I thought I said this as point 2 (maybe rock or glacial till was encountered . . .). emmgjld reinforces the point though.
In many locales, it is truly amazing at how much bedrock will differ in elevation in relatively short distances. At the plant site here, there was more than a 25 m elevation difference in bedrock over 35 m length. Very unstable (uneven) bedrock surface that even more boreholes would/could lead to even more confusion.