Complex sentence: The La Loche Formation marked the beginning of the deposition of the Elk Point Group at the onset of a marine transgression over the Canadian Shield, which had been exposed to a long period of erosion.
Split into simpler sentences: The La Loche Formation marked the beginning of the deposition of the Elk Point Group. It was deposited during the onset of a marine transgression over the Canadian Shield, which had been exposed to a long period of erosion.