What is this karst landform (N 47.21181 W 121.94142), how did it form, and what do the hatchered contours mean (that you can see in Acme Mapper topo tap)?

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What is this karst landform (N 47.21181 W 121.94142), how did it form, and what do the hatchered contours mean (that you can see in Acme Mapper topo tap)?