Gravels from 45m Depth below Ground Level

These gravels were extracted with great difficulty from a borehole 45m below the ground level at the upper stream of Baram River. How did they come about?
The River of the hilly area was the formerly deeply eroded valley during the ice age when sea level was low. The rise of sea level about 12,000 years ago and warming resulted in plenty of rain, brought down sands, gravels and boulders  to deposit at the upper stream of the submerged valley.
During boring,  clay,silt,sand could be easily washed out using pressured water flow but not the larger gravels. These gravels accumulated below the bottom of the borehole as the drilling got deeper. It needed some skill to get some of them out of the borehole. Sometimes, the drillers used core barrels and some times they used grouting.

But these gravels have serious impact on the Standard Penetration Tests at the test layers, very high blow counts!  So, engineers have to interpret the soil layers containing gravels,with caution. For example, N=50 blows per 300mm does not mean dense sand, it may be just medium dense sand.