Sink Holes
Others have given good info to you, but I'd like to suggest a second
possibility. If you have a 4 foot deep hole and don't see any rock, then you
must have a lot of sediment on top. If this is a dissolution cavety in the
country rock, then you might have a hard time finding it by digging, because
the surface feature may not be as well correlated to the feature in the rock as
one might like, owing to water movement in the sediment.
If the sediment you see down in this hole is very sandy, you might just have
some kind of wierd groundwater feature. This happens on occassion in very
sandy areas. I've seen it (drove a car into one, in fact) on Cape Cod in very
sandy sediment. This probably means that it is not as serious as a sink hole,
in that a very large sink hole is a VERY LARGE sink hole!, while a groundwater
feature in sandy substrate is probably not going to get real big.
If you have easy access to a pile of fill, you may just throw it into the hole
and see if the problem goes away over the course of the coming winter and
spring. Also, you can get a long metal spike-like device, either a probe like
the kind that bottle diggers (damn them, BTW) use, or even a piece of
reinforcing rod. This may help determine if the low-spots you've got are
hollow underneath. Jumping up and down on them (perhaps a questionable
practice!) may not tell you enough.
As suggested, you should be very careful about the possibility that you've got
an old septic system in place. I assume that either your house is hooked up to
sewer, or that if it has a septic system, you know where it is and can rule
that out. But perhaps one existed previously. It should be located in a
"logical" place such as "downstream" from your sewer outlet. You may check
around in the basement or below your house for old sewer pipes heading in that
direction.
The highly linear shape of this features strongly suggests to me that it is
either a septic system or a trench (for a pipe) that was filled improperly,
either with bad fill or without tamping. For this reason, you should be
careful about probing (don't use a sledge on the probing bar or spike, for
example) so you don't go through any pipes.
You might try this: If you have natural gas going into your house, and this
feature is anywhere near where you can reasonably suspect that the gas line is
layed, call the gas company and ask them to have a look at it. Similarly,
public water/sewer or possibly electric company, if your electric lines are
underground, may be called. It could be a trench coming off one of their
lines, or even one of their lines going through your property.
On the same note, if you have an easment map or plat map of your property,
check for easments for such lines running across your land, and see if they
line up.
Good luck. Let us all know what happens!
Cheers,
GTL
– Greg Laden, Written 13-Oct-1994 @ 19:52:26
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