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.
Our gardens are all raised beds, 4 X 10 with 1 inch PVC pipe bent into hoops with bird netting to keep the critters out. Our “soil” is harvested creek bed, plus bags of soil from the big box stores and compost from our veggie clippings.
Lately, we've been overrun with armadillos but our traps (and my .22 Marlin) have been able to cull these invaders.
Our house is on the Lake and regulations say no septic water is allowed to drain into where we boat and swim. Our system is called “LPP,” for low pressure pipe.
It features two 100 yard drain fields with perforated pipe and a timer to start the drain every 8-10 hours. It's worked since we built new in 2002. The pipes lay in gravel, buried in yellow clay about one foot deep.