An uncanny situation for the canines
Jim saw that his dogs were acting in a peculiar manner as they got closer to the peculiar mound that they were approaching. This behaviour became more noticeable as they moved closer to the mound. When you take into account the fact that these are two young canines who had gone to the place on numerous occasions in the past, you can see why this event was considered to be exceptionally strange.
The hounds were the source of a low and rumbling growl that could be heard emanating from their general location. No matter how hard he tried, his dogs would not allow him to get any closer to the field than he had previously gotten to it. This was the case regardless of how close he came to the field in the past.