Thermal storage enables concentrating solar power (CSP) plants to provide baseload or dispatchable power. Currently CSP plants use two-tank molten salt thermal storage, with estimated capital costs of about 22-30 $/kWhth. In the interests of reducing CSP costs, alternative storage concepts have been proposed. In particular, packed rock beds with air as the heat transfer fluid offer the potential of lower cost storage because of the low cost and abundance of rock. Two rock bed storage concepts which have been formulated for use at temperatures up to at least 600 °C are presented and a brief analysis and cost estimate is given. The cost estimate shows that both concepts are capable of capital costs less than 15 $/kWhth at scales larger than 1000 MWhth. Depending on the design and the costs of scaling containment, capital costs as low as 5-8 $/kWhth may be possible. These costs are between a half and a third of current molten salt costs.