Complex sentence: Then, during the Laramide Orogeny, the entire plateau uplifted until about six million years ago when erosion began to remove sediment from the plateau, which removed a load from the crust causing isostatic uplift and a second passive rise for the plateau.
Split into simpler sentences: Then, during the Laramide Orogeny, the entire plateau was uplifted until about six million years ago. This load removal resulted in isostatic uplift and a second passive rise for the plateau.