A fundamental question arising in the dynamics of thin-skinned fold-and-thrust belts concerns the role and origin of large-scale folds and thrust sheets that make up the composite structure, which may, at another level of approximation, be treated as a homogeneous “critical” wedge.  Information on the internal geometry of these bodies and their tectonic history may be derived by application of kinematic idealizations to geometric field data and from inversion of the history of erosion and deposition.  But for obvious reasons, such as the absence of quantities containing the essential physical dimension of mass from any kinematic description, the formulation of this and other questions and their broad investigation must be based on a complete mechanics capable of fully describing the underlying processes.  This is so whether such models are “mathematical” or analog models conducted in the laboratory.  A simple 2-layer model for folding of a layer overlying weak ductile salt as in the Central Appalachian Plateau Province and the Zagros Fold Belt requires 9 physical parameters (salt: n₁ – stress exponent, h₁ – effective viscosity, r₁g – specific weight, H₁ – thickness; sedimentary rock layer: K – yield stress, rg, H; D – topographic diffusion constant, D_xx rate of shortening) with dimensions of mass, length, and time, giving 6 dimensionless groups (n₁, R=2h₁|D_xx|/K, S = rgH/2K, T=H₁/H, E = DH²/|D_xx|, d = (r – r₁)/r.  The same rheological assumptions may be incorporated into a complementary critical wedge model.  A comparison of model results and field observations provides a test of internal consistency.