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
As Josh Lee and Colin 't Hart have said, you could optionally just use position: sticky; top: 0;
applying to the div that you want the scrolling at...
Plus, the only thing you will have to do is copy this into the top of your page or format it to fit into an external CSS sheet:
Just replace #sticky_div's_name_here
with the name of your div, i.e. if your div was #example { position: sticky; top: 0; }
.