From A Wiki of Ice and Fire
and the location of the Broken Arm
The Broken Arm, also known as the Broken Arm of Dorne, is a region in eastern Dorne where the Dornish peninsula juts into the narrow sea. East of the jagged coastline are the islands of the Stepstones.
According to legend, a land bridge called the Arm of Dorne once connected Westeros and Essos. The Broken Arm and the Stepstones were created when the Arm of Dorne was shattered by the magic of the children of the forest.
References and Notes