There are actually two different conditions that cause rash-like symptoms on the legs, plus good-old ordinary heat rash makes a third. (Heat rash is normally in areas covered by clothing, or in places where the skin rubs together, such as the upper thighs.)
The first condition is called Golfer's Vasculitis. This is the one that manifests with the line where the socks hit. This one is caused by a lot of unaccustomed walking, and as someone else pointed out, it causes the capillaries in the lower legs to leak and cause little blood spots and swelling. GV usually isn't very itchy, and it doesn't usually show below the sockline because the elastic in the socks keeps the fluid from pooling in that area.
The other rash that hits the legs is a reaction to unaccustomed exposure to UV radiation, and is called Polymorphic Light Eruption, or PMLE. This one is a histamine reaction to sunlight on exposed skin, and it is VERY itchy. Women are more susceptible to it; doctors say that the most likely sufferers are 4F: fair, far from home, (over) forty, and (usually) fat. It seldom happens to people except when they travel, because they are accustomed to the sunlight levels where they live. Taking preventative doses of antihistamine both before and during your trip usually helps to prevent it, as does "hardening" the skin ahead of time in a tanning bed. Once you get it you pretty much have to just wait for it to go away, but prevention works a bit better. Also, once you get PMLE, you are more likely to get it again and to get it more severely -- just like other allergic reactions.
It is possible to get both GV and PMLE simultaneously. You can also get PMLE on other exposed body parts, whereas GV only affects the legs.
