The islands are famous for their remains of Neolithic, Bronze Age, and Iron Age settlements. Of these, the best-known are Skara Brae, a Neolithic settlement first excavated by V. Gordon Childe in the 1920s; Maeshowe, a Bronze Age chambered cairn; and the Ring of Brogar, a henge-type monument. There are many brocks (round stone towers) from the Pictish Iron Age. Viking raiders first appeared in the eighth century; Viking settlers followed during the next few centuries. The emerging Viking-Norse culture, which became Christianized in the early twelfth century, continued until 1468-1469, when the Orkneys and Shetlands were transferred to Scotland as a pledge in lieu of a dowry for the daughter of King Christian I of Denmark and Norway when she married King James III of Scotland. This pledge was never redeemed and, in 1472, Scotland formally annexed the islands. Scottish settlers began arriving after 1230 and their increasing Presence influenced the local culture and language. During the late 1500s and early 1600s, the people suffered greatly under the tyrannical rule of the Scottish earls and the feudal system they imposed. During the nineteenth century, these restrictions were eased and agricultural improvements were Instituted, resulting in a greatly improved quality of life. In the eighteenth century, 75 percent of the employees of the Hudson's Bay Company in Canada were recruited in the Orkneys. Commercial agriculture in the early twentieth Century and the discovery of the North Sea oil in the 1970s brought a new type of economic prosperity to these islands.