At https://www.bbc.co.uk/news/science-environment-31356229 … we learn that a huge canyon on Iceland was created by a few days of catastrophic flooding. The Jokulsargljufur canyon is 28 km in length and up to 100m in depth – cutting directly into basalt (former outpourings of lava). It is home to a powerful waterfall – and this has cut back through the basalt (sometimes at an astonishly quick rate). How do they know this. Researchers investigating the canyon analysed the chemistry of the rocks in the canyon walls and discovered most of the change occurred during three very brief events – which they date two, five and nine thousand years ago. This is not your usual dating methodology, such as calibrated C14, so we might expect some adjustment to them, depending on what caused the events in the first instance (presumably the blowing of an ice volcano). Volcanoes beneath glaciers are a speciality of Iceland.
What they also found was that in between these three events there was relative stability – and erosion was minimal. Their conclusion is that brief but catastrophic events can shape landscapes. It gives a lie to the idea the landscape necessarily is formed gradually and slowly over time. In some instances it clearly was not formed slowly.
The three events involved water released from glaciers. Iceland sits on the tectonically active Mid Atlantic Ridge. The basalt walls of the canyon were eaten away so rapidly waterfalls were formed upstream by as much as 2km away (retreating at a rate of 100s of metres a day).
See also the National Geographic article at https://news.nationalgeographic.com/2017/03/channelled-scablands/