In this paper; I treat ice–water interface as a codimension-1 ridge endowed with a Z₂ sheet-exchange involution (choice of normal vector). The Stefan balance provides a sheet-even latent-heat flux diagnostic, while the signed normal heat flux (or signed curvature) furnishes a sheet-odd parity marker. Using a sliding-window analysis of high-frequency temperature and displacement measurements on a frozen lake, I apply the Bilax Return test (Eq. E3) to demand simultaneous closure of the even diagnostic and inversion of the odd marker under the ridge-pairing map. A complementary acoustic proxy (Pac) supplies an independent odd observable. Successful PASS outcomes certify that the freezing front behaves as a dual-sheet seam, establishing a rigorous field-theoretic description of lake-ice growth and its audible signatures. Water surface freezing as a codimension-1 interface embedded in 3D, with a thin near-surface layer where gradients and anisotropy concentrate. The “dual sheet” is the two-sided lift of that interface: choosing the normal vector n or −n- is a Z2 ambiguity. I applied the same Z₂-seam machinery that underpins our atmospheric curvature gate to the ice–water freezing front. The interface normal admits the binary involution ι :n ↦ −n, giving a two-sheet lift Σ~ and observable base Σ. Within each analysis window I extracted the four quadratic-echo amplitudes (7 Hz, 14 Hz, 8 Hz, 16 Hz) and formed the dimensionless seam kernel Q. Its argument yields the arrow-level clock θ; the axis-level clock θ ⁣ax is obtained by the Z₂ quotient θ↦θ+π. The odd observable was taken as the signed normal heat flux Pf, while the even diagnostic was the latent-heat release rate If. Using the palindromic pairing of windows I performed the Bilax Return test, obtaining tolerances εP=0.12 and εI=0.08 (bootstrap, B = 3000). Both the freezing front and the atmospheric gate passed the Bilax test, and the two standard nulls (phase-scramble, seam-off) each caused a complete loss of the palindrome certificate. This is the geometric origin of our ridge/seam involution. ι2=Id,ι:n↦−n. Terms & Conditions: The involution ι is not a physical mirror. It is the orientation flip of the local chart on the interface (the sheet exchange).