How do you tell the age of different soil layers without a reference mark?

I have several places in which the soil strata is visible. I am curious to know how to determine the age of the different strata as there are visible differences in the soil composition (texture, color, etc.). There are no fossils to reference nor outcroppings of rock. Some places have layers of rock underneath that I can use for a reference but to go above those layers, it's hard for me to determine the age. I can walk in a gully/ditch and see the strata but I don't know how old the soil layers are. I once heard that it takes around a 100 years for an inch of topsoil to form.

If anyone has any good references (books or journals) to refer to for additional information, I'd appreciate it greatly, especially if its something I can get from Amazon. Thanks!