Complex sentence: The chalk itself varies considerably in thickness, with a marine trangression progressing across into Dorset from the east, and evidence that the top of the Upper Greensand to the west is the same age as the oldest chalk in the Isle of Wight.
Split into simpler sentences: The chalk itself varies considerably in thickness, with the results of a marine trangression progressing across into Dorset from the east. There is evidence that the top of the Upper Greensand to the west is the same age as the oldest chalk in the Isle of Wight.