I live in Atlantic Canada with temperature ranges from -20 F - 100F. The frost layer is generally four feet deep.

I am rebuilding a retaining wall at the end of my driveway. There is a culvert that runs underneath the driveway and funnels water into a ditch. The retaining wall I will be building will cap the end of the driveway that is open due to the ditch. I will be be building a field stone wall (mostly dry) with some mortar added in areas for stability.

My question - when building the stone wall, do I need to excavate 4 feet deep for the base? Since the wall is field stone, would one or two feet of crushed rock suffice to alleviate concerns of the wall rising due to frost? I am building around an existing wall and do not have concerns of bulging outwards.