Error Instantiating Prefab in while look at end of row, Happens after a random amount of rows

I am working on a simple world generator using squares. But is its failing after a random number of rows, It does not instanate the last item at the end

Error:

 ArgumentOutOfRangeException: Argument is out of range. Parameter name: index System.Collections.Generic.List`1[UnityEngine.GameObject].get_Item (Int32 index) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:633) GenGround.Start () (at Assets/Code/GenGround.cs:93)

Code:

 using UnityEngine;
 using System.Collections;
 using System.Linq;
 //using System;
 using System.Collections.Generic;
 
 public class GenGround: MonoBehaviour {
  // Use this for initialization
  public GameObject[] terrainTypes;
  void Start() {
   int x = 0;
   int y = 0;
   int i = 0;
   GameObject[] itemList = new GameObject[10000];
   terrainTypes = GameObject.FindGameObjectsWithTag("Terrain").OrderBy(go => go.name).ToArray();
   List < GameObject > WaterDeepOpts = new List < GameObject > ();
   WaterDeepOpts.Add(terrainTypes[3]);
   WaterDeepOpts.Add(terrainTypes[4]);
   List < GameObject > WaterShallowOpts = new List < GameObject > ();
   WaterShallowOpts.Add(terrainTypes[3]);
   WaterShallowOpts.Add(terrainTypes[4]);
   WaterShallowOpts.Add(terrainTypes[1]);
   List < GameObject > SandOpts = new List < GameObject > ();
   SandOpts.Add(terrainTypes[0]);
   SandOpts.Add(terrainTypes[1]);
   SandOpts.Add(terrainTypes[4]);
   List < GameObject > GrassOpts = new List < GameObject > ();
   GrassOpts.Add(terrainTypes[0]);
   GrassOpts.Add(terrainTypes[1]);
   GrassOpts.Add(terrainTypes[2]);
   List < GameObject > StoneOpts = new List < GameObject > ();
   StoneOpts.Add(terrainTypes[0]);
   StoneOpts.Add(terrainTypes[2]);
   List < GameObject > Block1 = new List < GameObject > ();
   List < GameObject > Block2 = new List < GameObject > ();
   List < GameObject > Blocks = new List < GameObject > ();
   foreach(GameObject g in terrainTypes) {
    Debug.Log(g);
   };
   itemList[i] = Instantiate(terrainTypes[Random.Range(0, terrainTypes.Length)], new Vector3(0, 0, 0), Quaternion.identity) as GameObject;
   x = x + 10;
   i = i + 1;
   while (y < 1000) {
    if (i < 100) {
     //Debug.Log (i);
     if (itemList[i - 1].name == "WaterDeep(Clone)") {
      itemList[i] = Instantiate(WaterDeepOpts[Random.Range(0, WaterDeepOpts.Count())], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
     } else if (itemList[i - 1].name == "WaterShallow(Clone)") {
      itemList[i] = Instantiate(WaterShallowOpts[Random.Range(0, WaterShallowOpts.Count())], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
      //Debug.Log (itemList [i]);
     } else if (itemList[i - 1].name == "Sand(Clone)") {
 
      itemList[i] = Instantiate(SandOpts[Random.Range(0, SandOpts.Count())], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
     } else if (itemList[i - 1].name == "Grass(Clone)") {
      itemList[i] = Instantiate(GrassOpts[Random.Range(0, GrassOpts.Count())], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
     } else if (itemList[i - 1].name == "Stone(Clone)") {
      itemList[i] = Instantiate(StoneOpts[Random.Range(0, StoneOpts.Count())], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
     } else {
      itemList[i] = Instantiate(terrainTypes[Random.Range(0, terrainTypes.Length)], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
     }
    } else if (y > 0 && x > 0) {
     //Debug.Log(itemList[i - 1].name);
     //  Debug.Log(x);
     if (itemList[i - 1].name == "WaterDeep(Clone)") {
      Block1 = WaterDeepOpts;
     } else if (itemList[i - 1].name == "WaterShallow(Clone)") {
      Block1 = WaterShallowOpts;
     } else if (itemList[i - 1].name == "Sand(Clone)") {
      Block1 = SandOpts;
     } else if (itemList[i - 1].name == "Grass(Clone)") {
      Block1 = GrassOpts;
     } else {
      Block1 = StoneOpts;
     }
     if (itemList[i - 99].name == "WaterDeep(Clone)") {
      Block2 = WaterDeepOpts;
     } else if (itemList[i - 99].name == "WaterShallow(Clone)") {
      Block2 = WaterShallowOpts;
     } else if (itemList[i - 99].name == "Sand(Clone)") {
      Block2 = SandOpts;
     } else if (itemList[i - 99].name == "Grass(Clone)") {
      Block2 = GrassOpts;
     } else {
      Block2 = StoneOpts;
     }
     Blocks = Block1.Intersect(Block2).ToList();
     //Debug.Log(Blocks.Count());
     int d = Random.Range(0, Blocks.Count());
     //  Debug.Log(d);
     foreach(GameObject g in Blocks) {
      Debug.Log(g);
     };
     Debug.Log("Blocks = " + Blocks[d].name);
     itemList[i] = Instantiate(Blocks[d], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
    } else if (y > 0 && x == 0) {
     //  Debug.Log(x);
     if (itemList[i - 99].name == "WaterDeep(Clone)") {
      //  Debug.Log("BONG!");
      Blocks = WaterDeepOpts;
     } else if (itemList[i - 99].name == "WaterShallow(Clone)") {
      //  Debug.Log("BONG!");
      Blocks = WaterShallowOpts;
     } else if (itemList[i - 99].name == "Sand(Clone)") {
      //    Debug.Log("BONG!");
      Blocks = SandOpts;
     } else if (itemList[i - 99].name == "Grass(Clone)") {
      //    Debug.Log("BONG!");
      Blocks = GrassOpts;
     } else {
      Blocks = StoneOpts;
      //  Debug.Log("BONG!");
     }
     //Blocks = Block1.Intersect(Block2).ToList();
     //  Debug.Log("BONG!");
     int d = Random.Range(0, Blocks.Count());
     Debug.Log(d);
     Debug.Log("count=" + Blocks.Count());
     itemList[i] = Instantiate(Blocks[d], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
    } else {
     itemList[i] = Instantiate(terrainTypes[Random.Range(0, terrainTypes.Length)], new Vector3(x, 0, y), Quaternion.identity) as GameObject;
    }
    x = x + 10;
    i = i + 1;
    //Debug.Log(itemList[i]);
    if (x == 1000) {
     y = y + 10;
     x = 0;
 
    };
 
   };
 
  }
 
 
  // Update is called once per frame
  void Update() {
 
  }
 }