shader blending 2 texture script

Hi all;

I have a shader that allows me to blend two textures together. As a shader newbie, I'm struggling to try to word this carefully without too much confusion. I want to be able to blend between one texture to another on a plane asset, at one end of the plane is a snow texture and the other is a rock texture. I want to blending the middle of the plane so that if gives the effect of the first texture at 0.5 (snow) blending with the second texture (rock) - a snowy rocky blend. Here is a a picture to explain what I want to achieve:

The plane just blends the whole of the plane from one texture to another (not what I want). Any help is appreciated.

CODE:

 Shader "Custom/something" {
     Properties {
         // define texture, default white
         _MainTex ("Base (RGB)", 2D) = "white" {}
         // define 2nd texture default white
         _Texture2 ("Blend Texture 1", 2D) = "white" {}
         
         //http://answers.unity3d.com/questions/232106/shader-blending-multiple-textures-in-cg-code.html
         _Blend1 ("Blend between _MainTex and Texture2", Range (0, 1) ) = 0 //- Blend1 is the slider to go from snow (0) to rock (1)
         //_Blend1 (for (float i = 0.0; i < 1.0; i++;) = i // - test
     }
     SubShader {
         Tags { "RenderType"="Opaque" } // render opaque geometry
         LOD 400
         
         CGPROGRAM
         
         // surface shader, get output by calling surf function, use diffuse (lambert) lighting model
         #pragma surface surf Lambert
         
         #pragma target 3.0
         //#pragma surface surf BlinnPhong
 
         sampler2D _MainTex;
         sampler2D _Texture2;
         float _Blend1;
 
         struct Input {
             // get uv coordinates of _MainTex
             float2 uv_MainTex;
             // get uv coordinates of _Texture2
             float2 uv_Texture2;
         };
 
         void surf (Input IN, inout SurfaceOutput o) {
             //half4 c = tex2D (_MainTex, IN.uv_MainTex);
             
             // how to specify uv,
             // http://unitygems.com/noobshader1/
             // blend 0.5
             
             fixed4 mainCol = tex2D(_MainTex, IN.uv_MainTex);
             fixed4 texTwoCol = tex2D(_Texture2, IN.uv_MainTex);
             //fixed4 texTwoCol = tex2D(_Texture2, IN.uv_Texture2);
             fixed4 output = lerp(mainCol, texTwoCol, _Blend1);
             o.Albedo = output.rgb;
             o.Alpha = output.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"