New Delhi, April 20 -- The Ganga mostly flows east-southeast across the Indo-Gangetic alluvial plains towards its mouth in Kolkata. As it approaches Varanasi in eastern Uttar Pradesh (UP), it encounters something unusual: an outlier of ancient Vindhyan sandstone that juts northward into the otherwise soft alluvial plain.

The river cannot cut through this resistant rock as easily as through alluvium, so instead of continuing straight eastward, it deflects northward, running along the western flank of this Vindhyan rock ridge. It flows north for a short distance past the iconic ghats before eventually bending eastward again. This is what geomorphologists technically call lithological control over a river platform.

The northward flow is ce...