Currently reading through Bernard Bachrach's Early Carolingian Warfare and he made a point of highlighting how the early Carolingians used sword phalanxes. However he did not really go into detail how these formations worked, and it seemed pretty similar to a shield wall formation. However is the shield wall something that comes about later? If not, what is the difference?
The words "phalanx" and "shieldwall" are just terms for a general close order formation where each warrior is grouped in tightly next to the other. However, while every shieldwall is a phalanx, not every phalanx is a shield wall. The term shield wall is used for when the warriors have shields, which overlap one another forming a strong barrier. Renaissance pike formations did not utilize shields, but were still considered phalanxes when they closed up, usually for the purpose of resisting a cavalry or infantry assault. Likewise, whilst Sarissa pike-men had shields, they were too small to cover each other.
Pedantics aside (Phalanx is an organizational unit rather than a formation), the "Phalanx formation" is just a glorified shieldwall.