It is not a very good picture but it appears to be a concretion. The white rocks appear to be Quartz.the sand and mud appears to have cemented the various rocks together into one rock. These are formed in various ways. One of the most common is from alluvial flow from receding glaciers. One example of a concretion would be a pudding stone. See the puddingstone picture inserted here.
I tried posting a couple more pictures - it might be nothing and I am new to this but it seems so unique but not an expert at all :)
This is the top of the same rock
This is the other side of the 'rock'