TextBlock with ScrollViewer gets cut-off when text too long

I have a large amount of text in a textblock and I would like the scrollviewer to take effect when it reaches the limit of the grid.

Currently the text gets cut-off at the bottom of the grid and the scrollview doesn't identify when to take effect.

<StackPanel Grid.Row="1" Grid.ColumnSpan="2" Orientation="Vertical" ><ScrollViewer Margin="10,0,10,0"><TextBlock Name="TBlock_Desc" TextWrapping="Wrap" /></ScrollViewer></StackPanel>