Yeh, it would be chalk and when I add to the sparge I actually add it to the mash again just before I add the sparge water, stir and let it sit several minutes.
I'm just wondering how having 300 ppm chalk would effect the taste etc. of the beer? If you look at Dublin's water its within range on chalk being only 118 ppm yet its quite low on other minerals and has a high RA.