Smoothly deform terrain

Hi, I created a dig method for my terrain, which is supposed to decrease each point on the terrain within a certain radius x amount of units. It works, but when the hole becomes steep, random vertices begin popping up, probably in an effort to smooth out the terrain. How can I stop this? Here is my method :

     Vector3 tPos = transform.position;
     TerrainData tData = GetComponent<Terrain>().terrainData;

     Vector2 digPoint = new Vector2();
     digPoint.x = ((point.x - tPos.x) / tData.size.x) * tData.heightmapWidth;
     digPoint.y = ((point.z - tPos.z) / tData.size.z) * tData.heightmapHeight;
     float[,] heights = new float[radius, radius];
     for (int i = 0; i < radius; i++)
     {
         for (int j = 0; j < radius; j++)
         {
             int x = (int)digPoint.x + i;
             int y = (int)digPoint.y + j;

             float newHeight = Mathf.Max(tData.GetHeight(x, y) / tData.size.y - depth * Time.deltaTime, 0f);
             heights[i, j] = newHeight;
          }
     }
     tData.SetHeights((int)digPoint.x, (int)digPoint.y, heights);

Any help is appreciated!