Modifying UV Values in a Shader

I'm trying to create a shader that uses a vertical slice of rock stratigraphy and bends it using a deformation texture map. It compiles okay until I try to modify the V value of the stratigraphy sampler. At that point, I get an error message that references a line completely out of the program:

Shader error in 'Custom/Terrain': cannot implicitly convert from 'float3' to 'float4' at line 106

I've written a few shaders but this is driving me to distraction.

Shader "Custom/Terrain" {
Properties {
_Stratigraphy ("Rock Layers", 2D) = "white" {}
_Deformation ("Deformation", 2D) = "white" {}
_VScale ("Vertical Scale", Float) = 1
}

SubShader {
Tags { "RenderType" = "Opaque" }
CGPROGRAM
#pragma surface surf Lambert

struct Input {
float4 worldPos;
float2 uv_Stratigraphy;
float2 uv_Deformation;
};

sampler2D _Stratigraphy;
sampler2D _Deformation;
float _VScale;

void surf (Input IN, inout SurfaceOutput OUT) {
float vOffset = tex2D (_Deformation, IN.uv_Deformation).r * IN.worldPos.y *_VScale;
float2 uv = IN.uv_Stratigraphy;
uv[1] += vOffset;
OUT.Albedo = tex2D (_Stratigraphy, uv).rgb;
}
ENDCG
}
//Fallback "Diffuse"
}

 

Just an addendum: It's only when I add the line: uv[1] += vOffset; that the error shows up.

 

uv.y += vOffset; ?

 

I have a stratigraphy that's represented as a texture map. The second texture map is supplying the deformation to that basic stratigraphy. What I want to do is take the base position UV and shift it up or down in y based on the deformation value