Terrain drawing over itself at larger sizes - c++

Question

I have had a terrain generation algorithm for a while and have had a lot of success with it. However when I decided to start work on Level of Detail (Has not been implemented), I started to notice that my terrain would begin to draw underneath itself as it got larger (when it gets to around 257 by 257). I'm not too sure why it starts to do this however I have a feeling its either due to me converting the 2d vector into a 1d or when i generate my indices but i am not 100 percent. If someone could have a look that would be amazing
Terrain at 100 by 100
Terrain at 1000 by 1000
Code used to Generate Vertices
void TerrainObject::LoadFlatTerrain()
{
float F_terrainDimentions = terrainDimentions; //used to stop integer division
m_vertexCount = terrainDimentions * terrainDimentions; //terrainDimentions = the width and depth of the terrain
float halfWidth = 0.5f * F_terrainDimentions;
float halfDepth = 0.5f * F_terrainDimentions;
// project the grid into xz plane
float dx = F_terrainDimentions / (F_terrainDimentions - 1);
float dz = F_terrainDimentions / (F_terrainDimentions - 1);
float du = 1.0f / (F_terrainDimentions - 1); // texture co-ordinates
float dv = 1.0f / (F_terrainDimentions - 1);
//Resizes Vectors for Vertices
vertices.resize(m_vertexCount);
verts2d.resize(terrainDimentions);
for (int i = 0; i < verts2d.size(); i++)
{
verts2d[i].resize(terrainDimentions);
}
//Generates Vertices for 2d array
for (int i = 0; i < terrainDimentions; i++)
{
float z = halfDepth - i * dz;
for (int j = 0; j < terrainDimentions; j++)
{
float x = -halfWidth + j * dx;
float y = 0;
verts2d[i][j].Position.x = x;
verts2d[i][j].Position.y = y;
verts2d[i][j].Position.z = z;
verts2d[i][j].Normal = XMFLOAT3(0.0f, 1.0f, 0.0f);
verts2d[i][j].Tangent = XMFLOAT3(1.0f, 0.0f, 0.0f);
verts2d[i][j].TexCoord.x = j * du;
verts2d[i][j].TexCoord.y = i * dv;
}
}
//Converts to 1d array
for (int i = 0; i < terrainDimentions; i++)
{
for (int j = 0; j < terrainDimentions; j++)
{
vertices[i * terrainDimentions + j].Position = verts2d[i][j].Position;
vertices[i * terrainDimentions + j].TexCoord = verts2d[i][j].TexCoord;
vertices[i * terrainDimentions + j].Normal = verts2d[i][j].Normal;
vertices[i * terrainDimentions + j].Tangent = verts2d[i][j].Tangent;
}
}
// Create vertex buffer
D3D11_BUFFER_DESC vbd;
vbd.Usage = D3D11_USAGE_IMMUTABLE;
vbd.ByteWidth = sizeof(ComplexVertex) * m_vertexCount;
vbd.BindFlags = D3D11_BIND_VERTEX_BUFFER;
vbd.CPUAccessFlags = 0;
vbd.MiscFlags = 0;
D3D11_SUBRESOURCE_DATA vinitData;
vinitData.pSysMem = &(vertices[0]);
m_pd3dDevice->CreateBuffer(&vbd, &vinitData, &m_pVertexBuffer);
CreateIndices();
//Sets terrains Current position in the world to be the top corner
m_position = verts2d[terrainDimentions - 1][terrainDimentions - 1].Position;
}
Code to Generate Indices
HRESULT TerrainObject::CreateIndices()
{
UINT faceCount = (terrainDimentions - 1) * (terrainDimentions - 1) * 2; // each quad consists of two triangles
int index = 0;
indices.resize(faceCount * 3);
for (UINT i = 0; i < terrainDimentions - 1; ++i)
{
for (UINT j = 0; j < terrainDimentions - 1; ++j)
{
indices[index] = i * terrainDimentions + j;
indices[index + 1] = i * terrainDimentions + j + 1;
indices[index + 2] = (i + 1) * terrainDimentions + j;
indices[index + 3] = (i + 1) * terrainDimentions + j;
indices[index + 4] = i * terrainDimentions + j + 1;
indices[index + 5] = (i + 1) * terrainDimentions + j + 1;
index += 6; // next quad
}
}
m_indexCount = indices.size();
// Creates index buffer
D3D11_BUFFER_DESC ibd;
ibd.Usage = D3D11_USAGE_DEFAULT;
ibd.ByteWidth = sizeof(WORD) * m_indexCount;
ibd.BindFlags = D3D11_BIND_INDEX_BUFFER;
ibd.CPUAccessFlags = 0;
ibd.MiscFlags = 0;
D3D11_SUBRESOURCE_DATA iinitData;
iinitData.pSysMem = &indices[0];
return m_pd3dDevice->CreateBuffer(&ibd, &iinitData, &m_pIndexBuffer);
}