Explian about contact between granite and sandstone?

This is a nonconformity. the contact is between an igneous and sedimentary rock formation. Granite should be on the bottom and sandstone on top, If this is not the case then some type of over-turning has taken place. The depositional envornment allowed sand to acculate over the top of the granite and later consolodated into a rock. This is called a contact. Many years ( perhaps millions) passed) before the whole mass became a rock formation. Can you identify other contacts in the area? If so, you may be able to date this contact and correlate it to other areas in your vicinity. See if you can trace it and look on geologic maps to see if it is mapped. If not, you may have found something that no one has yet discovered.