessible by carriage. – Mike Scott Oct 17 '16 at 11:03
• Sure, but looking at the history of mankind open water is not exactly the kind of thing to stop us. People crossed the pacific at stone age technology level. Granted they would have to switch carriages for rafts and skiffs but it is not a real show stopper. If it is just water they will think of crossing it and manage to do so. – Ghanima Oct 17 '16 at 11:13
• This is a pragmatic answer. Once the surrounding area becomes swampy, nothing gets through it without considerable effort. – Tony Ennis Oct 17 '16 at 12:12
• @Ghanima Yakutsk, Russia is a real-world example of this: en.wikipedia.org/wiki/Yakutsk#Transportation - in the spring and autumn the Lena River is neither frozen enough for land transport to cross, nor free enough of loose ice for boats to cross safely. With sufficient technology anything is possible, of course - but I wouldn't count on a pre-industrial civilisation being able to find a solution. – gasman Oct 17 '16 at 20:32

The 'Pitch Black' story line comes to mind here -

A total eclipse allows creatures dwelling underground to surface and kill things, and soon as the sun comes up, they go back underground. You may be able to say the equator + surrounding x miles is the only section of the world that experiences the eclipse, making that region completely impassable for all but the baldest of muscle men. EDIT: They don't come out at night because there are multiple suns, so it never gets completely dark.

• Why don't they do this at night? It seems quite implausible even by movie standards. – John Dallman Oct 17 '16 at 16:04
• @JohnDallman I added an edit to the spoiler box – MooseLucifer Oct 17 '16 at 16:05

The Great Rann of Kutch in Gujarat, India is a lake that is dry during summer and flooded during monsoon.

             -> http://www.travelclassics.com/library/india_rann.shtml


An island with temporary ice sheet in winter.

You can access it for a few weeks in summer, after which ice forms on the sea, making it very difficult to board. In autumn, any boat would risk being caught in the ice, or sunk by an iceberg. In winter, the reaching the island would mean boarding on the ice, crossing a long distance on ice in blizzards, and in spring the melting would make the icebergs and the instability of the ice even more dangerous.

Similar to how some stations (like Dumont D'Urville) in Antarctica are difficult to reach outside of a few months in the warmer season.

1) The place is kind of like bermuda triangle which opens up/becomes navigable at the time of the "green flash" pirate lore.

2)Probably after an geomagnetic storm which makes it possible for aeroplanes to pass with least interference.

3) place surrounded by mountains filled with smog. Uncharted territory. It makes it impossible to for planes to pass. You have to rely on the migrating birds for guidance. (hmm...Pandora-ish)

4) A no-fly zone. Riparian states that fighting over a water source. So, Transportation is possible only when the source is dried up.

5) Mythological Areas: Certain areas are made off-limits certain times of the year as they would be visited by certain deities. (Fishing is banned at my place for few months to facilitate the fishes to breed. also to balance out excessive fishing.)

Frame Challenge

I'm trying to write a story where the two main characters decide to stow away with some travelers who come through their village annually, and I want the travelers to realize that these kids are there when it's too late to turn around and take them back, so the travelers have no choice but to go about their usual annual route before dropping the kids back at the same time next year.

Why is it the travelers' responsibility to escort those kids back?

If the kids manage to stay hidden for a couple days, I would be surprised to see the travelers going out of their way to get them back and get going again. Note that for each day the kids are hidden, the travelers would have to add two days to their travel, which might derail many plans and arrangements made.

Imagine, for example, that they have to catch a weekly or bi-weekly ferry across a great lake: be one day late, and you have lost a full week (or two). And this just compounds.

Since your group of travelers come every year, I expect that they have a rather set journey plan, and delaying it might significantly affect them. There may be events along their road that they are counting on (fairs, ...) and missing them would cost them money, maybe a lot of money.

On the other hand, keeping the kids and having them work for their food does not incur any delay, and is still morally "fair".

Note: There is also a "simple" way to increase the delay (and cost). If the kids are discovered rather quickly, but argue that they are going to the next town over to their uncle's house, the travelers might find it more convenient to just follow on. Of course, once they arrive in the next town after a few days and there's no uncle to be seen, it now is very expensive to get back to the village...

