From Regia Anglorum:
By c. 900 the Vikings had ravaged northern France to such an extent that there was little plunder to be found along the rivers which had formed their major avenue of attack. Ironically it was a Danish Army (under a leader called Hrolf or Rolf in some chronicles), which arrived in 911 to pillage the lower Seine Valley that created the Vikings’ only lasting impact on western Europe.
Hrolf attempted to besiege Chatres without success, but his army was such a threat to the Seine valley, that Charles, King of the Franks, negotiated a treaty at St. Clair-sur-Epte. Under this treaty all the land bounded by the rivers Brestle, Epte, Avre and Dives was granted to the Danes; effectively the land they already controlled. By 924 the Franks were forced to grant the Danes the districts of Bayeux, Exmes and Sees, and in 933 the Cotenin and Avranchin.