Complex sentence: Sometime before the last magnetic reversal of the Earth's magnetic field 700,000 years ago, the Minto Lavas created a large volcanic plateau formed from coalescing shield volcanoes that were heavily eroded by glaciers before Jefferson started to grow.
Split into simpler sentences: Sometime before the last reversal of the Earth's magnetic field 700,000 years ago, the Minto Lavas created a large volcanic plateau formed from coalescing shield volcanoes. They were heavily eroded by glaciers before Jefferson started to grow.