Hi Bhante, 
The writing of David Arnold on the history of cremation in India is informative in this regard. Burning Issues: Cremation and Incineration in Modern India - PMC
The universality of cremation as a historical method for disposing the dead in India is a bit of a myth.
I quote:
For instance, in Bombay in 1880 only 43 percent of the Hindus who died in the city were cremated—that is, 5,569 out of the 13,037 dead.
In Madras, too, burial rather than cremation was the more common means of disposing of the Hindu dead. In 1914, 5,362 bodies were cremated (all Hindus) compared to 18,812 buried, including 13,919 Hindus (Madras 1915: 59–60).
Cremation is expensive and fuel intense and has historically been inaccessible to the lower social classes. It also has some tricky points with rain (e.g. during monsoon) and wet weather as well as availability of firewood. Up until 1830, the policy of the British administration was to bury the bodies of Hindu prisoners unless they were high caste.
There has always been a diversity of practice in South Asia. The concept of the normative Indian Hindu (Sikh, Buddhist, Jain) cremation is probably a by-product of the creation of the Indian state and Hinduism as a global religion in the 19th-20th century. As a practice associated with the Rajputs and Marathas, and with Sikhism, as well as the Hindu elite, cremation gradually took on the status of being “the” Indian method of disposing the dead during the period of formation of Indian nationhood.
This has resulted in a more widespread uptake of the practice in general, which Arnold terms “Sanskritisation”.
This is not to say that cremation and specific forms of cremation are not deeply important to the Hindus who follow sastra on these points (as evidenced by the fact that that access to open pyre cremation has gone to court in the UK). But historically when it comes to cremation, Indian religion has split along class lines and sastra has been for the elite.
Universal access to cremation became important to some of the lower castes as part of a social reform agenda: Arnold references a 1920 call from the Mahar Parishad, for example, for the low class Mahars to cremate their dead.
From the Mahaparinibbana Sutta, we can see that the Buddha was given a top-notch funeral, not actually that of a common person. DN16
“But sir, how to proceed when it comes to the Realized One’s corpse?”
“Proceed in the same way as they do for the corpse of a wheel-turning monarch.
So we shouldn’t assume that cremation was actually the majority method of disposing of bodies even in the Buddha’s time. It likely wasn’t: it’s more probable that abandonment of bodies was actually the statistical norm (burial also has issues like topsoil and land availability, as well as watercourse pollution). Hence why the Satipatthana Sutta has invited contemplation in this way.
Wikipedia defines a charnel ground as, “an above-ground site for the putrefaction of bodies, generally human, where tissue is left to decompose uncovered”, which may or may not contain a crematorium. There is probably some logic to this definition where the term is used in a pre-modern sense.
