Looking for Surface Hoar

ECT N16 and N28. N16 failed with resistance 30 cm down, no propagation. N28 failed 35 cm down on what we suspect is the Jan 3 surface hoar layer, resistant and no propagation. Snow was stiff, facets on the bottom 30cm of the snowpack. We heard whumpfing on our exit out about 500m from where we dug the pit. We suspect the surface hoar layer is still active in isolated terrain within the area.