Some of the specific types of food they ate include; beans, sourdough biscuits and meat. These foods were mostly carried on chuck wagons. Most of the food that was eaten along the drive became better over the time with the invention of new cooking apparatus, such as Dutch ovens.
Don't forget cowboy coffee. Coffee beans were put in hot water over a fire and then were (somewhat) strained separating the beans from the drink we know as coffee. The longer the water and beans were left over the fire, the stronger the coffee