The sensory receptor responsible for detecting this sensation is; Mechanoreceptors
Mechanoreceptor is a type of somatosensory receptors that relays extracellular stimulus like touch, pressure, stretching through gated ion channels. The Merkels disk contained in the Mechanoreceptor is slow to responding to touch while the Meissner's Corpuscles responds fast to touch. that is is why it took some time for the skin to detect the sensation and the presence of the rock after sitting on it.
Hence the sensory receptor responsible for detecting this sensation is Mechanoreceptor.
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