During my recent field trip to the Konkan west coast one of the geologists leading the trip mentioned that all the famous Buddhist rock cut monuments situated in the Deccan volcanic province were excavated in lavas of the Bushe Formation.
I had never given any thought to this before. If you look at the various famous Buddhist rock monuments...Ellora, Ajanta, Bhaja, Karla, Elephanta ...they do occupy similar looking basalt slopes and escarpments. Those are the lavas belonging to the Bushe Formation confirmed the geologist.
The Deccan Basalt lava pile is subdivided into hierarchical geological units - each corresponding to an eruptive episode - based on their geochemical similarities. Here is the basic chemical stratigraphy:
The Bushe Formation is quite thick and made up of compound lava flows. That means that eruptive episodes were made up of blobs of lava units that spread and overlapped and coalesced with other lava units to form a compound unit, forming a humpy undulating lava pile in the process.
Like a overlapping stack of pancakes...like this one..with the Bushe Formation for comparison..
So why did Buddhist monks and artisans choose these lava flows out of the many rock slopes and escarpments from other lava flows available..?
My quick speculations:
1) East of the Western Ghat escarpment is the Deccan plateau which is dissected into spurs of Deccan Traps and flat valleys. The Bushe Formation is well exposed in these spurs forming slopes and thick rock escarpments.
2) The locations of the caves in this part of the Deccan where the Bushe Formation is prominent are along ancient trade routes. Patronage from sympathetic merchants and rulers provided sustenance for the monks and artisans.
3) The Bushe Formation occupies a convenient middle tier in the multi-storey Deccan landscape.
Landscape evolution has transformed the Deccan lava pile in to a series of steep slopes and escarpments. The Bushe Formation especially the escarpments are situated above forested slopes which grade into the valley floors. That would have made Bushe escarpments ideal locations... isolated enough from the roads and villages to suit the Buddhist monastic lifestyle but still accessible via the forested slopes. Many of these caves also acted as resting stops for traders and merchants and so making them not too inaccessible by going too far up the landscape might have been a consideration.
Here is a picture of the Bhaja caves near Pune...occupying a steep rock slope.
Formations younger than Bushe form the upper story escarpments of the Deccan landscape. Access to these may have been too tough to negotiate.
4) The workability of the rock material.
Maybe there is a difference in the way rock material of the Bushe gives way to hammers and chisels compared with other especially the younger lavas. The Bushe has been described as a blocky kind of a lava while the younger lava which also form escarpments have been described as having splintery properties that may not give way to form the rounded and blocky shapes the artisans wanted.
I don't know. This is a problem that some experimental Archaeology can throw light on.
These rock-cut monuments are a wonder. If you are in the area don't miss out visiting them.