http://cosmos.colorado.edu/astr1120/l9S4.htm
Recall that the Earth (and the other planets in the
Solar System) formed by the accretion of gas and dust
out of the protoplanetary disk.
http://cosmos.colorado.edu/astr1120/l9S4.htm
At the very end of this process, there were around
10 large planets -- more or less what we have now in
the Solar System -- plus a large number of smaller
objects called planetesimals. Planetesimals
are (were) like very large comets and/or asteroids
which could easily have been 10s to 100s of kilometers
in diameter.