I found a little bit about Stone/Rock Baskets and posted it up here: http://wakinguponturtleisland.blogspot.com/2010/09/what-is-it.html , wrote something more recently about similar stones in CT here:
Recently Alyssa Alexandria (these are all her photos, taken by Lake Shastina in Northern California) came upon a "newer" looking stone row with this boulder that I think looks as if someone started working on a stone basket, like the two on the right above, but for some reason never finished it."The question of mortars is one which presents some difficulty. The almost unanimous testimony of the Indians is to the effect that neither they nor their ancestors, within their recollection, ever made the finished, globular mortars of which so many hundreds have been found within the area occupied by the stock. In the manufacture of acorn-flour, they were accustomed to use either the smooth, flat surface of some large bowlder or ledge, or else a flat slab or block of stone of irregular shape sunk into the floor of their houses... the statement is made, moreover, by the Indians, that finished, widely and deeply excavated globular mortars (like these shown above) were found by them in certain localities in considerable numbers. These localities were gravel-banks along the edges of streams, the stream-beds themselves, or the surface of the ground in certain stony places . They claim that in a few spots large numbers of mortars were to be found; and some declare that the shamans know where these spots are, and go there at times to secure mortars, which, as will be seen later in speaking of religious ceremonials, were used for various mysterious and sacred purposes. Everywhere throughout the Northern Maidu area these mortars are regarded with considerable awe and veneration, and are feared quite keenly in many instances..." but then Joel leaps into describing yet another type of "mortar," even including this drawing:
"Mortars were used by the shamans as receptacles in which to keep their most powerful and precious charms, especially the "pains" which they shot at people to cause disease or death. Such "pains" were kept in a mortar, with another mortar inverted over the first, the whole carefully secreted in a hollow log or under a large stone at a considerable distance from all habitations or trails. In the initiation ceremonies of the Secret Society the sacred meal used for sprinkling the novices must similarly be kept in a mortar. Lastly, the mortars are generally known by the name of ku'kinim to'nior i'tum to'ni ("spirit or pain baskets"), and are often supposed to be themselves the abiding-places of powerful spirits; although the belief in the mortars' animation, and their ability to move of their own accord from place to place, is not held here, as it is among the Shasta. By some the mortars are said to have been made by the Creator, or the Coyote, at the time of the creation, and scattered over the world for the use of mankind. By others they are supposed to have been people originally, during the bSte'ito, or time of the "first people," who were turned into stone in this form at the coming of the Indian people, when the other "first people" became animals."
"Does Alyssa have any photos of this type of motar stone?" you might ask.
I'll show you two spectacular photos as an answer:
Above is the stone "pain basket" (and its displaced cover),
while below is the view of Mt. Shasta above it. Before a dam was built around 1930, there would have been a river in the valley where Lake Shastina presently is located...