An igneous dyke is where molten magma has forced it’s way up into a crack between existing rocks. Dykes can extend for many kilometers in lines on the surface.  There’s a dyke marked on the Sydney 1:250,000 geological sheet that cuts across the M1 and old pacific highway just south of the Hawkesbury River. It’s mentioned in the “Geology along state highways” section of The Field Geology of NSW. I’ve tried to spot it many times while driving down the M1, I think I’ve seen it but it wizes by so fast it’s hard to be sure.

Last weekend I went for a ride down the old Pacific Highway and I’m pretty sure I found it.

It’s a vertical channel about 4m wide that cuts through the surrounding Hawkesbury sandstone. The material in the dyke was heavily weathered and seems softer than the surrounding sandstone.