Secondary qauv dav hlau
Xws li cov qauv dav hlau yog tsim nyob rau hauv qhov kev txiav txim ntawm sab nraud quab yuam tom qab tsim cov pob zeb loj, nrog rau kev tshem tawm fissures, weathered fissures, theem nrab intercalation txheej thiab av nkos intercalation, thiab lwm yam.