Cave with Manitou

Saturday I went hiking at a conservation area in Smithfield,  RI.  This area was once a farm, and the trail went up a steep hill past a large outcrop with a shallow cave.

There are no visible marks on the cave walls. The cave faces east, and this is the view.

There is water further downhill. And this is what I saw right next to the cave.

A manitou about 6 inches thick standing at a slant,

and what looks like a niche.

There is something familiar about a cave with a manitou, and I remembered the shrine described by Norm Muller on Larry Harrop's June 23,2011 blog posting here. There aren't any stones inside the cave and the  floor is stirred up, as if kids were playing there.  However, the manitou and niche suggest Native spiritual or practical significance.
     The cave faces east, and could have been used to observe solstice sunrises or Pleiades rises.  Both summer solstice and equinox sunrises would have appeared over the crests of nearby hills. 
     In this same area are two other  manitous. This one is 100 feet  away to the northeast, also on a hill above a tumbling brook. The late afternoon sun shows it faces to the southwest.

And this one is about one mile away to the northeast, on a hill near a river.

None of these seem to have astronomical alignments.  Both of these sites were cleared for farming, so any other structures would have been removed.  I have looked at hundreds of pictures of agricultural structures such as pens, sheds, smokehouses and outhouses, and none of them would have incorporated these slabs. My best suggestion is that these manitous were set up by Natives, perhaps as markers for campsites or fields.  The farmers left them because the other end of a manitou is buried deeply and bracketed with other stones, making it difficult to remove.  Strangely enough, the cave and these two manitous are on a straight line, as if they marked a trail or territory.  If so, it would have been Native use. This area was sold to John Mowry by  William Minnion, a Massachuset of Punskepage, in 1666. The deed uses a chestnut tree, Nipsachuck Hill, a clump of pines, and the Woonasquatucket River as boundaries. There is no mention of stones.