I don't think abstraction reduces player agency. Uncertainty certainly does.
A player has at least as much agency if the whole affair is hand-waved as he would have with in-depth resolution. "I want to establish a keep here" is as effective if the answer is "OK. Six months later, the keep is complete, but unfurnished." or "OK. Step 1 is pick your hex... OK not that the keep is fully established, step 971 is to purchase furnishings".
What destroys player agency is uncertainty / shifting ground. Shifting ground can be used as a tactic for illusionism, after all.
The gorge can be problematic to a variety of play styles. (For those just tuning in, "the gorge" is shorthand for a check failure in a skill challenge resulting in the PC looking out over a gorge that previously did not exist in the area)
Player agency is harmed because the player cannot predict/mitigate the dangers of running into such a device. At that point the terrain surrounding the PCs becomes set dressing and effectively nothing the character can rely on. Uncertainty rises.
The same problem results from grid-less combat. The player's positional uncertainty or another person shifting the ground undercuts agency if the game engine insists on that level of detail. But if the game engine is focused at a more abstract positioning (say Strands of Fate where action areas are carved into zones but actual positioning doesn't matter) agency isn't undercut.
I outlined that particular Skill Challenge in extreme detail about 1.25 years ago
here and in the posts surrounding it. Rather than run that down again, I think I'm going to work my way through this using Dungeon World's Undertake a Perilous Journey (which maps precisely to the situation in question with the 4e Skill Challenge mechanics), the GM principles/best practices, and the system components for action declaration and resource deployment that delivers agency to the players.
Undertake a Perilous Journey in Dungeon World is a move that exists (1) when the players are moving through hostile territory and (2) when the PCs know where they are going. So whenever dangers/threats can/should become manifest and when the PCs have either traversed this trek before or they have clear means to divine their way from point A to point B.
When this happens, 3 prescriptive roles (trailblazer, quartermaster, and scout) each role a singular check to determine success for the entirety of the journey. On a 10 + the job is performed very well and a relevant boon is earned (eg the scout would spot trouble quick enough to let you get the drop on it. On a 7-9, the job takes the normal amount of time and neither side gets the drop on each other if the prospect of an encounter comes up (because of the following). On a 6 or less, a complication is "earned" whereby the GM should be framing the PCs into adversity with respect to the job that "earned" the failure (and the PC marks xp).
So what is the agency that the PCs have here? They get to pick who does what jobs, deploy any resources that will successfully (if that is what they wish) facilitate the conflict resolution of that specific job (such as divination fiat moves, perilous journey fiat moves, hirelings, moves that mitigate ration consumption or moves that take general or specific + 1 forward), and then they get to roll their 2d6 + n.
GMing principles sets out that you should have a low resolution maps whereby blanks are fleshed out in the course of play. So let us say that the PCs have something specific that provides them the means to get from point A to point B. Perhaps they have quickly journeyed through the vast, broken landscape one time before or perhaps they have a map or some kind of divining rod-like item. If they're just setting out to explore the wild, without explicit means to get from A to B (perhaps to try to find a specific place after you have set out), then you're not going to use Undertake a Perilous Journey. You're just going to let the fiction guide you and let moves follow moves until things unfurl themselves.
So let us say that the PCs need to go through these dizzying badlands to get to this specific place. Relative to the terrain, they don't have enough info or experience to make it a point A to point B scenario. They know "what they're looking for" (let us say a temple to Zehir near an oasis) but they possess little to no familiarity with the region/geography. So we're going to play it out. It isn't a situation where you would deploy Undertake a Perilous Journey move. On the way through the hostile terrain, they commit to the deployment of resources which expedite travel pace and stealthiness. They have 0 prospects for the Supply and Recover moves out here. The best they can do is the Camp and Take Watch. That means consumption of supplies and prospects for dangerous, resource depleting conflicts (hard moves by the GM on a 6 or less). In 4e, this would be not wanting to risk a mass loss of aggregate daily resource loss (Powers and Healing Surges) with no prospects for an Extended Rest until the mission is accomplished. So they deploy moves and spend resources which ablate their resources but they allow them to reduce total exposure.
The terrain is described to them in the low resolution detail required to facilitate fiction first play and to immediately resolve action declaration. They aren't going to have a perfect map of the territory on the way back. However, significant success in action declaration related to terrain navigation may earn them currency (such as Hold or a + 2 on a relevant Nature or Perception check on the way back in a 4e SC) to spend on the way back (on a related check). That is another aspect of agency.
On the way back we're going to Undertake a Perilous Journey. It is hostile territory, but the PCs have enough grip (even if just having traversed it once) on the region to attempt to navigate a point A to point B scenario. On the way back, the trailblazer triggers a major complication on a 6 or less roll (and they mark xp). As a GM, its my job to make a hard move against them that properly reflects the implications of a failure in this action declaration. A trailblazer guiding the group might earn them a complication by mistakenly wandering into a major threat (such as a hazard or a monstrous lair) or by improperly reading the trail signs and making a navigation error. The former might manifest as a sudden collapse sinkhole, lightning sand, or the territoy or an alpha predator; perhaps Ankheg or Bulette. Perhaps if a chase scenario has evolved, the latter may manifest as one of the many (or few) nigh-impassible gorges that scar the land.
Now let us rewind a bit. Let us say that along the way there, the PCs actually made an action declaration that involved mapping the area and specifically trying to avoid the gorge(s) that scar(s) the area so the scenario above doesn't happen. Great. Declare the action and lets resolve it mechanically. The fiction/subsequent moves by me would follow from the results of their move/action declaration. In Dungeon World (as in 4e), if they are successful in their move, then I'm not going to complicate their lives with "gorge adversity" in the future on a failed navigation check. However, if they roll a 6 or less, its my job to complicate their lives with related adversity. I don't have to do it immediately, but I need to do it. Perhaps I would put that in my back pocket for later deployment when the moment is right. The moment is definitely right in the case of a desperate chase when they're trying to get out of the treacherous badlands and the last thing they want is to be cornered due to their earned navigation blunder.
Anyway, that is the way agency takes shape in Dungeon World and 4e exploration (via Undertake a Perilous Journey and Skill Challenges).
