You are missing the height CSS property.

Adding it you will notice that scroll bar will appear.

.wrapper{ 
    // width: 1000px;
    width:600px; 
    overflow-y:scroll; 
    position:relative;
    height: 300px;
}

JSFIDDLE

From documentation:

overflow-y

The overflow-y CSS property specifies whether to clip content, render a scroll bar, or display overflow content of a block-level element, when it overflows at the top and bottom edges.