The shield wall was very common, a staple of warfare not only by the Norse but Saxons, Frisians and Angles for example. http://static.guim.co.uk/sys-images/Arts/Arts_/Pictures/2012/8/7/1344339127732/Bayeux-Tapestry-009.jpg see there for a famous example of the saxon shield wall at the battle of Hastings.

Breaking the shield wall by finding and widening a small gap was a major part of battles but I think many people under estimate the size of a shield which could easily be 75cm-1m across. Baring in mind people tended to be slightly shorter back then if you were to stand with knees slightly bent and legs apart to properly brace and keep your head low such that only your eyes are above the edge of the shield and you can quickly duck down further then the shield will still extend to about knee height. While that does leave your lower leg exposed anyone going down low enough to be able to get an clear thrus at the lower legs will also likely expose themselves to the person they are attacking and the people either side of them. As such while it is possible it is also risky. Another thing to consider is that it requires relatively little training to stand in a line and overlap your shields, this allowed the non-professional armies of the early middle ages to remain relatively affective and helped to limit their casualties.