The distribution of Caucasian DNA in Europe is frankly pretty unbelievable and perplexing. There is 10% in Belgians and only 4% in Dutch. It is damn near absent in Scandinavians, and over 10% in Polish people. And yet it is 3% in Russians. It is 20% in Sardinians, close to 10% in much of Spain AND 0% IN THE BASQUE COUNTRY. There is a wild amount of variability to this admixture and there is no straightforward cline, or vector we can point to for this admixture.
I would however argue that there is a likely vector for this admixture: the Early European Farmers who took the Danubian route into Europe. We know that one group took the Med, and one group went inland.
It's either that, or it maps to a particular type of Paleolithic/Mesolithic Hunter Gatherer