Raise a *circle* on a terrain

I'm trying to do some runtime terrain deformation. Now, I can raise (or lower) a square on the terrain easily enough, but I cannot figure out how to get a (smooth) circle.

Here is my code:

     Terrain terrain;
     TerrainData tData;
 
     int heightmapWidth;
     int heightmapHeigth;
 
     public float radius;
     public float maxHeight = 0.03f;
     float currentHeight;
 
     float[,] originalHeights;
     // Use this for initialization
     void Start () {
         terrain = Terrain.activeTerrain;
         tData = terrain.terrainData;
     
         heightmapHeigth = tData.heightmapHeight;
         heightmapWidth = tData.heightmapWidth;
 
         originalHeights = tData.GetHeights(0,0, heightmapWidth, heightmapHeigth);
     }
 
     void OnDestroy(){
         tData.SetHeights(0,0,originalHeights);
     }
 
     // Update is called once per frame
     void Update () {
     
         if(currentHeight < maxHeight) {
 
             Vector3 relPos = (transform.position - terrain.transform.position);
             Vector3 coord = new Vector3(relPos.x/tData.size.x, relPos.y/tData.size.y, relPos.z/tData.size.z);
 
             int terrainX = (int) (coord.x * heightmapWidth);
             int terrainY = (int) (coord.z * heightmapHeigth);
 
             Debug.Log(terrainY);
 
             int size = 10;
             int offset = size/2;
 
             /*
              * This raises a square
              * 
             float[,] heights = tData.GetHeights(terrainX-offset, terrainY-offset, size, size);
 
 
             for(int i = 0; i < size; i++){
                 for(int j = 0; j < size; j++){
 
                     heights[i,j] =  Mathf.Max(heights[i,j],  currentHeight);
 
                 }                
             }
 
             tData.SetHeights(terrainX-offset, terrainY-offset, heights);
             */
 
             //raise a  circle. sort of.
             float[,] heights = tData.GetHeights(0,0, heightmapWidth, heightmapHeigth);
             for(int x = terrainX - offset; x < terrainX + offset; x++){
                 for(int y = terrainY - offset; y < terrainY+offset; y++){
 
                     if(( new Vector2(terrainX, terrainY)-new Vector2(x,y) ).sqrMagnitude < 25){
 
                         heights[y,x] = currentHeight;
                     }
 
                 }
             }
 
             tData.SetHeights(0, 0, heights);
 
 
             currentHeight += 0.03f * Time.deltaTime;
 
         }
     }

When I use this, though, my circle is very, ah, blocky. How would I get a smooth one?