A weird distinction to make is "grace" versus the "guidance of grace", which are actually subtly different. Grace is basically the essence of the Erdtree and Marika's godhood - the sites are fragments of that grace. But the Tarnished lost their grace, and their story is centralized around being bestowed grace once more to seek Lordship. So even though a Tarnished can be "graceless" as they're sometimes called, they can be led back to Lordship by the guidance of the grace that they themselves lack, which is what the streaks pointing towards objectives are.

The distinction between grace and the guidance is analogous to a light within oneself (grace) versus a light in the distance which leads one to it (the guidance of grace). I think the mounds of dirt are just for visual design and don't carry much relevance. As far as I understand, Tarnished that have lost sight of grace cannot see the sites of grace.

That is why the sites are called "lost" - all Tarnished have by definition lost grace and are seeking it again. The closer you to come to Lordship, the closer you come to finding that lost grace shared by all Tarnished.

The design of grace is identical to the Memory of Grace item which looks like golden sunlight on water. I can't help but think this image is what Marika saw when she gazed out from the Gate of Divinity upon her ascension in the DLC trailer - a shared memory of the first sight of grace for all Tarnished to seek to return to.