• Hi, thanks for your answer, but I'm not completely sure what you're trying to add? Are you basically saying that it's best for the travellers to keep the kids with them instead of ruining their schedule? Because I'm pretty sure that's what I said I want to happen. In other words: I already have that bit planned. (BTW the kids are only something like 7/8, so I doubt the travellers are going to abandon them or set them off to find their own way home, meaning that when the travellers realise the kids are there and it's too late to turn around=the moment the kids become their responsibility.) – s.anne.w Oct 17 '16 at 14:02
• @s.anne.w: I am trying to say that there is no need for a major blocker that prevent the group from going back; just schedule pressure can be sufficient to keep the travelers going forward. – Matthieu M. Oct 17 '16 at 15:04
• At the level of technology implied in the question, you do not ever plan your schedule so tightly that being a single day late will cause major problems. Delays happen: a wagon wheel breaks, or a horse runs off, or any of a number of other things could cause you to lose a few hours, and those could easily add up to missing the boat. – Mark Oct 17 '16 at 17:54
• @Mark: I disagree, you have a rough schedule, but you still have one. And if you are already running late and trying to catch up, then you'll be even more reluctant to turn around. – Matthieu M. Oct 17 '16 at 18:06
• @MatthieuM. Oh yes, I understand now. Sorry, I was just off going to bed when I read it so it didn't sink in very well. Thanks for that, but if I can I would like to have a reason relating to the environment the story is set in too. Sorry for my misunderstand :) – s.anne.w Oct 17 '16 at 21:58

I see only a handful reasons which are all bound to the classical elements: Earth, wind and fire; water; plus radiation, as a modern "element".

Others have given more detailed ideas for some of them; in general though, the reasons for the "elements" to shift seasonally are bound to celestial movements, either of the planet in question, or of near-by celestial bodies like a twin planet or a moon. A double-star could be interesting. Radiation could come from a star emitting hard gamma which is most of the time shielded by another body.

The only other seasonal shift is geological, like a geyser or volcano with a long-time stable period of a few months; possibly in connection with tidal forces of a close body destabilizing the planet's crust.

• I can see how a band of old funky men people singing classics would repel people trying to reach the place. XD – njzk2 Oct 17 '16 at 14:58
• @njzk2 Lol... true... their heads may explode... Or an old folks nudist colony. Can't go there in summer, seasonality built-in. – Peter - Reinstate Monica Oct 17 '16 at 15:03

An easy solution to your dilemma would be a one-way-road. A stretch of land that is only passable in one direction. A wide fast flowing river surrounded by impenetrable forest would be an obvious candidate for this scenario.

In that case your travellers would travel in a circle and part of that circle would be the river: A very fast and convenient way to travel … but only in one direction.

Travelling on this river could easily be more than ten times faster than hiking back trough the (dangerous) woods. So the kids would only have to stay hidden for a few days, which is probably more realistic than waiting for the seasons to change.

An underground fire in a coal seam that contains heavy metals making the smoke that rises from it toxic. Such fires have been known to burn for centuries, like in abandoned coal mines.

The valleys near the coal deposit collect the smoke, which is heavier than air.

During the rainy season, runoff dampens the fire and reduces the output of smoke, making the way passable. Occasional droughts make the area inaccessible for four to five years at a time.

They travelled by ice boat. For hundreds of years ice boats were the fastest way to travel, however they only work when there is ice with little to no snow. In the past large cargo ice boats have been built and used to ferry cargo. I could see the kids being very interested in stowing away on one for the experience.

I was primarily going to answer as Zxryya did, so I am torn about answering at all with just the leftovers, but here you go:

If you travel by sea, then the state of the sea matters. A treacherous region of the sea because of rocks or reefs can become completely safe when tide is high. This isn't just a daily cycle, as a few others mentioned, and tide is not the only option for raising the local water level.

When the monsoons come, they fill the cave pass. When the summer comes, the melting snow fills the creeks and rivers with icy water. If that melting ice involves large chunks falling and diverting to various channels unpredictably, that's even weirder, since some areas might rise and lower.

