Complex sentence: The surface of the trail is paved with asphalt for the first 13 miles westward from Waukesha, then becoming crushed limestone for the remaining 39 miles.
Split into simpler sentences: The surface of the trail is paved in Asphalt for the first 13 miles when traveling from Waukesha Westward. Then the trail switches to crushed limestone for the remaining 39 miles.