Bill,
Good luck and it sounds like you are going about this as logically and intelligently as you can. Those sand lenses can be a real pain. Even in my area where we have clay deposits as deep as 200 feet due to prehistoric glacial action, occasionally you will run into sand lenses.
I dug a pit for my well distribution box (where I can feed water to each or all of the four ponds on the property with gate valves), and there was a perfectly round pocket of sand in the side of the pit. Fortunately I was putting a liner in the pit, however it's not unusual to have water elevate the liner when the pit is low of water, probably due to this sand lense bringing in surface water.
The way I understand it, up here is, the sand lenses are glacial streams where sand was deposited, but could be wrong.
What's really cool up here is on a couple of occasions that I know of, two different well drillers have brought up wood shavings from around 100 feet which indicated they are drilling in to a tree which had to be from the last ice age about 10,000 years ago. I find that very fascinating!