You also have storms: There are places where the storms are rather predictable. You might not know the specific dates that a storm will boil up, but during certain seasons, you can know to expect one within a window. If the trip required is longer than that window, people might know better.

As simple as a steady wind is, it can make navigation tricky. Imagine if the wind is harsh but there are dangerous rocks sticking up out of the sea all over the place. If the wind varies only a few MPH and angle, it still makes that treacherous. If it's less predictable than that, all you need is a narrow straight made up of tricky rocks to avoid, and you've got a very good reason not to go that way.

So, there are some more ways that things can become semi-prdictably inaccessible and linked to almost any schedule that you want.

(What I mean by this is that you can mix and match effects to make the trip accessible when you want it, pretty much.

e.g. The passage is a cave up a mountain side. It's a rather risky climb, what with the local wildlife like goats and wildcats, at the end of the dry season. At the beginning, well, it's underwater. You have to wait for the lake to dry a bit after the rains to open enough to get through. For about a week, once or twice a year, the passage is filled with enough water that shallow boats can simply sail in with cargo. This is their trade time.

e.g.2 The opening to the passage is underwater all year round, but the passage itself is mostly above sea level. The ice on that shelf does this strange thing where it peels back for a few days, revealing the passage. Locals rush out and harvest the various sea creatures during that time.

There are a few ways in which you could use this. In one case, you could have a derelict nuclear power plant spewing out radioactive waste. For most of the year, the leakage is confined to the immediate area around the plant, but certain seasonal extremes could make for difficult traversal alone, not to mention a storm of radiation at your face. These weather disruptions could occur for weeks or months at a time.

You could also keep the planet as it is and make the star highly radioactive with relatively predictable solar flares which scorch every living thing and piece of electronic equipment on the surface, requiring residents or travelers to bunker down during "radiation season." The frequency for these solar flares could be a quick succession lasting less than a day but anything exposed during that timeframe has no chance of survival.

The village is mostly surrounded by very steep mountains. One side of the village is open to outside travel. The ground beneath that opening is filled with natural gas that leaks out and makes the villagers sick and short of breath. They decided to burn the gas to prevent these issues. A giant fire rages almost year round, blocking entry/exit. A very small creek runs from the mountains into the village. They damn up the water and use it for their water needs. During the winter they get excess water and after a year of collection the damn is now full. This is the one time when they let the water flood out and put out the fire so that people can come and go. Once everyone has gone through they start the fire again as villagers start getting sick from the gas. It takes a year of collecting water in the damn before they have enough to put the fire out again. For a reality check on this idea see Burning fire pit

Water levels, but a very different answer than above:

The area is ringed with mountains sufficient to block normal travel. The only reason there is anyone living there is that there is a river that flows through the area, on both ends it flows through caves.

In periods of high flow you run out of ceiling. In periods of low flow your raft runs aground. Note that the safe flow rate for the two caves is different, this gives one window in which you can enter and one in which you can leave and they do not overlap.

Note that properly packaged cargo can be shipped during periods of high flow--pack it to take a beating and drop your container in the river, it's fished out by nets on the other side. The water flow over the ages has worn away any spots that would cause them to hang up.

You could just take a magic approach if a fantasy setting. The town only appears at x location for x amount of time before disappearing to a new location or back to its own dimension. There is a movie called "Krull" where the castle/fortress they needed to reach was only going to be at a specific location for a short time.

Or town is stationary, but protected by a magic veil/curtain/wall that is only passable under specific conditions. (a la "Mists of Avalon")

Or the region could be on a floating island that is only in range for a certain amount of time before it floats out of range again. Maybe it circumnavigates the world/floats with the ocean currents.

Two things come to mind for me that I didn't see mentioned:

• First, fire, a massive forest fire would certainly cut off a return path through a wooded area, and without modern fire fighting equipment they can burn for several months.
• The second is completely out of the loop from natural reasons to be cut off, but what about some sort of conflict: a war, a military action, an invasion. This wouldn't have to have anything to do with your current story, but it could make a nice sub story while providing the tool to muck up a return trip.
• Welcome to Worldbuilding! Please expand upon your answer and add more details. – fi12 Oct 19 '16 at 20:12