How devastating really was William the Conqueror's Harrying of the North in 1069-70? I've heard it described as anywhere from a wholesale genocide and slaughter to something more mild. What are the sources and evidence?

There are several primary, and roughly contemporary, sources for the rebellion. William of Malmesbury, the Anglo-Saxon Chronicle, and Oderic Vitalis all describe it in some detail.

William and Oderic were writing approximately a generation later from the events themselves, but they were quite clear in their condemnation of his actions at this time. Vitalis had this to say about the events themselves:

The King stopped at nothing to hunt his enemies. He cut down many people and destroyed homes and land. Nowhere else had he shown such cruelty. This made a real change. To his shame, William made no effort to control his fury, punishing the innocent with the guilty. He ordered that crops and herds, tools and food be burned to ashes. More than 100,000 people perished of starvation. I have often praised William in this book, but I can say nothing good about this brutal slaughter. God will punish him

Now it is impossible to fully quantify this and medieval estimates at casualties are nothing if not exaggerated, but the dire picture that he paints is matched by William of Malmesbury's description:

He then ordered both the towns and fields of the whole district to be laid waste; the fruits and grain to be destroyed by fire or by water, more especially on the coast, as well on account of his recent displeasure, as because a rumour had gone abroad, that Canute, king of Denmark, the son of Sweyn, was approaching with his forces......Thus the resources of a province, once flourishing, and the nurse of tyrants, were cut of by fire, slaughter, and devastation; the ground, for more than sixty miles, totally uncultivated and unproductive, remains bare to the present day. Should any stranger now see it, he laments over the once-magnificent cities ; the towers threatening heaven itself with their loftiness ; the fields abundant in pasturage, and watered with rivers : and, if any ancient inhabitant remains, he knows it no longer.

So the picture that the contemporary sources is quite bleak indeed. The question then is can we rely on these accounts or are they exaggerating the scale of the destruction? The other question is how much of this devastation is due to raiding by the Danes and Scots who were also active militarily in the region at the same time?

The unfortunate truth is that it is impossible to answer these questions satisfactorily. Many historians have put the claims made by the primary sources under scrutiny, questioning the amount of soldiers that William could spare for such an operation, the amount of time they were able to be deployed in the field, and conflicting accounts in the Doomsday Book that do not shed any light on the condition of the area.

However given the strong terms in which William was denounced for his actions in the subsequent years, it is undeniable that the events left a black mark on his reign and were widely remembered and condemned as excessive.