I think we need to remember that it's not just the stuff in the pit
which fell into disuse, but the pit itself.  Farmers had to redig ice
pits, tobacco ordering pits, privies, etc  and the old pits had to be
filled.  The old pits may have collected a lot of garbage over a short
period of time simply because farmers were anxious to have a potential
hazard filled up.  So, old pits became targets for garbage and soil
from the new hole, and the appearance of a subassemblage may be an
artifact of this fact.  Many of the ice pits I have seen are filled
with early+ 20 c garbage because there was no new pit dug, so there
was no soil from a new hole available to partially or completely fill
the old hole.  So it's just a hole with mason jars, car parts, bricks,
shoes, you name it.  Why no new hole?  Ice boxes and commercial ice
became more reliable and more widely available after the first decades
of the 1900s, plus after about 1912 the climate in the Mid Atlantic to
upper south got too warm for ice to be gathered in any significant
quantity from farm ponds.  Rovner's comment also notes the need to
consider how stuff gets into and moves around in an open pit,
especially if it is later sealed.  A lot of small stuff and organics
could work way down in a pit left open for any length of time.  In
short, my point is that  we have to treat the pit as an artifact with
a context  before we can understand the artifacts that went into it.