We spent the rest of the day in Þjórsárdalur, the valley that runs up toward Hekla. It is one of the old settled valleys of the south, farmed since the saga age, and it holds a run of waterfalls fed by the river Fossá, a tributary of the Þjórsá. We had already been at the top of the country all week, in the highlands, and this was gentler ground, green and lower, the land beginning to soften on the way back down toward the coast. We had three falls in mind, one after another, and a late lunch at the end.

Our first stop was Hjálparfoss, a short walk off the road. It is not tall, but it has an unusual shape. The river Fossá splits in two here and comes down as a pair of falls side by side, over a curved lip of dark basalt, into one round pool at the bottom. The rock around the pool is columnar basalt, the six-sided pillars that form when thick lava cools slowly, and here they lean and twist rather than standing straight. The two streams, the curved rock, and the shared pool make it feel more arranged than most falls, almost designed. It was a short stop.

Then we went off the pavement again and drove up the valley to Háifoss and Granni. These two come down side by side into a deep gorge that the Fossá has cut into the rock. Háifoss is the big one. It drops about 122 metres in a single fall, which makes it one of the tallest waterfalls in Iceland. Granni is its neighbour, a narrower fall a little to the side, quieter and thinner, but coming off the same cliff into the same canyon. Standing at the edge, you look down a long way to where both of them hit the bottom.

The valley they sit in has its own history, and it is worth knowing. Þjórsárdalur was farmed in the saga age, a thousand years ago, with steadings up and down it. Then in 1104 Hekla erupted and buried the whole valley under pumice, and the farms were abandoned. What is now empty upland used to be lived-in country. The same eruption is the one that buried the farm at Stöng, further down. So these two falls, and the green gorge around them, sit on top of a valley that people left in a hurry nine hundred years ago and never fully came back to.

We ended the day with a late lunch at Farmers Bistro, in the village of Flúðir.

The bistro is built on a working farm, and the food comes off it. The farm is Flúðasveppir, the only mushroom farm in Iceland. They grow three kinds of mushroom, white, chestnut, and portobello, and they grow a lot of them. About eleven tons a week come out of six growing houses, which is enough to supply the whole country. If you have eaten a mushroom in Iceland, it most likely came from here.

They grow more than mushrooms. In greenhouses nearby they raise bell peppers and tomatoes all year, under electric light, across about 5,000 square metres of glass. Out in the fields they grow cauliflower, broccoli, cabbage, red cabbage, kale, and carrots, along with fresh herbs. Growing this range of food on a farm this far north takes heat and light that the climate does not give for free, and both come, one way or another, from the ground. This is the same story we have been running into all over Iceland: geothermal heat is what makes the food possible.

The menu is built around what the farm produces, so the mushrooms are the centre of it. It was a good place to end a long day of driving and waterfalls, sitting down to eat food that had been grown a few steps from the table.