Put the big rocks on the OccludingObjects layer

UOP1_Project/Assets/Prefabs/Nature/Rocks/BigRock3.prefab

       propertyPath: m_StaticEditorFlags
       value: 4294967295
       objectReference: {fileID: 0}
+    - target: {fileID: 919132149155446097, guid: 3fdd92505d7ca4634b00be350b9649af,
+        type: 3}
+      propertyPath: m_Layer
+      value: 11
+      objectReference: {fileID: 0}
     m_RemovedComponents: []
   m_SourcePrefab: {fileID: 100100000, guid: 3fdd92505d7ca4634b00be350b9649af, type: 3}

UOP1_Project/Assets/Prefabs/Nature/Rocks/BigRock4.prefab

       propertyPath: m_StaticEditorFlags
       value: 4294967295
       objectReference: {fileID: 0}
+    - target: {fileID: 919132149155446097, guid: 2fe53c47199a241a1b09db8aea39ad9d,
+        type: 3}
+      propertyPath: m_Layer
+      value: 11
+      objectReference: {fileID: 0}
     m_RemovedComponents: []
   m_SourcePrefab: {fileID: 100100000, guid: 2fe53c47199a241a1b09db8aea39ad9d, type: 3}