This mud hole has existed for the entire 4 years I have owned this property. I would welcome and advice, questions, concerns, comments about this project that has been a long time coming.
Well, I had collected 'distressed' bags of concrete mix from LOWES against some future need as they were asking less than half price and once or twice, two dollars a 'bag.'
Turns out I don't know my concrete storage solutions very well and, over time (Months? a year?) the packages seemed to have hardened/solidified.
I had a small issue with erosion behind the shed they had been stored in and decided to simply lay the bags in the depression because "couldn't hurt" right?
After first reading your post I thought to suggest big rock (Rip Rap) first layer, then Ball Rock (3") then washed ABC. Also, that, if you succeed, the low spot will become a high spot and the water will collect 'elsewhere.' So, think of where it comes from - how it gets there - and be sure now to impede the new flow path to the new retention pond (mud hole).
Then, I thought about the sacks of sakrete. It is my understanding that keeping 'the pour' moist helps cure teh concrete - making it stronger still. So, laying unopened sacks about the desired pathway might help create a relatively uniform base of even better quality than the Rip Rap etc. As the bags are of uniform size and shape, they could be place almost like super tiles. Given the quantity needed, you might be able to qualify for a wholesale price.
A Super 'cobblestone trailway!'