1. Head of National Road Implementation Agency for Central Java - D.I. Yogyakarta Region, 2018, “South Coast Route to Grow Kebumen Beach Tourism (in bahasa)” https://binamarga.pu.go.id/balai-jateng-diy/berita/jalur-pansela-menumbuhkan-wisata-pantai-kebumen (accessed on June 2021).
2. Ministry of Public Works and Housing, 2021, “Ministry of Public Works and Public Housing Targets 427 Km of New Toll Roads to be Completed Throughout 2021 (in bahasa)” https://pu.go.id/berita/kementerian-pupr-targetkan-427-km-ruas-tol-baru-tuntas-sepanjang-tahun-2021 (accessed on June 2021).
3. Directorate General of Highways, 2019, “South Coast of Central Java DIY Handled 247.17 Km (in bahasa)” https://binamarga.pu.go.id/index.php/berita/pansela-jateng-diy-tertangani-24717-km (accaessed on June 2021).
4. S.D. Kurniawan, “Study of Karangbolong Ancient Volcano Facies in Ayah Subdistrict and Surroundings, Kebumen Regency, Central Java Province (in bahasa),” undergraduated thesis, Universitas Gadjah Mada, 2018.
5. de Vallejo, L. I. G., and Ferrer, M., “Geological Engineering” (CRC Press/ Balkema, Leiden, Netherlands, 2011), 678 p.