They both looked down at the clots swirling in the purpled water. 'What carnage is this?' Carnelian said and ground his teeth.

'A poetic name for such filthy commerce.'

'Commerce…?' The word was unexpected. Carnelian gazed across the stream.

'And a fitting occupation for the Wise.'

Carnelian watched the creatures struggling on the other bank. 'Are those sartlar?'

'Of course. Repulsive, neh?' He pointed an arm upstream. 'No doubt we shall have to follow their stinking march through that.'

Carnelian followed his finger. The valley was still in shadow but he could see that it seemed to be set with dark jewels. The stream came out of it through something that looked like a wall stretched across its mouth.

Aurum sent the Marula into the stream with a motion of his hand. As they crumbled the banks into the water their aquar stumbled, jolting their riders. Then they were wading up against the flow with the water foaming red against their legs. The baggage animals were next. Seeing it was safe, the Masters urged their aquar after them.

Carnelian followed Jaspar. He watched him sway as his aquar slid down the bank. Then it was his turn. He gripped the chair rim but was still thrown about and almost pitched forward into the eddying stinking water. Soon he was making his way up the channel with the others.

They reached the sartlar and pushed into their march. He could see that it was spiral ammonites that filled their baskets. The aquar jostled the sartlar out of the way. Carnelian saw shells knocked out of the baskets being trodden into the water by the sartlar's spade feet. He wanted to see their faces but not a single one turned to watch them pass.

The wall was close now, so close Carnelian could see it was a scree. Little avalanches clinked down and he realized the whole slope was made up from fragments of ammonite shell.

He felt a change in his aquar's footfalls. Each impact was sharper. Peering down into the water he saw the channel was now lined with stone. Two buttresses rose up to hold back the hills of shells. Between them was a gap out from which the stream flowed. Soon masonry had risen on either side. The other Masters were in the noisy space with him, being carried along by the sartlar tide whose burdens formed a mosaic that hid them from view. Carnelian saw a leathery hand fumble up to steady a basket. One of the sartlar became entangled in the legs of Jaspar's aquar and was knocked down. Its shellfish load pattered into the water like stones. Other sartlar bleated and tried to get out of the way. The aquar's plumes flared. It trumpeted. Its claws churned the sartlar's body into the stream as it tried to keep its footing. There was a gurgling shriek, some crunching sounds that might have been crushing shells. Jaspar pulled his aquar back into control, then moved on.

Carnelian's aquar stepped around the floating mess. He glimpsed a face into which a spiral had been branded so deeply, the nose was in several chunks. He could not look away, even though he had to lean out dangerously to keep the corpse in sight. The baskets meshed and it was gone. He looked ahead, troubled, to find that they had come through the gap into a valley that was carved and gouged into a complex honeycomb awrithe with sartlar.

Jaspar affected a cough. 'It is unusual to find oneself in a drain.'

The channel swelled into a great sink whose walls were streaked abattoir-brown and pierced by many dribbling mouths. Gutter lips wedged into the top of the wall were disgorging thick dark jets. Meaty chunks made them spit and splutter. Carnelian watched a hunched sartlar using a paddle as if it were a broom to brush a sodden lump over the edge so that it splashed into the pool below. That pool poured its bloody mixture past their aquars' feet. Carnelian felt as if he were choking.

Sartlar were hauling their baskets up a stairway that rose out of the channel. The Marula were sent to clear a way. With the other Masters, Carnelian urged his aquar up the steps.

As his aquar neared the top, Carnelian emerged into a red world, an amphitheatre with tiers of cisterns, walkways, channels. The air hazed with flies. Everywhere there were piles of ammonite shells. As each sartlar emptied its basket on a pile it caused a rattling avalanche. Sartlar sat surrounded by baskets like women at a market. They cracked the ammonites open on rocks, used their claws to scratch out the creatures inside and then discarded the empty shells. These were gathered by other sartlar who shuffled off towards the rubbish wall. There, amidst a frenzy of gull wings and screeches, the shells were thrown away.

'Filthy industry,' Jaspar cried above the clatter.

The Marula were trying to wave away the flies. As these spat against him, Carnelian became thankful for his mask and cloak.

A brown man was battering his way towards them through the sartlar using the handle of a bladed whip. He made his hands into a tube and shouted through it, 'You've come the wrong way.'

The lances of the Marula pricked him out of their path. They started moving through scattering sartlar, spilling baskets. Other overseers confronted them, shaking their whips, slapping the lance heads away from their faces. 'Wrong way,' they cried. 'Wrong way.'

'We should return,' shrilled Vennel in Vulgate. 'Clearly there's no road through here.'

Suth said something to one of the Marula. The man advanced and lifted his lance to point up the valley. ‘Show the way to upper land.'

The overseers shook their heads and showed grimaces of brown teeth. Any sartlar that bumped into them they cracked upon the head.

Aurum pushed his way through the Marula. 'Show me the route.' His voice carried even over the noise.

They waved their whips and scowled at him. A whip handle clacked against Aurum's saddle-chair, causing his aquar's eye-plumes to flutter. The Master lunged forward past its neck. The men drew back, loosening their lashes ready to strike, but the beginnings of fear distorted their faces. Aurum slipped back his cowl to reveal his gold face. They hid behind their arms as if that face were brighter than the sun. They cringed back, dropping their whips, tripping over baskets and sartlar. Aurum straightened, hid his face within the shadow of his cowl. The overseers cowered till he commanded them to approach. Then they fawned on him and strained to hear his every word.

Cringing guides led them up the valley. They wound through a maze of vats and channels. Carnelian watched sartlar stir a rot of shelled ammonites. Each stir gave off a stench and oozed out more yellow liquid that darkened as it flowed. Sluices let water in from above or allowed the liquid to filter down from one tank to the next. With each descent the colour deepened. In one tank he saw it had become almost black but that its edges were the colour of blood. He understood and looked around with wonder. The whole valley was a dyeworks for extracting fabled, precious purple.

A whole run of tanks was flushed clean by a flood released from higher up. Red to their waists, sartlar leaned against the current as they scraped at the rotting matter that was sticking to the sides. Carnelian knew this sewage would be channelled down to the sink from where it would stream red off to the sea.

'Do my Lords think it reasonable that I should now be given explanations?' Vennel said over the scrabbling of aquar claws.

Carnelian was exhausted by the effort of keeping in his seat. During the long, hard climb his saddle-chair had been giving him a constant bruising batter. He would be glad to stop even for a short time.

Aurum looked up at the sun. 'We must be far from here before we camp tonight.'

'Camp…?' said Vennel.

'Aurum, we have almost reached the land above,' said Suth. Carnelian saw his father turning round to look for him. 'And it might profit us to rest our beasts before we go much further.'