While I would like to hear the response from Sage, I can shed some light on the possible reasons for the till testing program. Let's assume you've mapped a gold bearing quartz vein along surface until it disappears into a swamp. There are at least 2 parameters you want to pin down before positioning a drill rig to core the vein underneath the swamp. First you want to establish the depth to bedrock since the thicker the swamp/till layer then the further away you need to place the rig to intersect bedrock at the projected vein location under the swamp. So you draw a line along the vein strike direction across the swamp ( the risk is lower if you can locate the vein on the other side of the swamp).Then you carry out a till testing program along strike right above the vein location. This will tell you the thickness of the till and whether or not the quartz vein is there. You can sample for gold in the samples as well depending on the type of rig used. Remember that the quartz veins occur with a pinch and swell geometry( thinning and thickenening) , so the till testing will reduce the risk of drilling barren bedrock. These are the reasons I can think of but we'll see what Sage has to say.