Make all Meshes under y Invisible - and filll the hole

Hi
i think its easy but i am a noob in shaders
I want a cut throught the hole Scene and show only all Meshes under a specific Y. This is working fine, but I want to fill a plane in the cutted mesh.
This is how it looks now:

And this is how I want it:

The Problem is that in the First Picture I can see every Cave in the Mountain. In the Second i can't see anything because the plane is simulating the Blocks

How could i ,make it with shaders?

This is the Shader I am using now:

Code (CSharp):

1. Shader "Custom/BlockShader" {
2.     Properties  {
3.          _Color ("Main Color", Color) = (1,1,1,1)
4.         _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
5.         _LetzteSichtbaresY("Letzte sichtbare Ebene", Float) = 50.0
6.         }
7.     SubShader {
8.         Tags { "RenderType"="Opaque" "Queue"="Geometry+1"}
9.        
10.        
11.         Pass{
12.             CGPROGRAM
13.             #pragma vertex vert
14.             #pragma fragment frag
15.             uniform sampler2D _MainTex;
16.             uniform float _LetzteSichtbaresY;
17.            
18.              struct vertexInput {
19.                 float4 vertex : POSITION;
20.                 float4 texcoord : TEXCOORD0;
21.                 float4 position_in_world_space : TEXCOORD1;
22.              };
23.              struct vertexOutput {
24.                 float4 pos : SV_POSITION;
25.                 float4 tex : TEXCOORD0;
26.                 float4 position_in_world_space : TEXCOORD1;
27.              };
28.            
29.              vertexOutput vert(vertexInput input)
30.              {
31.                 vertexOutput output;
32.    
33.                 output.tex = input.texcoord;
34.                 output.position_in_world_space = mul(_Object2World, input.vertex);
35.              
36.                 output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
37.                 return output;
38.              }
39.            
40.             float4 frag(vertexOutput input) : COLOR
41.              {
42.               if(input.position_in_world_space.y > _LetzteSichtbaresY)
43.                     discard;
44.                 return tex2D(_MainTex, input.tex.xy);  
45.              }
46.            
47.             ENDCG
48.            
49.         }
50.     }
51. }
52.