I would like to create a div, that is situated beneath a block of content but that once the page has been scrolled enough to contact its top boundary, becomes fixed in place
The info provided to answer this other question may be of help to you, Evan:
Check if element is visible after scrolling
You basically want to modify the style of the element to set it to fixed only after having verified that the document.body.scrollTop value is equal to or greater than the top of your element.