An epoch refers to a subunit division of geological time used to describe portions of earth's history as a component of the geologic time scale. It represents a smaller segment relative to the larger eras, periods and eons delineating stretches of planetary change.
Some key attributes help define geological epochs:
- Duration - Epoches generally span several million years or more, for example the Miocene epoch lasted about 20 million years from 23-5 million years ago. Boundary dates get estimated from radiometric dating of reference rock strata.
- Climate & Geography - Epoches capture consistent paleoenvironmental conditions before climate regime shifts result in new epoch classification, i.e. the transition from the Pliocene's warmer forests to Pleistocene ice sheet expansion 2.6 million years ago.
- Fossils - The rise and fall of plant and animal families, plus physical markers and assemblages tracing evolutionary branches and extinctions help characterize ecological eras signifying changing epoch periods.
- Stratigraphy - Standardized epoch boundaries coincide with agreed upon global reference points marking transitions preserved as visible signatures in geological sections and successions of deposited rock layers.
By recognizing epoch distinctions using physical, chemical, floral and faunal markers - improved chronologies emerge clarifying the sequence of evolutionary and environmental changes unfolding on earth across deep history through today's Holocene interglacial warmth.
Epochs represent distinct time intervals within the relative geological time scale, quantitatively defined by radiometric dating and events preserved in strata successions helping split earth history into coherent chapters. Updated epoch delineations depend on scientific review of accumulated evidence and consensus.
For example the Pliocene epoch spanning 5-2.5 million years ago was originally undifferentiated until magnetic reversals and faunal analyses allowed formal subdivision into Early and Late Pliocene sections. This distinction captured the onset of Northern Hemisphere glaciation which altered global conditions. Refined classification thus enhances contextual resolution.
Likewise Plio-Pleistocene boundaries shifted from 1.8 million years ago to the currently accepted 2.6 million years ago based on ice core data signaling the key climate transition toward cyclic icehouse-greenhouse Earth variability that came to define the ensuing Pleistocene pattern up to the Holocene interglacial.
Careful geological detective work further located the aptly named Golden Spike stratigraphic marker bed in Italy to anchor the formal Eocene-Oligocene transition 33.9 million years ago when rapid Antarctic ice sheet growth starkly cooled global temperatures heralding the dawn of Neogene climate phases.
Thus epochs represent pragmatic temporal units qualitatively separated using identified physical, biological and chemical evidence sampled worldwide. These units provide geohistorical structure aiding global correlation and scientific narratives explaining earth system changes through deep time.
As the Himalayas uplifted during the Cenozoic era from the collision of the Indian and Eurasian plates, Nepal’s dramatic high mountain topography preserves a rich record of environmental shifts that allow demarcating major epoch transitions within the regional geology.
For example, characteristic sedimentary sequences found along the Sub-Himalayan range front contain fossilized pollen and leaf assemblages indicating Nepal hosted subtropical, warm-temperate forests during the early-mid Miocene epoch. But by the late Miocene, large ice sheets expanded as the monsoon intensified, helping carve massive valley glacier systems.
This radical Pliocene climate transition towards colder icehouse conditions left clear geologic signatures - as seen in the enormous fluvioglacial outwash deposits, tills, and lacustrine beds peppering foothill zones. These formations represent epochal Pleistocene glacier advance-retreat cycles that extensively remodeled Nepal’s peaks and valleys over 2 million years.
Thus Nepal almost tangibly manifests epochs, with Lydekker’s Line famously delineating the subcontinent’s northward drift marking eras. And Holocene sediments continue accumulating, with anthropocene signatures interjected by extensive rice farming terraces carved over centuries. Epochs thereby enable contextualizing Nepal’s landscape evolution stages framed by identifiable climate shifts.
The region’s rich sequential geology has much to clarify regarding the discontinuities and thresholds signifying epoch-level global change scales that transformed Asian paleoenvironments over millions of years - imprinting signatures still readable across Nepalese bedrocks.