hi, that is my paper of choice for drawing. i usually pick it up at pearl paint if i need it right away, but you can order it online. i got some from dickblick, they had the best price at the time. it has a good weight to it and accepts pencils very well. i never tried pastels on them, other than for white highlights. i use either canson mi-teintes or wallis pastel paper. i'm sure someone can help you with their pastel experience on stonehenge. good luck.
'Lord, grant that I may always desire more than I can accomplish' - Michelangelo