Between 900 and 1100, the Thule Tradition spread westward. The efficiency of housing was improved as they spread to the west and hunting methods were improved due to the use of dog sleds, umiaks, and kayaks. This enabled the hunters to travel further to hunt and follow the migration of the large game and sea mammals. After 1000, the practice of using polished slate for tool making continued to spread to the Aleutian Islands. The methods of pottery making also spread and replaced the Norton tradition in Southern Alaska. There were differences between the areas to which the tradition migrated. Houses in the more eastern region were more above ground and round with stone platforms to sleep on. The shape and support for the buildings came from whale bones. Eastern populations preferred soapstone domestic items instead of pottery and developed the use of dogs to pull sleds.