Firstly id be checking to ensure that it is an old hydraulic sluice field, for reasons, (1) there should be a large dam area or some hand cut channels that weave there way to that area from surrounding hillsides to supply the monitor with enough water to blast. It may just be a large shallow surfacing area, i cannot be sure.
(2) the trees in the background above where you can see the bench(your 2nd pic), appear to resemble a similar size to ones in the area that they Hyd sluiced,normally there would be quite a size difference between untouched trees and new growth(more of a tree guru answer that one i think)
(3) there is a possability that gold was richer underneath the conglomerate then actually in it, so you can determine this by looking for some old stamper battery footings, some may have dodgy looking concrete as a base, some used large timber squares dug into the ground, either way they would of left some sign, even a large 20-30mm black bolt sticking out of the ground, also look for very fine sand deposited in a small area, this can be waste from the battery sluice box. Search old maps and data sometimes they logged where the stampers were located.
Now the hard questions, "will detectors find gold in a 1mt piece of conglomerate.....Yes absoloutly, if the gold was big enough, or it was a couple of inches under the surface of the conglomerate, however keep in mind that conglomerate is the base of an old river bed so you have a concentration of heavy materials and minerals, and a lot of these will and do react with your detector.
Where would be the best spot to start detecting.......uuugh, thats a tough one, the higher ground could mean that the gold pinched out, or it could mean they ran out of water, or another gold rush popped up a few miles away and it was easier digging than where they were, i certainly would have a good detect in that immediate area, if no joy personally i would follow the bench cut along to the next rise in the landscape and start detecting there, mirror imaging what they have done but with your detector.
Im no legend on the coils mate so its just a suggestion.