 Along the Acasta River the CSBC includes the Acasta Gneisses with a protolith age of about 4.03 Ga, one of the oldest dated rock units on Earth.These gneisses are polymetamorphic and have a tonalitic and gabbroic composition.The rest of the CSBC is younger with a central core <3.5 Ga and the remaining craton with detrial and protolith ages ranging from 3.4 to 2.8 Ga.