Key takeaways
- Miocene limestone: firm enough to stand, weak enough to work.
- Rainwater dissolves it from above, waves break it from the side.
- And arches become stacks when the span finally gives way.
- Not on a human timescale, but not finished either.
The rock itself
Everything on this coast follows from what the cliffs are made of. They are limestone, laid down in the Miocene, which puts them somewhere in the region of ten to twenty million years old.
Limestone is calcium carbonate, and its defining property for our purposes is that it dissolves in weakly acidic water. Rain picks up carbon dioxide as it falls and again as it passes through soil, and the result is acidic enough to eat rock given time.
It is also mechanically middling: firm enough to hold a vertical face sixty metres high, soft enough that waves can quarry it. That combination is exactly what produces dramatic coastal scenery, and it is why the Algarve looks like this and a granite coast does not.
The colour comes from iron in the rock and from the bedding. The bands you see in a cliff face or a cave wall are the layers it was deposited in, and where the sea has cut a clean section you are looking at a few million years of seafloor stacked up.
It is also why the water is that colour. Limestone dissolves rather than crumbling into mud, so the sea here carries very little suspended sediment, and clear water over pale sand is what produces the turquoise everybody photographs.
How a sea cave starts
Rock is not uniform. It has joints, fractures and bedding planes, and those are lines of weakness running through it.
Waves arriving at a cliff do two things. They hit it, with real force, compressing air into every crack and blowing it apart as the water retreats. And they carry sand and gravel, which acts as an abrasive against the base of the cliff.
Both are concentrated where the rock is weakest, so erosion does not proceed evenly across a cliff face. It follows the joints, cutting inward along them and leaving the sound rock either side standing.
What starts as a notch becomes a slot, becomes a chamber. Add the dissolution working from above through the same joints, and you get a cave that is being enlarged from two directions at once.
A sea arch cut through a headland
From cave to arch to stack
The sequence is one of the most legible things in geology and this coast displays every stage of it within a few kilometres.
A cave cut into a headland from one side keeps going. If the headland is narrow, or if a matching cave is being cut from the other side, the two eventually meet and the sea passes right through. That is an arch, and there are several on this run that a small boat can pass under.
An arch is unstable by definition: it is a span of rock with nothing under its middle. Weathering works on it from above and the sea from below, and eventually the span fails and drops into the water.
What is left is the far side of the arch, standing alone offshore. That is a stack, and once you know the sequence you can look at any tower on this coast and see the headland it used to be part of.
Timescales for the last stage are shorter than people assume. An arch that has stood for centuries can fail in a single winter, and several well-known Algarve formations have collapsed within living memory. The coast in the photographs is not permanent.
How a roof opens
The skylight in the Benagil cave is not part of that sequence, and it is worth separating.
The chamber was hollowed by the sea from the side. The opening came from above, where rainwater working down through joints in the limestone dissolved a shaft, and the thinning roof between shaft and chamber eventually collapsed.
Geologists call that kind of feature a collapse doline: a hole in the ground formed by the failure of a roof over a void beneath. They occur all over limestone country, usually inland and usually without a beach at the bottom.
What makes the Algar de Benagil unusual is the combination. A collapse doline that happens to have opened over a sea cave, so that a hole in the ground and a hole in a cliff turn out to be the same hole, with daylight coming in one way and the ocean the other.
Miocene
The limestone here was laid down roughly ten to twenty million years ago. Everything the sea has done to this coast since has been working on that one rock.
Is it still going on
Yes, on a timescale that will not be visible to you on a boat trip.
Every winter storm removes some rock. Every rainfall dissolves a little more. The debris fans at the base of the cliffs and the fresh unweathered rock surfaces in places are the visible evidence that this coast is not a museum piece.
Occasionally it is dramatic. Cliff collapses happen on this coast, which is why the clifftop paths have fences set well back from edges that look solid and are not, and why signs warn against sitting under overhangs on the beaches.
The practical version for a visitor is simple. The coast you are looking at is a snapshot of a process, every arch is a cave that got through and every stack is an arch that did not, and the one you photograph today is somewhere in the middle of the same sequence.
