"James Waldby" wrote in message
...
On Sun, 07 Sep 2014 11:14:20 -0700, etpm wrote:
On Sun, 07 Sep 2014 12:24:35 -0400, Ed Huntress wrote:
... It's more like the earth
pushing them up from below to fill any gaps you leave by removing
them.
...
If large items and small items are put in a container and vibrated
the
large items will rise to the top. I wonder if something similar is
why
the large rocks keep appearing. Freeze/thaw cycles, wet and dry
cycles, that kind of thing. Whidbey Island is a glacial till
formation
that is 3000 feet thick and so the geology mixed. My garden, which
I
have been working for about 12 years, is sandy loam with lots of
rocks. However, where the garden is the loam is only about 10 feet
thick. The next layer is a 40 foot thick layer of hard pan. I
wonder
how many years I'll need to till to get all the rocks out of the
top
layer of loam.

The "something similar" / "Freeze/thaw cycles" / etc is pretty close
to
the usual explanation. See for example
http://en.wikipedia.org/wiki/Granular_convection#Geology which
says
"In geology, the effect is common in formerly glaciated areas such
as
New England -- new stones appear in the fields every year from
deeper
underground. ... Underground water freezes, lifting all particles
above
it. As the water starts to melt, smaller particles can settle into
the
opening spaces while larger particles are still raised. By the time
ice
no longer supports the larger rocks, they are at least partially
supported by the smaller particles that slipped below them. Repeated
freeze-thaw cycles in a single year speeds up the process."

--
jiw

In spring the saturated upper few inches of soil that thaws during the
day and refreezes at night can break loose and arch up like bubbles.
It pulls free of boulders, leaving holes, but lifts the smaller stones
up. I haven't observed if this phenomenon moves rocks up through
absorbent black topsoil since my property doesn't have any. Rocks
don't accumulate on the surface of the sandy glacial till here.
-jsw