What is limestone ? Limestone : It is a calcareous sedimentary rock. Limestone or Calcium Carbonate is generally not soluble in water. But it gets readily dissolved in rainwater or acid solutions when it is transformed to Calcium Bicarbonate, which is not resistant to erosion and highly permeable. Surface water sinks underground through holes on the surface of the earth carved by solution actions of rainwater in Limestone regions. Limestone may be white, grey, green or blackish in colour. It is used as a raw material in making cement and in the iron and steel industry. What is sandstone? Sandstone : It is a sedimentary rock rich in sand. It is resistant to erosion in spite of being highly porous and permeable. Architects use it in making epitaphs and monuments. It is yellow, orange, red, pink, white or grey in colour. Soil in regions dominated by Sandstone is saline and infertile. Many architectural wonders in India, like the Red Fort, the temples in Udaygiri-Khandagiri and K