Dolerite layers in lavas



In East Houghton on Waterworks Road there is a large basalt boulder, likely a big piece of one of the Keweenawan lavas.  This boulder is cut by three dolerite layers which formed inside the lava flow, late in its cooling history, after the top and bottom of the flow had solidified, but when the middle of the flow was at least partly molten. Dolerite is a coarser and more volatile rich basalt layer which was molten late in the solidification.  Large lava flows in the Keweenawan may have taken centuries to solidify.  The large size of this boulder reflects the massive geometry and denser, more resistent flow interior.  The lava flows solidify at their base and then upward and from the top, migrating downward. From many field observations, dolerite or pegmatites tend to form near and slightly above the mid point of lava flows, the last part of the lava to cool. They are trapped in a tabular geometry by the solidified top of the flow.  It may be possible to infer which side of the dolerite is the former top.


Dolerite or pegmatite layers


Glacial striae on the boulder



Can you suggest from looking at the dolerite layers which way is toward the top of the lava flow?