Generally, Mudstone has relatively indistinct joints and generally exist in massive bedding or interbedding with other sedimentary bedrocks.
But, these samples are hardened and irregular, as if being joined together from pieces with bonding agents. One sample looks like conglomerate with same materials. These rock samples are from a gold mine at Siniawan near Bau near Kuching.
Imagine, millions of years ago this bed of sedimentary rock was intruded by hot lava below which also brought the gold ingredients to the surface. As the sedimentary rock experienced high temperature and pressure, the soft rock got baked, shrunk and cracked, forming thousands of micro-joints. Chemical solutions then seaped in and closed all the joints.
So, there it is, our fractured metamorphosed Mudstone near the surface as humans excavated down to look for gold.
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