Top figure: a column density map fitted from the Herschel Hi-Gal data with two filament extraction regions superposed in cyan and purple. The purple extracted position-velocity diagram is shown in Fig. 10. Bottom figure: a position-velocity slice of the 68 km s^-1 cloud, shown in cyan in the left figure, which includes an 8 μm-dark cloud and the interaction region with the W51C supernova remnant. (bottom figure, top panel) H₂CO1₁₀ − 1₁₁ observed optical depth (bottom figure, middle panel) H₂CO2₁₁ − 2₁₂ observed optical depth (bottom figure, bottom panel) ¹³CO 1−0 emission from the Galactic Ring Survey (GRS Jackson et al. 2006) with H₂CO1₁₀ − 1₁₁ contours superposed. The weakness of the H₂CO absorption on the right half of the cloud corroborates the geometry inferred from comparison of the 1₁₀ − 1₁₁ and 2₁₁ − 2₁₂ lines in Fig. 8. The ¹³CO emission without corresponding H₂CO absorption at offset 0.2 degrees is primarily background material in the 51 km s^-1 cloud (see Fig. 11. These figures were made using wcsaxes¹² and pvextractor¹³).