It still seems to me he made a fire to get rid of evidence. The photos show there was a fire at the dirt location or as BW said, fire pit.
Are the gullies moist or swampy? He may have thought the dirt location was the best spot for the fire, if the fire was his focus and also out of site as FM has described, around the corner a bit.
Does a pit show preparation? I feel he went ahead of time set it all up took her in his SUV backed up into that area and by then she was either dead or would be soon and he was clearly familiar with it. Knowing now exactly where she was how close now was she to the house closest?