I'm looking for a story about an ark on land where the people who live in the ark are pulling the ark along. If they travel forward of the ark, time speeds up, if they travel in a reverse direction to the ark, time slows down.
This is Inverted World by Christopher Priest.
As chance would have it, we're discussing this book at our next SciFi reading group meeting. I first read the book as a 14 year old and I really loved it. I heartily recommend it to 14 year olds of all ages!