This "standing stone" appears to be a natural protrudence from the bedrock:
http://www.flickr.com/photos/british_isles/2742929916/in/photostream/
And this "cairn within standing stones" doesn't seem to fit the description:
http://www.flickr.com/photos/british_isles/2742093461/in/photostream/
Show us something prehistoric and we'll probably identify it for you.
;o)
baz