rocks ? sigh, you must be real close to where a glacier 'stopped' eons ago ! Have two neighbours down the road, guy to the east live on a sand pit, guy just west ,maybe 1000', WORSE than your place !
I ran tach at 2500-2800 todig and dump HIS 'rock collection' last year. A thumb would have been nice....
Always dig a little deeper( +6" ?), maybe use a 'joint' or something on the BH as a reference so after 3-4 hours 'muscle memory' will automatically do the measuring for you .
Also take breaks otherwise your butt will be sore !