Only when the little hill at the junction of the River Thames with the River Lea, somewhere about sixty miles from the open sea, was reached could any such crossing be made. We said that in the earliest days of London there was, facing the hill, a great flat which at high tide became a wide lagoon, stretching southwards to Sydenham. Now this was quite shallow; moreover, a long tongue of fairly firm gravel ran right out northwards from the firmer ground till it came to a point nearly opposite the Llyndin Hill. This firm bed enabled the Britons to lay down, across the marsh, some sort of a road or causeway joining up with the main Kent road, and so gave them another lower and practicable crossing of the River, which, of course, meant a shorter road to the Midlands and the north. This crossing—in all probability a ferry—laid the foundation-stone of the prosperity of London town, and the building of the first bridge cemented that foundation. Why? Simply because such a bridge, in addition to being a passage _across_ the River, became a barrier to any passage up and down the stream. Bridge-building was not at a very advanced stage, and, of necessity, the arches were small and narrow. This effectively stopped traffic passing up from the seaward side. On the other hand, the small arches meant a very great current, and this, with any considerable tide, rendered the “shooting” of the bridge by smaller boats an extremely dangerous affair: thus traffic from the landward side came to a standstill at the bridge. This meant that ships, bringing goods up the River from the sea, must stop at the bridge and discharge their cargoes: also that goods, coming from inland to go to foreign parts, must of necessity be transhipped at London. It was inevitable, therefore, that once the bridge was in position a commercial centre must arise on the spot, and almost certain that in time a great port would grow into being. So that we may say quite truly that _the Thames founded London_. CHAPTER TWO _How the City grew_ (_Roman Days_) Who built the first bridge? We cannot say for certain; but it is fairly safe for us to assume that the Romans shortly after their arrival in Llyndin set to work to make a strong wooden military bridge to link up the town with the important road from Dover. Thousands of Roman coins have been recovered from the bed of the Thames at this spot, and we may quite well suppose that the Roman people dropped these through the cracks as they crossed the roughly constructed bridge.