Sorry for the poor writing again and I don't know why but my brain always conflates Justin with Jerome. What I said above about Justin was from Jerome. Geez my brain....
Touchofgrey....yep in some Rabbinic legend Melchizedek aka Shem (yes, they are linked in many legends) was given the skull for burial in the 'navel' of the world Jerusalem. In some legends the rest of the bones were distributed to other parts. And yes, by the time of Jerome the very physical phenomenon of blood reaching to the skull of Adam was popular. He objected , preferring the legend that Adam was buried in Hebron that was then circulating. Anyway, Jerome had issues with the Jerusalem church and pilgrimages to 'sacred' sites. That may explain his preference for the Hebron legend. (PDF) Constructing the Sacred in Late Antiquity: Jerome as a Guide for Christian Identity
There is suggestive evidence that the Hebron legend was actually created to refute the Christian sacred site that connected Adam with Jesus. So that might suggest the Jerusalem burial legend predates the Hebron legend but who knows.
Since the distribution of the Gospels there have been various theories about the meaning. Jerome said it was because of skulls lying about, some thought that a 'round' bump of ground, using an extended etymology might have been involved. Both are unlikely. Interestingly Epiphanius specifically denied the latter theory.: There is nothing to be seen on the place resembling this name; for it is not situated upon a height that it should be called (the place) of a skull, answering to the place of the head in the human body." He felt the 'place of the skull' was a reference to the skull of Adam.
The idea that some stone structure that resembled a skull shape with eyes etc. is a very recent 19th century fantasy. The idea that a modern feature could be the site becomes absurd when we realize the limestone was actively quarried in the first century and the soft limestone erodes rather quickly. I compared pictures of a waterfall we saw on our honeymoon with a recent one and in just a few decades the rocks have eroded/changed significantly. Imagining a limestone formation would be unchanged for 2000 years is just silly.
The site of the Temple of Venus by Hadrian famously was rebuilt as a Christian church now called the Church of the Holy Sepulcher by Constantine. And below it was created/discovered the Chapel of Adam. The religious history of the site is long and complicated, but briefly said, the legends surrounding a 15' bump of rock encased in glass inside the reconstructed church led to its being venerated by billions. In reality the bump was almost certainly a leftover from quarrying due to fissures in it, or as some suggest made (outside originally) when the church was built.
The earliest, and IMO the best, 2nd/3rd century explanation was rather that the site was that of the skull of Adam. Whether there actually was a site connected with that legend by ancient Jews is unknown. If there was, it might explain Hadrian's choice to locate his Temple to Venus there as part of his campaign to eradicate Judean Judaism. If there was only a legend and no sacred site designated by Jews, the Markan writer could simply have been linking his story with the legend literarily and had no specific geographic place in mind. This seems most likely given the writer's distance temporarily and geographically from the story's setting. Interestingly it appears there was no designated place for executions, which would have freed the Markan writer to be creative. Regardless, a century or two later the site of Hadrian's temple was clearly linked with both Adam and Jesus by Christians which is why Constantine built his Christian church there.