Terrain and PerlinNoise Glitch

I use this Script to create random terrain heightmap:

 using UnityEngine;
 using System.Collections;
 
 public class TerrainMesh : MonoBehaviour {
 
     public TerrainData terrainData;
     public float heightScale = 0.001F;
 
 
     public Vector2 position;
 
     // Use this for initialization
     void Start () {
         GenerateTerrain ();
     }
 
     void GenerateTerrain () {
         int xSize = terrainData.heightmapWidth;
         int zSize = terrainData.heightmapHeight;
         float[,] heights = terrainData.GetHeights (0, 0, xSize, zSize);
         for (int x = 0; x < xSize; x++) {
             for (int z = 0; z < zSize; z++) {
                 heights [x, z] = Mathf.PerlinNoise (((position.y * xSize) + x) * heightScale, ((position.x * zSize) + z) * heightScale);
 
             }
         }
         terrainData.SetHeights (0, 0, heights);
     }
 
 }
 

and the result seems to be buggy ( there meet the two terrain) :

Can u tell me what is wrong with it?