Edited: Common Whitlowgrass (Draba verna). I initially suggested This one really has me stumped: 1) It does not have notched petals, so I eliminate Draba; 2) the overall inflorescence and petal form suggest Arabis. Possible a) Slender Rockcress (Arabis pycnocarpa); b) Hairy Rockcress (Arabis hirsuta), c) Arabis caucasica (based on toothed basal rosette leaves). Growing on limestone pavement alvar, moss substrate, white petals, Lime Kiln trail, Nepean, ON, May 14/21