I would expect the bedrock to be generally flat except in areas where fractures, folding, intrusions, faults, and upthrusts have occurred.
In other words hard to tell without a survey of the site in person.
The gravel and boulders are most likely deposited from glacier run off creating an ancient stream or river bed and the density of the blue clay would have most likely prevented much of the placer gold from settling all the way down to bedrock. Therefore I would expect to see more gold in and above the clay than under it but you never know until you dig.
Even though the blue clay is not worth it's weight in gold, it is however quite valuable in it's own right as a healing medium depending upon it's radium content.
Blue clay is widely used in hospitals of Germany and Sweden for T.B. treatment.
The curative spectrum of blue clay is much wider in folk medicine: cancer, leukemia, adenoids, polypi, goiter, lymph nodes inflammation, anemia, headache, atherosclerosis, paralyses, nerve disorders, epilepsy, brain disturbances, otitis, eye inflammations, women’s diseases (mastitis, mastopathy, prolapses of the uterus, fibromyoma, polypi, cysts, irregular periods, vaginitis, different discharges), men’s impotence and testicles induration, diabetes, digestion disorders (gastritis, constipation, intestinal colic, enterocolitis, prolapsus of the rectum, piles), inflammation of the liver, jaundice, nephritis, urinary bladder diseases, heart disorders, bronchitis, pneumonia, pleurisy, T.B., cough, blood spitting, rhinitis, antritis, nose bleedings, sore throat, skin diseases (eczema, psoriasis, erysipelas, warts), alopecia, rachitis, arthritis, arthrosis, rheumatics, spurs, injuries, varicose veins, bones and joints diseases, ulcers and so on.
From the ancient times people knew that blue clay doesn’t contain any bacteria.
It absorbs all liquids and gaseous toxins, odors and gases, and also kills morbific microbes.
Go for the gold
GG~