Good Find ! The "dump" may be more recent, as others have suggested, but keep in mind that people dumping their everyday garbage in the 30's & 40's often were cleaning up and throwing out older garbage. We have an area here that we call the "40's Dump" which we go to in the Spring while there is still snow at higher elevations and we often pull out much older bottles and other artifacts. Good Luck.
Keep on searching and digging Dtruex, you will soon find the mother lode of pre-1900 bottles!! One of those rare instances when a homeowner appreciates his land being "trashed!!"
I found a huge dump site in Chicago underneath the old Riverview Amusement Park, built in 1904 but closed to build a shopping center in 1967. Excavations for new buildings started, and up came beautiful bottles!! I know the excitement of finding such a huge source of treasures!! The good thing about your site is you can take your time and dig to your heart's content. Construction companies started closing their sites - including this one - in 1973-4. Had only about 3 digging seasons but I found some little treasures!!
Most of what I see is dating from the 1940s-60s. There's a massive bottle dump about 8 miles from my house that's the same. Thousands of bottles, headlights and junk dating from the 40s-60s. Not worth much in value, but still fun to dig and even better to recycle the glass and get it out of the dirt and back into use.