Past builders have developed very low-embodied energy construction techniques optimizing the use of local building materials. These techniques are a source of inspiration for modern sustainable building. Unfortunately, this know-how was orally transmitted and was lost as earth construction fell into disuse during the 20th century in European countries. The absence of written documents makes necessary to use an archaeological approach in order to rediscover these construction strategies. Micromorphological analysis of thin sections collected in earth building walls was used for the first time to describe cob construction technique and highlighted several typical pedofeatures allowing to clearly identifying this process. Finally, a first comparison of the cob and rammed earth micromorphological features permitted to identify two key factors to distinguish these two techniques, the manufacturing state (solid or plastic) and the organization of the material in the wall.