A river runs through it
In the Morni Hills of Haryana, smooth, rounded stones lie scattered at elevations up to 1,220 meters (4,000 feet). These are not ordinary rocks. They are river gravels, the kind of water-worn cobbles found on the bed of a powerful river. Their presence high in these hills records a dramatic geological upheaval and a river system that has long since vanished. The hills themselves are part of the Siwalik range, the outermost foothills of the Himalayas, composed of sediment eroded from the greater mountains to the north.
The gravels are part of a geological layer known as the Boulder Conglomerate Formation, the youngest unit of the Upper Siwalik Subgroup. This formation dates to the Early and Middle Pleistocene, roughly 1.8 million to 500,000 years ago. The deposits consist of massive beds of conglomerates—pebbles, cobbles, and boulders cemented together in a sandy matrix. The size and rounded nature of these stones indicate they were transported a significant distance by a high-energy, braided river system, typical of the environment at the foot of a rising mountain range. The river that deposited these gravels was a major waterway, likely a precursor to the modern Yamuna river. It flowed before the Morni Hills had risen to their current height.
Tectonic shift
The landscape of northern India was once very different. Before a period of intense tectonic activity, the river that laid down these gravels flowed through this area. Then, beginning around 2.5 million years ago, immense pressure from the ongoing collision of the Indian and Eurasian plates caused the land to buckle and rise. This uplift, associated with major fault lines like the Himalayan Frontal Thrust, literally lifted the old riverbed into the air.
This process is "topographic inversion," where a feature that was once the lowest point in a landscape—a river valley—becomes a high point. As the surrounding, softer rock eroded away over millennia, the hard, cemented river gravels remained, preserving the channel as a hilltop. The original river was forced to change its path, a process known as river capture. Evidence suggests that tectonic events caused the ancient Yamuna to shift its course eastward, eventually becoming a tributary of the Ganges. The Morni Hills gravels are the physical record of this powerful geological event, a fossil not of an animal, but of a long-dead river drainage pattern frozen in stone.