How did sand dunes appear in the midst of a fertile region?
It appears the sands at Talakad are the result of a ecological disaster that occurred in the 17th Century. At the time, a dam was constructed north of Talakad on Kaveri River which caused the river to be diverted. This exposed a sand deposit in the river which was subsequently moved to Talakad by monsoonal winds. The following quote is from this source AB Roy &amp; MS Sethumadhav, 2014:


  It is thought to have originated due to an ‘eco-disaster’ that
  lashed the region during the 17th century. That was the
  time when the normal flow of the Kaveri River was
  impeded because of the construction of a dam on the up-
  stream side, north of Talakad and subsequent diversion of
  water in a different direction. As a result, the water level
  at the point bar on the concave side of the Kaveri meander southwest of Talakad was significantly lowered leading to the buildup of sand at this place. The accumulated
  sand was then actively reworked and carried by the
  northeasterly monsoon winds, which were subsequently
  deposited around Talakad engulfing the temples and other
  settlements in the region. The yellow coloured, uniformly fine-grained and homogeneous nature of the sand
  without any clay material in it provides further proof that
  the air-borne sand deposited as dunes