Column density maps of ¹³CO (frame a) and C¹⁸O (frame b), and their ratio (frame c) normalised to the expected ¹³CO/C¹⁸O from Wilson & Rood (1994). Frame d: Excitation temperature of ¹²CO. Frame e: Integrated ratio. The sources s1454 (north clump) and s1446 (NGC 2264 H) are indicated in each frame. The ellipses in frame a show the GCC sources with the same colour code as in Fig. 2: red for protostellar, blue for starless, and black for undetermined. A scale bar is shown in frame c. The structures named “the arc” and “the bar” are shown in frame e.