It looks like the clay core is settling. To me that means not enough compaction of that area. Whether the lifts were too thick to properly compact, or there was too much or not enough moisture in it to properly compact it I don't know. If I am wrong, then I have no idea what is going on.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).