Here's some excellent details on how to find those completely covered bottle dumps and privy holes using a long home-made ground probe. As it says in the article, you're not looking to "feel" bottles with it - rather you're looking for that odd difference in ground texture that usually is a void rather than a solid ground feel:
http://my.net-link.net/~prostock/bottle_digging_part_1.html
http://my.net-link.net/~prostock/Bottle_digging_II_III.html
http://my.net-link.net/~prostock/bottle-digging_IV.html
http://my.net-link.net/~prostock/bottle_digging_V.html
Often times old community bottle dumps can be found cascading down river front slopes - found a couple here in Minnesota along the Mississippi myself. Walk along the river at the water line a look up the slope - if there was one there you'll see all sorts of bottles and broken glass cascading down the slope.
A pitch fork is a common tool for getting deeper into the soil -
gently insert and twist to break open the soil.
Be sure and check YouTube for some excellent bottle digging videos too. I gotta' tell ya, bottle digging videos are about 1000 times better than watching metal detecting videos!