In his reconstruction of the supercontinent cycle, Rogers proposed that the continent Ur formed at about 3 Ga and formed East Gondwana in the Middle Proterozoic by accretion to East Antarctica; Arctica formed around 2.5–2 Ga by the amalgamation of the Canadian and Siberian shields plus Greenland; and Atlantica formed around 2 Ga by the amalgamation of the West African Craton and eastern South America. Arctica then grew around 1.5 Ga by accretion of East Antarctica and Baltica to form the supercontinent Nena. Around 1 Ga Nena, Ur, and Atlantica collided to form the supercontinent Rodinia.