Are silver coins usually deeper than clad?
If all other things are equal, then yes. An introduction to archaeology class (or maybe some YouTube videos) would help out here. In general, various things deposit on top of the ground and form new soil over time. So, older things are usually buried deeper. This deposition usually occurs as a yearly cycle because of wet and dry seasons. Stuff gets deposited in some times of the year and weathered away at others. In most places, there is more deposition than erosion, so the ground slowly builds up. In some places, there is more erosion, so the ground wears away and naturally exposes stuff that has been long buried.
To further complicate matters, if there were a ditch, something might deposit in the ditch and, because of its hollowness, debris will deposit faster in the ditch than around it, filling it up. If that happens, relatively new items can get buried at an accelerated rate. There's also the possibility that someone has dug holes in any given place in the past, completely jumbling the stratigraphic nature (order) of the deposits.
To complicate things even more, there is also liquifaction. This is the one that normally throws people for a loop. It occurs when the ground flows like a fluid and happens very slowly in most places. It can be caused by heavy water drainage, the slow effect of frost heave, or anything else that can make the ground flow. Whenever the ground flows, all objects in the ground will start to become bouyant. Objects that are heavier than the flowing soil will sink deeper. Objects that are lighter will float to the top. Have you ever heard of people picking stones out of a farm field every single year because they "just keep coming back?" The lighter weight rocks literally float to the surface in the winter time because of the frost heave.
This usually takes place very slowly. If frost heave is the driving force, then the pace of the objects will depend on how many times the ground has gone from warm to cold and back. Sometimes the soil on top will freeze every night and thaw every day for a few weeks. Every freeze and thaw nudges all objects in the ground slightly. This being said, we still haven't considered the complicating effects of roots or of ground strata that vary in density.
To make a long story short, every little piece of ground has its own history. With experience, one can learn to read this history just by looking and to read much more by digging a few holes. The ground underneath trees does not act like the ground away from trees. Roots tend to compress things and hold them in place, while the organic matter from the trees above ground will increase the local rate of deposition. Pay attention primarily to vegetation and to paths of water drainage. Where a certain kind of plant grows can tell you about the soil and the water is what shapes the soil in the first place.
