Among the many ant species in northern Washington State are these thatching ants, Formica obscuripes. In some cases, the activity of the ants causes plants to flourish in a ring around the hill. This effect seems to work with different species of plants, like these hills ringed in grass and a flowering herb.
I noticed that the ants at the hill in image 1 congregated in a crescent-shaped area defined by the shadow of the vegetation.
In the video (6), note a morph with a smaller, darker head emerging from the hole near the end. A similar morph also appears in this image. These morphs are not separate castes as in some other ant species.
Thanks to James C. Trager for the species ID on bugguide.net.