And the hardy Men of Norway Could cross the marsh On a causeway of corpses.

From King Harald's Saga by Snorri Sturluson. 

85. The Battle of Fulford

WHEN King Harald saw that the English flank was advancing down the dyke and was now opposite them, he sounded the attack and urged his men forward, with his banner, ‘Land-Waster’, carried in front. The Norwegian onslaught was so fierce that everything gave way before it, and a great number of the English were killed. The English army quickly broke into flight, some fleeing up the river, and others down the river; but most of them fled into the swamp, where the dead piled up so thickly that the Norwegians could cross the swamp dry-shod.

Earl Morcar lost his life there.  In the words of the poet Stein Herdisarson:

Many were lost in the water;

The drowned sank to the bottom.

Warriors lay thickly fallen

Around the young Earl Morcar.

Harald’s son, young Olaf,

Pursued the fleeing English

Running before King Harold.

Praise the brave prince Olaf.

This is from the poem which Stein Herdisarson composed in honour of King Harald’s son, Olaf, and it makes it clear that Olaf took part in this battle with his father. This is also mentioned in the Harald’s Poem:

Waltheof’s warriors

All lay fallen

In the swampy water,

Gashed by weapons;

And the hardy

Men of Norway

Could cross the marsh

On a causeway of corpses.

Earl Waltheof fled with the survivors towards the town of York, and there was great carnage there.

The battle was fought on the Wednesday before St Matthew’s Day.