I am slightly confused by these forums so please excuse me if this post is in the wrong area.

I've been trying solidworks out for a few days now, and am struggling to resolve this issue I currently have. Basically I want to extrude a radius into surface, but if I go to extrude a radius, it turns into a triangle? I believe I am approaching this wrong.

Here, highlighted in blue is the radius I would like to extrude along the model. As you can see the model currently has a flat top, the aim is to replace that flat top with a 12" radius.

Here I have the radius selected, and am Extruding the cut along the top.

Once I confirm the cut, it renders the radius into a triangular shape, rather than a curve as you can see here.

Here is another view.

Any help is appreciated.

I ask if possible, please make your explanation as simple as possible, I am a complete newb to Solidworks.

Thanks

JP