Is there any reason we can't have a casual, early-afternoon ceremony and a formal reception with dinner and dancing later?
You can definitely do this. The only issue: Your guests might be confused about what to wear. Some will probably even want to change outfits for your formal reception. Consider the time lapse between the two events, and where guests can go to freshen up after the nuptials. If everyone's staying at a hotel where all parts of the wedding will be taking place, this isn't an issue -- guests can easily run up to their rooms, change and hang out to kill some time. But to your relatives who drive an hour-and-a-half to attend the ceremony and then need to find a place to change before going to dinner, it's inconsiderate. Arrange for your guests to have a place to reboot, and you're golden.