The Damuda series of Gondwana system has three stages, namely, Raniganj, Barren Rocks and Barakar. The middle stage is called Barren because

The Damuda series of the Lower Gondwana system is divided into three stages: the Barakar (lower), the Barren Measures (middle), and the Raniganj (upper). The middle stage is specifically termed 'Barren' because it is devoid of workable coal seams, unlike the coal-rich Barakar and Raniganj formations [4]. Geologically, the Barren Measures (also known as the Kulti formation) consist of thick sequences of carbonaceous shales and sandstones [5]. A distinguishing feature of this stage is the presence of iron-stone shales, which contain iron ore in the form of siderite and limonite [5]. While these iron deposits are often of inferior quality compared to other sources, their presence alongside the total absence of exploitable coal justifies the name 'Barren' in the context of coal mining. Thus, the stage consists of iron-bearing formations but lacks the coal found in the surrounding stages [5].

Sources

1. [2] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 1: Geological Structure and formation of India > THE MESOZOIC ERA (THE GONDWANA SYSTEM) > p. 16
2. [4] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gondwana
3. [5] https://www.lkouniv.ac.in/site/writereaddata/siteContent/202003291621086820vibhuti_Stratigraphy.pdf
4. [3] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 1: Geological Structure and formation of India > Panchet Series > p. 18