Yes , glacial alluvial deposits do make a significant difference in topography and the ability to grow crops.
My former 5 acre farmette sat on heavy clay; it was a chore just to find adequate drainage for a septic distribution system
while across the street there was a hillside of #2 washed stone which the transportation department wanted for construction of an expressway.
My current residence, about 3/4 mile away sits atop a shale shelf with 4-6 inches of good topsoil, then 6 feet of heavy clay, over the shale bedrock.
I was unaware that you had a rock garden - the best alternative is raised beds, or decades of building a rock wall around the perimeter.
I have done both,.. and have the backache to prove it (!), although my ol Ford 8N with double mouldboards bounced around a lot jumping over rocks.
Yes, the Mennonites do raise fantastic crops, even in urban locations - and they have the talent and many hands to make the work and the crop seem easy.