* Unconformity: An unconformity in geology represents a gap in the geologic record, indicating a period of erosion or non-deposition. It signifies that the layers of rock above and below the unconformity were not deposited continuously.

* Nonconformity: A nonconformity specifically refers to an unconformity where sedimentary rock (like sandstone) lies directly on top of igneous or metamorphic rock (like granite). This indicates that the granite was formed, cooled, uplifted, and eroded before the sandstone was deposited on top of it.

In summary: The one million year old sandstone layer resting on top of the 1.5 billion year old granite signifies a long period of time where the granite was exposed to erosion before the sandstone was deposited. This gap in time is represented by the nonconformity.